Creating Variation: Meiosis, spermatogenesis, oogenesis,
fertilization, genomic imprinting

By Mai Stern, Nandu Mohan, and Matt Ostrow














Meiosis
































Stages of Meiosis (Click here for an animation)










































^Video Created by Matt Ostrow




  • Sexual reproduction results in a greater degree of variation than asexual (clone-copy) reproduction; the different combinations of two parents’ genes allows for greater genetic variation (E)
  • Animals, for example humans, pass on their genes through chromosomes, which can be mapped by karyotypes.
    • o Of the 46 chromosomes each human somatic (body) cell has, two determine gender characteristics, which are known as the sex chromosomes. Girls have a homologous pair of X chromosomes (XX), whereas men have an X and Y chromosome (XY)
    • o We inherit 23 chromosomes from each our parents; half of the 46 chromosomes we have are paternally derived and the other half are maternally derived. (E)
    • o Sperm cells and ova (egg) cells are haploid cells as they only contain 23 chromosome each, consisting of 22 somatic chromosomes and 1 sex chromosome (either an X or a Y). The haploid number of humans is referred to as n and n=23 chromosomes.
    • o Via sexual intercourse, the haploid sperm cell from the father approaches and fuses with a haploid ovum of the mother through the process of fertilization or syngamy (E)
    • o A zygote is a fertilized egg that contains both the maternally and paternally derived genes in the form of all 46 chromosomes. A zygote, like all other cells with two sets of chromosomes, are diploid cells , with a diploid represented by 2n=46 (E)
    • o Sperm and ova cells are known as gametes and are derived from the gonads (the testes and ovaries, respectively). Unlike somatic cells, they are not produced by mitosis as each round of fertilization would double the number of chromosomes present in each cell, causing a variety of genetic complications. (E)
    • o Meiosis¸or the cell division of gametes, occurs in the ovaries or testes in order to produce the haploid sperm cells and ova cells.
  • Differences of Sexual Life Cycles

life_cycles.jpg
Life Cycles (E)
    • oAnimal Life Cycle
      • § A diploid multi-cellular organism reproduces through the production of haploid gametes (meiosis). These gametes fuse during fertilization to form a diploid zygote that will develop into the mature, multicellular diploid organism (E)
      • § The timing of meiosis in mammals greatly differs between males and females as a result puberty. For example, male germ cells (spermatogonia) in the testes do not undergo meiosis until after puberty (A).
      • hlm.png
        Human Life Cycle (E)
    • o Most fungi and some algae
      • § After the gametes fuse into a diploid zygote, meiosis occurs before offspring develop in order to produce haploid cells that then divide by mitosis to create a haploid multicellular adult organism. This organism then produces gametes via mitosis to restart the process through a diploid zygote. (E)
    • oPlants and some algae
      • § These organisms alternate generations through haploid (gametophyte) and diploid (sporophyte) multicellular stages. Meiosis of sporophyte produces haploid cells known as spores, which creates another organism without fusion to another cell. Via mitosis, the spore divides to generate the multicellular gametophyte organism and later uses mitosis to produce gametes that again form a sporophyte zygote that develops into a diploid multicellular organism (E)
  • Process of Meiosis

how_meiosis_reduces_chromosome_number.jpg
Diploid to Haploid in Meiosis (E)
    • o Meiosis can be divided into two consecutive cell divisions, meiosis I and meiosis II in order to result in four haploid daughter cells
    • o In prior to Meiosis, the homologous pairs of chromosomes in the diploid parent cell have been replicated. Thus, the parent cell has pairs of replicated chromosomes where each pair is made up of 2 sister chromatids. (E)
    • o In Meiosis I, the homologous chromosomes divide from each other and form haploid cells, each with replicated chromosomes.
    • o In meiosis II, the sister chromatids separate in order to form haploid cells with unreplicated chromosomes, for a total of 4 haploid cells.
  • Detailed Summary of Meiosis 1: Separates homologous chromosomes

pp3.png
Meiosis 1 (E)
    • o Interphase: Before meiosis 1 occurs, there is a period of interphase of the cell cycle in which chromosomes replicate. For each of these chromosomes, there are two genetically-identical sister chromatids. These sister chromatids remain attached at the centromeres. Additionall, the centrosomes also replicate and divide. (E)
      • § Synapsis occurs-in which a protein structure known as the synaptonemal complex- binds the homologues together in a tetrad, four-chromatid cluster. This tetrad cluster is also known as a bivalent (A).
      • § Due to the overlapping criss-cross pattern of the chromatids (the contact points are known as chiasamata) crossing over or recombination occurs. Not only do the chiasmata hold the chromosomes together until anaphase I is reached, but chromosomes trade gene segments at similar loci during this genetic recombination.
      • § Cells that are unable to properly form chiasmata may not have the ability to segregate their chromosomes during anaphase I and therefore cause the formation of aneuploid gametes with abnormal numbers of chromosomes (A).
As Prophase I is the longest part of the process of meiosis, cytologists have divided it into multiple stages based on the appearance of the choromosomes. (A)
  • Lepotene stage (thin threads)
  • Zygotene (paired threads)
  • Pachytene (thick threads)
  • Diplotene (two threads)
Additionally, more information regarding chromosome alignment can be found in the additional information section in the Meiosis section.
  • oMetaphase I
    • § Chromosomes in homologous pairs then align on metaphase plate double file, where each chromosome in each pair becomes attached to a kinetochore microtubule. One chromosome pair is connected via microtubules to one pole and the other pair to the other pole
    • oAnaphase I
      • § Like mitosis, the spindle apparatuses lead the movement of chromosomes to opposite ends of the cell. Yet, each pair of sister chromatids remains connected at their centromeres and move towards the same pole, thus separating the homologous chromosome pairs.
      • § Before the pairs of homologous chromosomes split to be contained in different daughter cells, meiosis cohesions and chromosome crossovers must reach completion in order to separate the arms of the sister chromatids. The failure to split the chromosome pairs to unique daughter cells is called nondisjuction and can cause aneuploidy, abnormal chromosome numbers. If chromosomes are present in triplicate, this is known as trisomy (Down syndrome is caused by an extra chromosome 21. Any additional sets of chromosomes is known as polyploidy.(E) (A) Aneuploidy increases with maternal age, and can be estimated as frequent as 10% to 30% of all meiosis divisions. (A)
    • oTelophase I

      • § The chromosome pairs eventually reach the poles of the cell, thus concentrating the haploid chromosome set of two sister chromatids.
    • oCytokinesis
      • § Two daughter cells are formed when a cleavage furrow (animals) or cell plate (plant) forms in the center of the cell and “pinches” the cellular membrane into two separate entitities
  • Meiosis II

  • pp4.png
    Meiosis 2 (E)

    • oProhpase II
      • § During the prophase stage of meiosis II, the chromosomes of each of the two daughter cells move towards the metaphase II plate of their respective cells while the spindle apparatus forms.
    • oMetaphase II
      • § The chromosomes are positioned on the metaphase plate in mitosis-like fashion, with the kinetochores of each of the sister chromatids directed towards opposite poles
    • oAnaphase II
      • § The centromeres connecting the sister chromatids breaking and move toward the opposite poles of the cell
    • oTelophase II
      • § Nuclei from at the opposite poles of the cell
    • oCytokinesis
      • § Cytokinesis results in the formation of four daughter cells that each have the haploid number (23) of unreplicated chromosomes. (E)
  • Independent Assortment (Mendel's law)

indep_assor.jpg
Independent Assortment (E)
    • o The orientation of the homologous chromosome pairs during metaphase I and metaphase II is random which leads to a 50% chance that a specific daughter cell will receive the maternal copy of a chromosome, likewise for the paternal copy. Given that there are four sister chromatids, there are four combinations of possible daughter cells as a result of meiosis I thus giving us 2n=4, n=4.
  • Given the haploid number of human chromosomes is 23, the resulting number of combinations of parental genes can be expressed by 223= approximately 8 million possible assortments of chromosomes. (E)
  • This law of independent assortment states that during gamete formation, the alleles of a gene on two separate chromosomes will segregate independently of other genes. However, linked genes do not assort independently. (I)
  • Related to this is Mendel's alw of segregation that relates that during gamete formation, the two alleles for each trait are segregated randomly.

Independent Assortment (F)
Independent Assortment (F)

  • Crossing Over/Recombination

external image crossingover01.jpg
    • o During early prophase I, homologous portions of two non-sister chromatids trade places in a process known as crossing over. This process is also known as recombination , in which alleles from homologous chromosomes of two non-sister chromatids replace each other, and thus create variation. (A)
    • o Recombinant chromosomes , or chromosomes that contain a combination of maternal and paternal genes, are formed and increase variation.
    • o Meiosis-specific enzymes are key to the process of recombination. The recA gene encodes a protein used in invading the DNA strand. RecB, recC, and recD genes also code for three polypeptides that fuse form the protein complex RecBCD, which unwinds double-stranded DNA and cleaves the strands. During this, genes ruvA and ruvB help form enzymes that initiate branch migration admist the resolving of Holliday structures by resolvase formed by the ruvC gene. (B)
    • crossover.png
      Crossing Over, Genetic Recombination at Chiasmata (E)

Random Fertilization


    • o Each sperm cell or egg cell is one of approximately 8 million different variations of that particular gamete cell, which increases the number of possible zygotes to be formed. Approximately 70 trillion diploid combinations are possible when solely taking account of independent assortment and random fertilization (2^23 sperm X 2^23 egg). Crossing over and other mutations also add to this number. (E)

Additional Information

Alignment of homologous chromosome pairs is difficult to show experimentally. However, new research on human and yeast cells denotes that chromosomes are unable to pair with one another until DSBs (double stranded breaks) are present in DNA after being caused by proteins with topoisomerase activity. For yeast, a resembling Spo11 protein initiates similar activity, whereas mutant versions of this protein are unable to sporulate. Next, DNA trimming that results in a 3’-leftover sequence that connects to a homologous section on another chromatid. As this strand extends, a synaptonmeal complex develops around the homologous pair and initiates synapsis. (A)
Recombination Models (B)
There are two primary models of recombination
  1. 1. Single Strand- After homologous chromosomes are aligned, a break is placed into one DNA strand of each chromosome, resulting into two free ends, and each end crossover over and invades the other chromosome. This intersection is known as a Holliday junction. (B)Following Holliday junction formation, branch migration occurs as the junction itself moves down the strand of DNA, where it is resolved vertically (causing recombination) or horizontally (not causing recombination). (B)
  2. 2. Double stranded breaks- The ends of the breakpoints are transformed into single strands through the inclusion of 3’ tails to these points. These ends then engage in strand invasion, produce two Holliday junctions, and follow the single-stranded model (B)


Holliday Junction Structure (B)
Holliday Junction Structure (B)
Recombination Enzymes
Recombination Enzymes





Additional Videos:

How Meiosis Works

Random Orientation of Chromosomes During Meiosis

Unique Features of Meiosis



Spermatogenesis


Process of Spermatogenesis:
definition: the production of mature sperm cells

It is a continuous and prolific process in adult males that occurs in seminiferous tubules of the testes. Gonadotropin-releasing Hormone (GnRH) from the hypothalamus stimulates the pituitary to release FSH and LH. It starts as LH (leutinizing hormone) induces the interstitial cells of the testes to produce testosterone. Testosterone combined with FSH (follicle stimulating hormone) then induces the maturation of the seminifirerous tubule. This stimulates the beginning of the sperm production.


Spermatogonia, the stem cells that give rise to sperm, are located at the periphery of the seminiferous tubules. (Primordial germ cells of the embryonic testes differentiated into spermatogonia). The spermatogonia give rise to primary spermatocytes (diploid) which undergo meiosis 1 and then become secondary spermatocytes (haploid). Those then undergo meiosis 2 which results in four haploid gametes called spermatids. Those develop into mature spermatozoa.
The developing cells undergoing meiosis move toward the lumen (central opening) The spermatids make their way tothe epididymis where they become motile.

The spermatozoa consists of a head, which contains the nucleus, a midpiece packed with mitochondria, and a tail (flagellum) to propel the sperm. In front of the nucleus is the acrosome containing enzymes which help the
sperm penetrate the egg.
(G) (H) (I)
external image sperm1.jpgmature sperm


-external image Vg4a3_V2CFcbJeLjM36g6pRLhI0rutUz5kaIOvkf3i-5IeqVSHtKatu_3uj6jrsuubz2yrnOmyCow_0dY60noznqwHlLrTQDPq55sfAuqOF59spArno
Process of Spermatogenesis

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Spermatogenesis


































Video about Spermatogenesis, Oogenesis, and Comparison







Oogenesis


The Process of Oogenesis:
defintion: development of ova-mature unfertilized egg cells-
Oogenesis begins prior to birth. The embryo contains oogonia, stem cells that give rise to egg cells, which are diploid. They multiply and begin meiosis. The cells temporarily stop at Prophase 1 and these cells are called primary oocytes. They remain in small follicles in the ovaries until puberty.
Primary oocytes (diploid) become reactivated by the hormone FSH (follicle stimulating hormone) periodically. FSH stimulates the primary oocytes to finish meiosos 1 and begin meiosis 2; at this point they are now secondary oocytes (haploid). The first polar body is formed and one haploid secondary oocyte result from one primary oocyte.
Secondary oocytes are released at ovulation and stop again at the metaphase 2 stage until fertilization. Once released during ovulation, the secondary oocytes in humans only finish meiosis after being triggered by human sperm. The result is one haploid ovum and a second polar bod.
Oogenesis produces 1 ovum for every oogonia.The cytokinesis stage of meiosis does not split equally; it creates one large cell and two small polar bodies
that will disintigrate.
(G) (H) (I)
Process of Oogenesis

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Fertilization



  • Fertilization activates the egg and brings together the nuclei of the sperm and egg

Fertilization: Penetration of the Egg
Fertilization: Penetration of the Egg

  • o Fertilization functions to combine the haploid set of chromosomes from two individuals into a single diploid cell, the zygote. (I)
  • o When a sperm contacts the surface of an egg, it initiates metabolic reactions within the egg that trigger the onset of embryonic development
  • o It is important to note that the secondary oocyte will only complete meiosis II (which the oocyte is stuck in at metaphase II) if it is fertilized by a sperm. The sperm contact restarts the cell cycle, activates anaphase-promoting complexes, and breaks down M-phase promoting factors. All of this takes place within a follicle, the fluid filled area of cells surrounding the newly developing egg. (D)

Sperm Capacitation


    • o Newly ejaculated sperm cannot easily fertilize an egg and must undergo capacitation, the process by which adherent seminal plasma proteins are removed (C). Additionally, it includes an influx of calcium ions from the extracellular space, a decrease in intracellular pH levels, and an augmentation in cyclic AMP (C). This process has a duration ofa few hours during sperm residence in the female reproductive tract. Capacitation causes a sperm to express hyperactive motility and destabilizes its membrane to prepare for the acrosomal reaction. (C).
  • Acrosomal reaction of sea urchins

Steps of the Acrosomal Reaction (E)
Steps of the Acrosomal Reaction (E)
    • o They have external fertilization
    • o A sperm cell is exposed to molecules from the slowly dissolving jelly coat that envelopes the egg
    • o The acrosome, a vesicle at the tip end of the sperm, releases its contents to the egg via exocytosis. This vesiculation exposes the acrosomal contents and leads to the leakage of acrosomal enzymes from the sperm head. (C) By the moment the sperm has traversed the zona pellucid, denudation of the anterior surface and inner acrosomal membrane of the sperm occurs.
    • o The acrosomal reaction is characterized by the release of hydrolytic enzymes that cause the acrosomal process, an elongating structure, to break through the jelly coat of the egg. The tip of this structure is covered in a protein that particularly attaches to a specific receptor molecule under the jelly coat located on the vitelline layer that its outside of the plasma membrane of the egg. This lock and key method insures same-species fertilization (I)
    • o Fast block to polyspermy is the result of membrane fusion causing ion channels to open in the egg cell’s plasma membrane, leading sodium molecules into the cell and causing a depolarization that limits the egg to be fused with only one sperm cell. Without it, multiple sperm could fuse to the membrane and cause the zygote to have abnormal numbers of chromosomes (E)
  • Cortical reaction

Wave of Ca2+ during Cortical Reaction
Wave of Ca2+ during Cortical Reaction
    • o Fusion of the egg and sperm membranes leads to this series of changes in the cortex of the egg cytoplasm. It triggers a signal-transduction pathway that leads to the release of calcium from the endoplasmic reticulum of the egg into the cytosol.
    • o The high Ca2+ concentration changes the vesicles known as cortical granules that fuse with the plasma membrane and release their contents into the perivitelline space, which is located in between the plasma membrane and the vitteline layer. Enyzmes from these granules then separate these layers while mucopolysaccharides creating an osmotic gradient that draws water in and swells the space. Thus, the vitelline layer becomes the fertilization envelope, a structure that resists the entry of additional sperm. (E)
    • o The voltage of the zygote returns to normal and the fast block to polyspermy becomes inactive. However, the fertilization envelope and changes in the surface of the egg cell acts as a slow block to polyspermy.
    • o The zona reaction is an alteration in the structure of the zona pellucid caused by proteases of the cortical granules. First, the zona pellucida hardens to prevent further fertilization. Second, the zona pellucida’s sperm receptors are destroyed to no longer allow sperm binding. (C)

Activation of the Egg


    • o The contact of the sperm cell to the egg cell increases the egg’s cell metabolic processes in preparation for the creation of the zygote.
    • o After approximately 20 minutes, the sperm nucleus merges with egg nucleus, thus creating the diploid nucleus of the zygote. DNA synthesis commences and is followed by the first cell division (E)
  • Fertilization in mammals

Fertilization in Mammals (E)
Fertilization in Mammals (E)

    • Diagramed Steps (E)
    • 1) The sperm breaks through the coat of the follicle cells and reaches receptor molecules to which it binds in the egg's zona pellucida
    • 2) After the receptors have bound themselves to the sperm, the acrosomal reaction begins and the sperm excrete hydrolytic enzymes into the zona pellucida.
    • 3) Membrane proteins of the sperm bind to the egg membrane's receptors after the sperm has reached the egg's plasma membrane.
    • 4) The sperm cell enters the egg after membrane fusion.
    • 5) A block to polyspermy occurs when cortical reaction enzymes aid in the hardening of the zona pellucida
    • o Mammals tend to have internal fertilization and have secretions in females that promote sperm motility towards an egg, located at the extracellular matrix of the egg known as the zona pellucida, which consists of proteins such as ZP3 that aid in the sperm’s degradation of the egg’s plasma membrane.
    • o A tract through the zona pellucid is formed by the consistent propulsive force of the sperm’s flagellating tail in addition to the results of acrosomal enzymes. (C)
    • o A receptor-ligand interaction that has a high degree of species specificity is an integral part of the binding of sperm to the zona pellucida. Sperm receptors of the zona pellucid are made up of glycoproteins. (C)
    • o In humans, microvilli of the egg cell engulf the entirety of the sperm into the egg, where the basal body of the sperm’s flagellum divides into 2 centrosomes and associated centrioles in the zygote
    • o Once the zona pellucid is penetrated, the sperm binds and fuses with the plasma membrane of the oocyte at the posterior, post-acrosomal region of the sperm head. Unlike other animals, humans do not use fertilin as fusion-inducing chemical.
    • o In vertebrates, sperm components such as mitochondria are degraded and are not incorporated into the embryo. (C)
    • Fertilization typically occurs in female humans in the Fallopian tubes and implantation takes place in the uterus

  • Chromatin from both the egg and sperm become encapsulated in a nuclear membrane and form the pronuclei. Each pronucleus consists of a haploid genome. The two pronuclei migrate towards one another, break down their membranes, and combine the two genomes into chromosomes in order to form the diploid organism (C).



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Genomic Imprinting –


According to Mendelian genetics, alleles from both parents will be given to the offspring throughout generations. Each allele is either recessive or dominant for a certain trait whether it is a maternal (inherited from the mother) allele or a paternal (inherited from the father) allele. Either parent can pass on a recessive or dominant allele for a certain trait. Using Mendelian genetics as a basis one would think that any allele would have the same effect on the offspring independent of which parent it came from. Now geneticists have disproved this part of Mendelian genetics by finding two or three-dozen traits in mammals whose expression in the offspring depend on which parent the allele came from. Genomic imprinting is the variation in phenotypes of the offspring depending on whether the allele is maternal or paternal. Genomic imprinting occurs in the formation of gametes and since the genes are imprinted differently in the sperm and the egg the zygote will only express one allele of that gene, either from the mother or the father. These imprints will help in the development of the offspring as it will pass to every cell during the early stages of development and be expressed in every cell of the resulting offspring. If an allele is a maternal one, generation through generation, it will always be imprinted for maternal expression. (J)

Diagram_1.png
First example of genomic imprinting - ( J )



One of the first imprinted genes ever to be identified was insulin-like growth factor 2 (Igf2) in mice. It was found after experimentation that only the paternal allele for this gene is expressed where as the maternal allele is hidden. (J)
The Igf2 gene can be seen in the above diagram. This is the one of the first evidences of genomic imprinting discovered. In the first part of the diagram a mutant gene trying to make the mouse offspring dwarf is passed from the mother. It can be seen that the mouse is not dwarf because the allele for size in this mouse is an imprinted. The maternal allele is not expressed, so the dwarf trait is not expressed in the offspring, only the normal sixe trait from the father is. In the second part of the diagram the father passes on the mutant gene and therefore all the offspring will be dwarf because the size trait is completely dependent on the paternal allele, which in this case is dwarf.
A genomic imprint is the gene that is imprinted in genomic imprinting that consists of methyl groups that are added to the cytosine nucleotides of one of the alleles. Because of this methylation, the alleles may become silent. For most genes, methylation does make it inactive, but in some genes methylation can in fact make the gene active. For the Igf2 gene methylation on the cytosine’s on the paternal chromosome leads to the expression of the paternal Igf2 allele. (J)
Diagram_2.png
Methylation - http://www.google.com/imgres?q=dna+methylation&um=1&hl=en&client=safari&sa=X&rls=en&biw=1238&bih=684&tbm=isch&tbnid=JKakQ7XHEtAm8M:&imgrefurl=http://cerch.org/research-programs/chamacos/chamacos-epigenetics/&docid=OQlaZAqtKAJAwM&imgurl=http://cerch.org/wp-content/uploads/2011/02/DNA-methylation-image.jpg&w=500&h=684&ei=gTVdT73gMsTr0gG0_s3XDw&zoom=1&iact=hc&vpx=411&vpy=153&dur=765&hovh=263&hovw=192&tx=118&ty=123&sig=107658057137550201945&page=1&tbnh=133&tbnw=97&start=0&ndsp=21&ved=1t:429,r:2,s:0



As the methyl groups are added to the DNA genes are silenced and repressed to determine the activity of the gene in the offspring. This methylation and turning on and off of genes is what the study of epigenetics and genomic imprinting is centered around.
Genes that are imprints are under greater selective pressures than normal genes are because of the existence and activity of only one copy at all times. Unlike other genes, imprinted genes don’t have a second copy to mask any mutations or variations. Because of this, imprinted genes evolve at a faster rate than regular genes; therefore, there can be very big differences in very closely related species because of the variations in genes that are silenced and activated during gamete formation. An example of this is when lions and tigers mate in captivity to produce hybrid offspring such as ligers and tigons. These hybrid offspring depend completely on what animal the mother is. Ligers are the biggest of big cats and are produced from a male lion and a female tiger. On the other hand tigons are produced from a male tiger and a female lion and they are the same size as the parents. These differences in the offspring are due to the imprinted genes of the mother. (K)

Liger.png
Liger - http://www.snopes.com/photos/animals/liger.asp#photo
Tigon.png
Tigon - http://www.google.com/imgres?imgurl=http://lion_roar.tripod.com/patrick_liger3.jpg&imgrefurl=http://lion_roar.tripod.com/Liger_Tigon.html&h=436&w=383&sz=58&tbnid=gZuPK-g_sUpWxM:&tbnh=100&tbnw=88&prev=/search%3Fq%3Dtigon%26tbm%3Disch%26tbo%3Du&zoom=1&q=tigon&docid=CvadZqQGAgswnM&sa=X&ei=-jNdT8GWFsj50gGA7uiWDw&ved=0CEEQ9QEwAg&dur=90



It is evident that genomic imprinting is a key factor to embryonic development and what genes and traits are expressed in the offspring. Genomic imprints have three stages in their life cycle: establishment, maintenance, and erasure. Establishment is when the genomic imprints get implanted into the male and female germ line and then go through further development ready to be passed on to the offspring. In the males, the genomic imprints become fully established and finish developing in prospermatagonia by the neonatal stage. In the females, the imprints are implanted to the germ line layers of the oocyte and then completely develop by the time the oocyte is almost completely developed. Next is the maintenance stage. Once the imprints are completely ready to be expressed they are passed on to the offspring by fertilization. But before the embryo implants, the imprints go through some more changes as the protamines are replaced by histones in the paternal genome. Once the active de-methylation of the paternal genome and the subsequent passive de-methylation of both parental genomes happen then the imprints will be able to maintain themselves. The last step of an imprints life cycle is erasure. Finally, the epigenetic imprints are erased in the primordial germ cells, making sure that the sex dependent imprints can be expressed in later embryonic development. (L)


Diagram_3.png
Life Cycle of a Genomic Imprint - http://www.nature.com/cr/journal/v21/n3/fig_tab/cr201115f1.html#figure-title





Multiple Choice Questions


1. If one cell has 36 chromosomes at the beginning of meiosis, it will have _ daughter cells each with _ chromosomes at the end of meiosis.

A) 2, 18
B) 4, 36
C) 4,18
D) 4, 9
E) 2, 36

2. Which of the following regarding meiosis is FALSE?

A) during prophase I, crossover occurs
B) between meiosis 1 and II, there is no replication of chromosomes
C) the longest phase of meiosis is prophase
D) spindle fibers are attached to the centriole in plants
E) during prophase I, synapsis occurs

3) The completion of which stage of meiosis results in the first set of two haploid cells?
A Cytokinesis 1
B Telophase 2
C Metaphase 1
D Cytokinesis 2
E Anaphase 1

4)What is the name of the site of crossing over in meiosis?
A Kinetochore
B Chiasma
C Recombinant Chromosomes
D Centrosome
E Synaptonemal complex

5) How many gametes result from a single primary oocyte vs. primary spermatocyte?
A 4;4
B 1;1
C 4;1
D 1;4
E 2;4

6) Which of the following are true?
I. All spermatozoa are created before birth
II. All ovums are created before birth
III. Ovums fully finish developing after fertilization
A I only
B II only
C III only
D II and III only
E I, II, and III

7)Which statement is not true?
A Four spermatids result from one primary spermocyte
B Spermatozoa are produced in the seminiferous tubules of the testes
C New ovum are produced throughout life after puberty
D New spermatozoa are produced throughout life after puberty
E Oogenesis only results in one ovum

8) Why are imprinted genes under greater selective pressure?
A They are not
B They control major life characteristics
C There are many copies of the same gene
D They are more adaptive
E There is only one copy, so it cannot check itself for mutations

9)What are the three stages in genomic imprints?
A Fertilization, differentiation, gastrulation
B Prophase, maintenance, erasure
C Meiosis 1, meiosis 2, gamete production
D Establishment, maintenance, erasure
E Deletion, Replacement, Maintenance

10) Which of the following are true?
I Methylation may silence alleles
II A gene that is imprinted consists of methyl groups added to cytosine nucleotides
III Genomic imprinting does not occur
A I only
B II only
C I and II only
D II and III only
E I, II, and III









AP Essay Question


A) What are the major differences between meiosis and mitosis?

B) What are two Mendelian methods by which variation in offspring arise?

C) Describe in detail three of the following four processes and how they relate to creating variation –

  • Spermatogenesis
  • Oogenesis
  • Fertilization
  • Genomic Imprinting









Sources



A) O'Connor, Clare. "Meiosis, Genetic Recombination, and Sexual Reproduction." Nature.com. Nature Publishing Group. Web. 28 Feb. 2012. <http://www.nature.com/scitable/topicpage/meiosis-genetic-recombination-and-sexual-reproduction-210>.
Nature.com Genetic Recombination and Sexual Reproduction- A great article on genetic recombination, sexual reproduction, and the process of meiosis.

B) Clancy, Suzanne. "Genetic Recombination." Nature.com. Nature Publishing Group. Web. 01 Mar. 2012. <http://www.nature.com/scitable/topicpage/genetic-recombination-514>.
Nature.com Genetic Recombination- A fantastic source on the different recombination models, recombination enzymes, and the orientation of chromosomes during meiosis.

C) Bowen, R. "Fertilization." Arbl.cvmbs.colostate.edu. Colorado State University. Web. 01 Mar. 2012. <http://www.vivo.colostate.edu/hbooks/pathphys/reprod/fert/fert.html>.

Colorado State Fertilization Video - A comprehensive overview of sperm capacitation, the acrosomal reaction, the zona and corticle reactions, and post-fertilization events.


D) Kimball, J. "Sexual Reproduction in Humans." Sexual Reproduction. 5 Dec. 2011. Web. 01 Mar. 2012. <http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/Sexual_Reproduction.html>

Sexual Reproduction in Humans An interesting set of articles regarding the many different parts of human sexual reproduction. Excellent articles on fertilization, oogenesis, and spermatogenesis.



E)Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. San Francisco: Benjamin Cummings, 2002. Print.- Our biology textbook


F)IEUPI Department of Biology. 10 Feb 2000. http://www.biology.iupui.edu/biocourses/N100H/ch9meiosis.html
Meiosis and Formation of Eggs and Sperm -Great summaries of fertilization, oogenesis, and spermatogenesis. It also includes step by step visuals of meiosis and discusses inheritance



G) Campbell, Neil A., and Jane B. Reece.""Animal Reproduction" Biology. 6th ed. San Francisco: Benjamin Cummings, 2002. 985-986.Print.

H)Meiosis, Spermatogenesis, and Oogenesis. PDF.
<http://staff.bcc.edu/pslavin/pdf/Meiosis%20Spermatogenesis%20and%20Oogenesis.pdf>.

I)
Goldberg, Deborah T. Barron's Ap Biology. 3rd ed. Barrons Educational Series, 2012. 347-49. Print.
J) Campbell, Neil A., and Jane B. Reece.""Genomic Imprinting" Biology. 8th ed. San Francisco: Benjamin Cummings, 2002. 300-301.Print.
K) "Genomic Imprinting." Ligers and Tigons. Copyright 2012. Web. 01 Mar. 2012. http://learn.genetics.utah.edu/content/epigenetics/imprinting/
L) Sasaki, Hiroyuki. "Genomic imprinting in mammals: its life cycle, molecular mechanisms and reprogramming." 1 Feb. 2011. Web. 01 Mar. 2012.
http://www.nature.com/cr/journal/v21/n3/full/cr201115a.html