Creating Variation

by Liza Evans, Isabelle Levenson, and Varuna Nangia



Meiosis is a process which converts diploid cells to haploid gametes and which causes a change in the genetic information of those cells to create diversity in the offspring.


A haploid cell has only one set of chromosomes whereas a diploid cell has two sets. Most cells are diploid, while gametes are haploid. This allows them to combine and form a diploid zygote, which would have two sets of chromosomes, one set from each haploid gamete. (3)

Asexual reproduction lacks the mechanism of meiosis, and there is therefore no genetic diversity created in that process. The oversimplified diagram above
details the fact that all that can be created in the process of asexual replication are copies of the original cell, which lack any sort of combination of chromosomes or variation that is inherent in sexual reproduction. (3)


Break it down now! Below is Liza and Izzy's favorite song about the process. Starts at 4:42. Not that detailed, but so catchy.


Interphase: During interphase, which precedes meiosis I, each of the chromosomes replicates in a similar process to the replication in mitosis. The results of this phase are two genetically identical sister chromatids attached at their centromeres, as well as replicated centrosomes.

Prophase I: In prophase I, the chromosomes begin to condense, and homologues, which consist of two sister chromatids, pair up. What follows is a process called synapsis in which a protein structure, called the synaptonemal complex, attaches the homologous chromosones tightly together along their lengths. In late prophase when the synaptonemal complex disappears, each chromosome pair is visible as a tetrad or cluster of four chromatids. The chromatids of homologous chromosomes are crisscrossed at chiasmata, which hold the homologous pairs together until anaphase I. At these chiasmata, segments of the chromatids are traded in a process called "crossing over". Also in prophase I, as in the prophase of mitosis, cellular components prepare for the division of the nucleus. The centrosomes move farther away from each other while spindle microtubules form between them. Meanwhile, the nuclear envelope and nucleoli disperse and spindle microtubules capture the kinetochores that form on the chromosomes. The chromosomes then begin to move to the metaphase plate. In comparison to the prophase of mitosis, the prophase I of meiosis I is much more complex. This phase, prophase I, lasts for days and occupies more than 90% of the time required for meiosis.

Metaphase I: In this phase, chromosomes are arranged on the metaphase plate in homologous pairs. Attached to one chromosome of each pair are kinetochore microtubules from one pole of the cell. Microtubules from the opposite pole attach to the homologue.

Anaphase I: The spindle apparatus guides the movement of the chromosomes to the poles. The sister chromatids remain attached at their centromeres through this process and move together towards the same pole, though the homologous chromosome moves toward the opposite pole. This differs from mitosis in that in mitosis the chromosomes are individuals on the metaphase plate rather than as pairs, and the sister chromatids separate.

Telophase I and Cytokinesis: The members of each pair of homologous chromosomes continue to move apart until they finally reach the poles of the cell. At this point, each pole has a haploid chromosome set, but each chromosome still has two sister chromatids. Cytokinesis, the division of the cytoplasm, occurs at the same time as telophase I. Cleavage furrows in animal cells and cell plates in plant cells create the division. There is no further replication of the genetic material prior to the second division of meiosis, although in some species the chromosomes decondense and the nuclear membranes and nucleoli form again.

Prophase II: In this phase, as in the prophase of mitosis, a spindle apparatus forms and the chromosomes progress toward the metaphase II plate.

Metaphase II: As in mitosis and metaphase I, in metaphase II the chromosomes are positioned on the metaphase plate with the kinetochores of sister chromatids from each chromosome pointing towards opposite poles.

Anaphase II: The centromeres of the sister chromatids finally separate in this phase. The sister chromatids of each pair, which are now individual chromosomes, move to the opposite poles of the cell.

Telophase II and Cytokinesis II: At the opposite poles of the cell, nuclei form as cytokinesis occurs. Once cytokinesis is finished, there are four daughter cells that each have a haploid number of unreplicated chromosomes. (25)

And now, an animation to put together all the steps of meiosis into a fluid process.


Prophase I: 0:00-0:43
Metaphase I: 0:43-0:50
Anaphase I: 0:50-1:00
Telophase I and Cytokinesis: 1:00-1:12
Prophase II: 1:12-1:20
Metaphase II: 1:20-1:28
Anaphase II: 1:28-1:33
Telophase II: 1:33-1:49

Gametes are produced by meiosis in the gonads (ovaries for females and testes in males). After meiosis occurs in the gonads, gametes are left with haploid sets of 23 chromosomes. Therefore, when a sperm (male gamete) and ova (female gamete) fuse together in fertilization, the resulting cell will have to diploid number of 46 chromosomes. The process to make sperm is called spermatogenesis while the process to make an ovum is called Oogenesis (25).


Spermatogenesis is the production of mature sperm cells. The basic function of spermatogenesis is to turn each diploid spermatogonium into four haploid sperm cells (25). Spermatogenesis takes place in the male reproductive organ, the testes. The testes are made up of thin, tightly composed tubules known as the seminiferous tubules. Sperm, which are male gametes, are produced within the walls of these tubules; as they develop and mature, the sperm make their way toward the center of the seminiferous tubules, called the lumen (5,7). Throughout the seminiferous tubule are Sertoli cells, which act like the nurse of sperm cells. Sertoli cells are extremely important in protecting germ cells, concentrating androgens near the developing germ cells, feeding the growing cells, and cleaning up/eating up the excess of the developing sperm (7).

As the picture above shows, spermatogenesis takes place in the seminiferous tubule of the testes (16)

Spermatogenesis takes about 64 days to complete and can be divided into 5 different stages: spermatocytogenesis (mitosis), spermatidogenesis (meiosis I), spermatidogenesis (meiosis II), spermiogenesis, and spermiation (7)

Spermatogenesis starts out with a diploid, primordial germ cell. A germ cell is any biological cell that forms gametes for sexually reproducing organisms. The primordial germ cell then differentiates to become a diploid spermatogonium. The spermatogonium enter the first stage of spermatogenesis: spermatocytogenesis (mitosis). In this stage spermatogonium are essentially stem cells that divide through mitosis to produce large numbers of themselves and large numbers of cells that will become mature sperm by then end of spermatogenesis. There are 3 different types of spermatogonium cells: type Ad, type Ap, and type B. Type Ad spermatogonium are the cells that replicate to make sure that there is a constant supply of spermatogonium. Type Ap are the cells that go through mitosis to produce type B spermatogonium. Type B spermatogonium are the cells that become primary spermatocytes. Type B spermatogonium become primary spermatocytes through differentiation and the onset of meiosis I (7).

As the picture above shows, spermatogonium are stem cells that continually divide in order to maintain their own numbers and to form cells that will eventually become future sperm. (14)

The next stage of spermatogenesis is spermatidogenesis (meiosis I). In this stage, the primary spermatocytes complete meiosis I to form two haploid cells with duplicated chromosomes called secondary spermatocytes. The next stage in spermatogenesis is spermatidogenesis (meiosis II). In this stage, meiosis II occurs and the two haploid cells divide to form four haploid cells, each with 23 single chromosomes, called early spermatids. Meiosis is now complete in the spermatogenesis process (5, 7,8,25). However, at this time, the early spermatids are just a bunch of round haploid cells, sharing cytoplasm with other nuclei. The maturation and differentiation of these early spermatids into spermatozoan (sperm cells) is called spermiogenesis. In this stage, excess cytoplasm and materials in the cells are "eaten up" by Sertoli cells, tails begin to develop, and the DNA is repackaged. In the cytoplasmic space not taken up by the Sertoli cells, mitochondria (used to provide energy for the sperm cells that must swim long distances to get to the female ovum) and an acrosome cap (an organelle with digestive enzymes) develop. When the cell has undergone these developments, spermiogenesis is complete and the cells are now spermatozoan (sperm cells). Spermatozoan are now mature enough to be released into the lumen of the seminiferous tubule (5,6).

The picture above shows the maturation of sperm from spermatids onwards (17).

However, although mature, the spermatozoa (sperm cells) are still not functional in the testes; in order to become fully mobile, the sperm must go through one last process: spermiation. In spermiation, the sperm cells move from the testes to the epididymis, where they acquire motility. But how do the sperm get to the epididymis if they cannot move? The sperm are moved through tubules to the epididymis by peristaltic contractions of a smooth muscle layer that lies just outside the Sertoli cells. By the time the sperm reach the epididymis, they are fully developed and completely mobile (6).

This picture is a visual overview of spermatogenesis.(15)

Here is a very clear and concise animation on Spermatogenesis.


Oogenesis is the production of mature ova, which are unfertilized egg cells. Oogenesis takes place in the female reproductive organ, the ovary. Oogenesis can be divided into 2 main stages: oocytogenesis and ootidogenesis (9).
Oogenesis starts with a primordial germ cell which differentiates into Oogonium. Oogonium are stem cells that proliferate through mitosis so that by the seventh month of gestation, about 7 million oogonium have formed. Most oogonium die, however, so that the remaining number decreases to 1-2 million by birth. The oocytogenesis stage of oogenesis is when oogonium differentiate to form primary oocytes. Like the oogonium, the primary oocyte is a diploid cell, containing two complete sets of chromosomes (9,10). The next stage of Oogenesis is Ootidogenesis. Ootidogenesis, is the process in which a primary oocyte develops into an ovum, the haploid result. Ootidogenesis encompasses both meiosis I, and meiosis II. Primary oocytes go through meiosis I, but become arrested in the prophase stage (diplotene stage) in small follicles until puberty. The cells are reactivated to grow and induce the primary oocyte when stimulated by the hormone FSH (follicle-stimulating hormone)(9, 25). When this happens, the primary oocyte completes meiosis I, forming two haploid cells, through unequal cytokinesis. Because of unequal cytokinesis, one of the two cells contains hardly any cytoplasm, whereas the other cell has nearly the entire volume of cellular components. The smaller cell is called the first polar body, and the larger cell is referred to as the secondary oocyte. While the polar body may or may not divide again, the secondary oocyte does complete meiosis II. However, at the metaphase stage, the cell arrests until a sperm enters it. The entry of a sperm results in the completion of meiosis II where unequal cytokinesis occurs once again forming two or four (if the first polar body divides) haploid cells. The results of meiosis II on the secondary oocyte result in a second polar body and an ovum, a female gamete. Unlike spermatogenesis where four haploid sperm cells are created, in Oogenesis, only one mature ovum develops, while the other three polar bodies disintegrate (9,10,25).

Above is a brief overview of Oogenesis (19)

However, in order to fully understand oogenesis, it is also important to understand the ovarian cycle. The ovarian cycle can be split up into 3 distinct phases: the follicular phase, ovulation, and the luteal phase. The follicular stage, which begins due to the release of the hormone FSH, begins during ootidogenesis(meiosis I )when oogonium turn into primary oocytes. However, the primary oocytes become restructured so that each one enveloped by a single layer of follicular epithelial cells called an ovarian follicle. During the follicular phase, the developing egg in each of these follicles enlarges and the coat thickens. However, from all the follicles that started out developing, only one ovarian follicle continues to grow and develop while the rest break down. The ovarian follicle that continues to mature develops an internal fluid-filled cavity and gets very big. The ovulation phase begins when the follicle and adjacent wall rupture, releasing the secondary oocyte into the fallopian tubes where it is available to be fertilized. The follicular tissue that remains in the ovary after ovulation turns into the corpus luteum. During the luteum stage, endocrine cells in the corpus luteum secrete female hormones that help a new ovarian cycle to begin(25).

This picture shows how oogenesis and the ovarian cycle interact (20)

Although this a short video, it combines Oogenesis and the ovarian cycle pretty well.


information from our textbook, source 25

Fertilization, or syngamy, is the union of gametes. The main purposes of fertilization are to combine the haploid sets of chromosomes of a father and mother to create a single diploid cell and to activate the egg. Fertilization occurs when the haploid gamete of the father or sperm cell reaches and fuses with the haploid gamete of the mother or ovum. The result of this union is a fertilized egg, called a zygote. (25)

Fertilization in sea urchins (similar to vertebrate fertilization):


  1. Contact: The gametes are released into the water, and a sperm cell is exposed to the molecules from the dissolving jelly coat surrounding the sea urchin egg.
  2. Acrosomal reaction: A vesicle at the tip of the sperm called the acrosome discharges its contents via exocytosis. The resulting acrosomal reaction releases hydrolytic enzymes which enable a structure called the acrosomal process to penetrate the jelly coat.
  3. Growth of acrosomal process: The tip of this acrosomal process is coated with a protein which adheres to receptor molecules that are located on the vitelline layer under the jelly coat. A recognition mechanism ensures tha
    t eggs will be fertilized by only sperm from the same species.
  4. Fusion of the plasma membranes: The fusion of the sperm and egg plasma membranes follows the acrosomal reaction. This fusion causes ion channels to open in the egg's plasma membrane, therefore allowing sodium ions to flow into the egg cell and change the membrane potential, which signals a membrane depolarization. Since this membrane depolarization prevents more than one sperm cell from fusing with the egg, it is often called the fast block to polyspermy.
  5. Cortical reaction: The cortical reaction is a series of changes in the cortex of the egg cytoplasm. The fusion of the sperm and the egg triggers a signal-transduction pathway that causes the endoplasmic reticulum of the egg to release calcium into the cytosol, resulting in the production of the "second messengers" like DAG and IG3, which opens ligand-gated calcium channels in the ER membrane. With more Ca^2+ released, the opening of other channels is triggered. The high concentration of calcium causes a change in the vesicles that lie just under the egg's plasma membrane, which are called cortical granules. The cortical granules fuse with the plasma membrane and release their contents into the perivitelline space between the plasma membrance and the vitelline layer. At this point, enzymes from the granules separate the vitelline layer from the plasma membrane and mucopolysaccharides produce an osmotic gradient which pulls water into the perivitelline space and causes it to swell. This swelling pushes the vitelline layer away from the plasma membrane. Other enzymes harden it. The vitelline layer then therefore becomes the fertilization envelope, which blocks the entry of additional sperm. A this time, the voltage across the plasma membrane has returned to normal, so the fast block to polyspermy no longer functions, but the fertilization envelope functions as a slow block to polyspermy. (25)

This video shows the process of fertilization in a sea urchin:


Activation of the egg:
The rise in the Ca^2+ concentration of in the egg's cytosol also incites metabolic changes with the egg cell, which unfertilized has a very slow metabolism. After fertilization, the rates of cellular respiration and protein synthesis increase within minutes, so therefore the cell is said to have been "activated". In sea urchins and some other species, the DAG that was produced during the cortical reaction activates a membrane protein which transports H+ out of the egg. This makes the egg slightly alkaline; this change in pH is said to indirectly responsible for the metabolic responses of the egg to fertilization. Sperm cells do not contribute any materials required for activation.
While the egg increases its metabolic activity, the sperm cell's nucleus that is within the egg starts to swell. It then merges with the egg nucleus to create the diploid nucleus of the zygote. Now, DNA synthesis begins and cell divisions start. (25)

Fertilization in mammals:

Many of the events are similar to those in sea urchins, although there are important differences.
  • Fertilization in terrestrial mammals is generally internal
  • Capacitation: secretions in the female mammal reproductive tract alter molecules on the sperm cell surface and enhance sperm function
  • The mammalian egg is cloaked by follicle cells released with the egg during ovulation. A capacitated sperm cell must go through this layer of follice cells before reaching the zona pellucida or extracellular matrix of the egg, which consists of three different glycoproteins that form filaments in a 3D network.
  • One of the glycoproteins in the zona pellucida, ZP3, functions as a sperm receptor, and the binding of the sperm head to such receptors induces the acrosome to release its contents
  • Acrosomal reaction enables the sperm to penetrate the zona pellucida and reach the plasma membrane of the egg and exposes a protein in the sperm membrane that fuses with the egg membrane
  • The binding of the sperm and egg triggers the depolarization key to the fast block to polyspermy. A cortical reaction then occurs. Enzymes from the cortical granules catalyze changes in the zona pellucida, and these changes act as the slow block to polyspermy.
  • Extensions of the egg cell, microvilli, take the sperm cell into the egg. The basal body of the sperm's flagellum divides to form two centrosomes with centrioles in the zygote because unfertilized mammalian eggs do not have their own centrosomes.
  • The haploid nuclei of the sperm and egg do not fuse immediately. The sperm's and egg's nuclear envelopes disperse and the chromosomes from the two gametes share a common spindle apparatus. After the first mitotic division, as diploid nuclei form in the two daughter cells, the chromosomes from the parents finally join together in common nuclei to become the genome of the offspring. (25)



In biology, and specifically genetics, epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence It refers to functionally relevant modifications to the genome that do not involve a change in the nucleotide sequence.
Examples of such changes are DNA methylation and histone modification, both of which serve to regulate gene expression without altering the underlying DNA sequence.
Lamarck and Darwin's Theories
Opportunities for Control of Gene Expression in Eukaryotes including Epigenetics


Methyl groups (-CH3) attach to DNA bases after DNA is synthesized.
In flowering plants and animals, cytosine is usually the methylated base.
Inactive DNA is usually highly methylated, leading to hypothesis that methylation turns off genes and demethylization can turn them back on.
Ex. Mammalian x chromosomes, tissues
Some genes, de-methylating turns on
In both plants and animals, methyalation enzyme deficiency leads to abnormalities in embryonic development
During mitosis, methylation enzymes act on DNA sites of daughter strand after every round of DNA replication, therefore, patterns are passed on.
Tissue cells’ methylation patterns act as chemical record for what occurred during embryonic development.

Methylation on Cytosine
Two Types of Epigenetics

Genomic Imprinting

An Animation on Genetic Imprinting
In Genomic Imprinting, the allele has a different effect depending on whether it was inherited from mother or father, but this is not sex linked inheritance, but something different.
The gene on one chromosome is expressed while its allele on a homologous chromosome is turned off during spermatogenesis and oogenesis. This is one type of epigenetic silencing. In mammals, genes can have different effects depending on if they’re from the mother or father. Next generation, maternal and paternal imprints are “erased” and gamete re-imprinting occurs after fertilization with a certain gender in mind. After that, the genetic tags rarely change during the organism’s life and the epigenetic tags influence development.
Basically a genomic imprint is the pattern of methyl groups added to cystosine nucleotides of alleles.
Hypothesis: Methyalation silences gene, use non methylated allele
Twenty genes on 9 chromosomes have yet been discovered in which genetic imprinting plays a role. There might be hundreds more to be discovered
With imprinted genes, we only receive one functional copy, either from the mother or the father. Genetic Imprinting is unique to flowering plants and mammals.

Methylation Process
Genetic Tags- Methylation

Case Study One: Prader Willi Syndrome and Angelman Syndrome

Prader-Willi and Angelman syndrome are both caused by issues with the same area on chromosome 15. Those with Angelman Syndrome have learning problems, speech difficulties, seizures, jerky movement, and an unusually happy disposition. This is caused by missing a gene that is normally inherited from the mother or having two paternal copies. Prader-Willi Syndrome sufferers have learning problems, short stature, and eat compulsively. Those with this disease are missing a gene normally from the mother or have two paternal copies.

Can you guess which boy has either syndrome?

Case Study Two: The Igf2 Gene

The Igf2 gene codes for a hormone that causes fetal and embryonic growth. Usually, methyl tags silence the maternal Igf2 gene. When an epimutation leaved the maternal gene un-methylated, there are two copies of the gene so overgrowth occurs. This is called Beckwith-Wiederman Syndrome.

Igf2 Receptor- how it is passed on

Genetic Imprinting and Cloning

Epigenetic tags such as those involved in genetic imprinting have been one of the greatest obstacles to cloning. In cloning, the differentiated cell already has the epigenetic tags in place and the process for erasing epigenetic tags has a high error rate, so the methyalation process gets messed up.

cloned sheep

Imprinted Genes and Genetic Pressure

Case Study Three: Ligers and Tigons

Imprinted genes are under greater selective pressure, since only one copy is expressed at a time, so all variations will be expressed. Imprinting patterns evolve so rapidly that the imprinting patterns in closely related organisms are often drastically different. In Tigers, the gene for growth is imprinted on the female, whereas in lions, it is imprinted on the male. Therefore, in hybridization, a male tiger and a female lion produce a liger, which is the biggest of the big cats. However, a female tiger and a male lion produce a tion, which is smaller than either of its parents.

Many of such genes involve development of embryo as well as growth and metabolism.

But Why? It all has to do with promiscuous cats, Darwin and the genetic conflict hypothesis.
You see, in cats and many other animals, more than one male can father offspring in one litter. For male cats, it is in their interest for their offspring to grow larger and outlive their twin siblings from a different father. That is why maternal growth genes are often imprinted while paternal genes are expressed. For mothers, it is in their interest for all the offspring to survive, on Darwin’s standards. She needs to divide her resources (warmth, nutrients, energy) among her kittens. Therefore, it makes sense that metabolic genes are often imprinted paternally and expressed maternally.

just look at that promiscuous cat!


Meiosis info:
Picture of Haploid and Diploid Chromosome

Simplified Diagram of Asexual Reproductio
Meiosis tutorial that was very straignt-forward and simple. Great for basic information.
Very clear diagram for Meiosis I and II.
Probably the best website at explaining spermatogenesis in a clear and concise way.
6. up any confusion and superfluous info other websites give on the maturation of a cell; very clear and easy to understand.
7. detailed. While used much during this project, it can be very confusing and overwhelming.
8. very simplified version of spermatogenesis that helps if one just wants to know the basics.
9. site gives extremely detailed info on Oogenesis. Can be overwhelming and confusing to a reader..
10. website for getting basics of Oogenesis in a clear way. Extremely helpful!

Genetic Imprinting Info
11. A Great Source on Genetic Imprinting by University of Utah
12. a simple definition of epigenetics to help gain understanding that might help understanding the context in this page
13. is a picture of a mature, motile sperm.
14. picture is clear at showing how the spermatagonium are stem cells.
15. is a picture that gives a really clear and detailed overview of Spermatogenesis.
16. is a picture that shows that spermatogenesis takes place in the seminiferous tubule.
17. picture shows the maturation of sperm in all of its stages.
18. s a picture of a mature ovum.
19. is a picture that gives a basic overview of Oogenesis.
20. is a picture that gives a detailed overview and shows the connection between Oogenesis and the ovarian cycle.
21. animation gives an excellent overview of the basics of Spermatogenesis.
22. youtube video, while bad at explaining Oogenesis, is great at showing interaction between Oogenesis and the ovarian cycle.
23. Meiosis animation, very detailed:
24. Our favorite meiosis song!:
25. Biology by Cambell the sixth and eighth editionsOur bio textbook.
Photo for fertilization in sea urchins
Photo for fertilization in humans
Sea urchin fertilization breakdown video

Multiple Choice

1.An imprinted gene

a. is methylated and therefore will be expressed
b. is methylated and therefore will not be expressed
c. is not methylated and therefore will be expressed
d. is not methylated and therefore won’t be expressed
e. gets passed on by one parent and selects gender

2. Epigenetics

a. were discovered by Darwin and involve selective pressures
b. Were discovered by Mendel and involve heritability
c. Were discovered by Lamarch and prove the heritability of changes over a lifetime
d. Are heritable changes in gene expression not caused by DNA
e. Are considered a type of pseudo-science

3. How does selective pressure work on imprinted genes?

a. selective pressures do not work on imprinted genes
b. selective pressure work on imprinted genes less than other genes
c. selective pressures work equally on imprinted and non imprinted genes
d. selective pressure work slightly more on imprinted genes than non imprinted genes
e. selective pressure work much more rapidly on imprinted genes than non imprinted

4. During spermiogenesis, which cells are the first to become haploid?

a. spermatids
b. spermatogonium
c. primary spermatocytes
d. secondary spermatocytes
e. sperm cells

5. The best answer for where spermatogenesis takes place is?

a. testis
b. seminiferous tubules
c. epididymis
d. urethra
e. prostate gland

6. A difference between spermatogenesis and oogenesis is ?

a. the mature ovum is diploid while the sperm is haploid
b. the mature ovum is haploid while the sperm is diploid
c. spermatogenesis involves both mitosis and meiosis while oogenesis only involves meiosis
d. only one mature ovum is produced in oogenesis while 4 mature sperm are produced in spermatogenesis
e. the production of ovum stops at puberty while the production of sperm is continuous

7. What is the purpose of polar bodies in oogenesis?

a. they are a result of unequal cytokinesis and serve no function
b. they can produce mature ovum
c. they produce sets of multiple births
d. they ensure the ovum will have most of the cytoplasm
e. they rid the body of defective chromosomes, leaving the good set within the ovum.

8. Which of the following actions occur during prophase I of meiosis?

I. Chromosomes condense
II. Synapsis
III. Crossing over at the chiasmata
IV. Chromosomes begin moving towards the metaphase plate

a. I only
b. I and II
c. III only
d. I, III, and IV
e. I, II, III, and IV

9. Which of the following regarding the differences between meiosis I and meiosis II is FALSE?

a. Meiosis I is preceded by interphase, while meiosis II is not
b. Sister chromatids remain attached in meiosis I but separate in meiosis II
c. Crossing over occurs only in meiosis II, not in meiosis I
d. Chiasmata and tetrads exist only in meiosis I
e. All answers are true

10. Which of the following is an accurate summary of the fast block to polyspermy?

a. The cortical reaction causes swelling of the perivitelline space and results in the vitelline layer pushing away from the plasma membrane and hardening to become the fertilization envelope
b. The cortical reaction causes the vitelline layer to disintegrate and the plasma membrane to replace it as a more selective fertilization envelope
c. The fusion of the egg and sperm plasma membranes causes an influx of sodium ions, which signals a membrane depolarization
d. The fusion of the egg and sperm plasma membranes causes an influx of calcium ions, which work to stiffen the fertilization envelope
e. Hydrolytic enzymes released during the acrosomal reaction destroy any sperm cells that attempt to fuse with the egg

11. Which of the following is not a key difference between vertebrate and mammalian fertilization?

a. Capacitation
b. The zona pellucida
c. The absorption of the sperm
d. The slow fusion of the haploid nuclei
e. The cortical reaction


Write an essay about meiosis, making sure to cover the following topics:

  1. the stages of meiosis
  2. the differences between oogenesis and spermatogenesis
  3. how meiosis leads to variation