Chromosomes and gametes

Question 1

What are the fundamental principles of evolution?

Answer 1

• The defining feature of all evolving living organisms (even bacteria) is the ability to reproduce.
• Through reproduction, genes are passed on to a new generation (inherited)
• A second principle fundamental to evolution is variation; the replicating system must undergo changes to survive (adaptations).
• Variation allows organisms to adapt to changes within their environment; Darwinian concept of survival of the fittest.
• Each new generation in turn reproduces or dies out = selection of the fittest

Question 2

What is the typical karyotype of human chromosomes?

Answer 2

• Karyotype = number and appearance (structure) of human chromosomes
• Each chromosome has a constriction point called the centromere, which divides the chromosome into 2 sections or “arms” = short (p) and long (q).
• On chromosome 3, the constriction point (centromere) is quite centred, so the P and Q arms are quite similar in length, but sometimes it can be very distinctive. The centromere location gives the chromosome a characteristic shape and is used to describe the location of the gene. There are lots of genes on all of these chromosomes and there are alleles.
• The ability to reproduce begins with DNA and RNA. Genes are on chromosomes. Karyotyping chromosomes allows us to visualise them and this ability can be exploited.
• 22 pairs of autosomes (one inherited from each parent) and 1 pair of sex chromosomes = 46 chromosomes in total (in a human being).

Question 3

What are the requirements of DNA for genes to be functional? How does this sexual reproduction differ to cloning?

Answer 3

• For genes to be functional, DNA must be able to:
  1) replicate
  2) separate its 2 copies at mitosis
  3) maintain itself between generations
• For sexual reproduction, the DNA requirements are different to cloning; each parent will pass on one allele (version of a particular gene) to the offspring. Any abnormalities in those alleles can also be passed on, but it can be compensated for depending on what is inherited from each parent.
• Consanguineous relationships lose the ability to filter out harmful genes (common mutations). When the parents are closely related, they may have the same mutations that are then passed on to the next generation.
• Copy number variants (CNV) occur when the number of copies of alleles varies between people, i.e. one, three or more copies of alleles.
• If Alleles are heterozygous, the phenotype of the trait can be dominant or recessive.
• Human Genome contains only 20,500 genes. Variety and functionality can also be introduced at the level of transcription, translation or protein modification.

Question 4

Human Genome contains only 20,500 genes. Variety and functionality can be introduced at the level of transcription, translation or protein modification.

Briefly summarise these processes.

Answer 4

• The other way that differences are introduced is about how transcription and translation of DNA occurs. This process can be exploited to bring about changes in function of various proteins.
• Promotor and coding sequence transcribed into a gene product (RNA). In cells, genes consist of a long strand of DNA that contains a promoter, which controls the activity of a gene, and a coding sequence, which determines what the gene produces. When a gene is active, the coding sequence is copied in a process called transcription, producing an RNA copy of the gene’s information. First the primary transcript of mRNA is produced, then mature mRNA.
• Introns are removed from exons by splicing (exons come together)
• Mature transcript of mRNA exported out of nucleus
• Translated into proteins in ribosomes i.e. complexes of tRNA and proteins. This RNA can then direct the synthesis of proteins via the genetic code. However, RNAs can also be used directly, for example as part of the ribosome. These molecules resulting from gene expression, whether RNA or protein, are known as gene products.
• Proteins then folded into unique 3D structure that determines function.

Question 5

How can the same gene be tissue specific? Give an example.

Answer 5

• By having alternative promoters
• Most genes contain non-coding regions that do not code for the gene products, but regulate gene expression. The genes of eukaryotic organisms can contain non-coding regions called introns that are removed from the messenger RNA in a process known as splicing. The regions that actually encode the gene product, which can be much smaller than the introns, are known as exons.
• E.g. CYP19A1 (coding for aromatase) uses different promotors in breast, ovary and brain. Aromatase is made in the granulosa cells of the follicle in the ovary (androgens are made in the theca and transported into the granulosa cells where aromatase is present); it is the enzyme that catalyses the conversion of androgens to oestrogens. However, the gene that produces aromatase is also present in breast, placenta, adipose tissue etc. The product aromatase is exactly the same, because the coding region (always exons 2 to 10 encoding aromatase) of mRNA is then translated into the protein. This gene is interesting because it has a splice site at the start of exon 2 where different exon 1’s can be attached. These different exons can have different promoters. In the ovary, it is promoter 2; this promoter in exon 1 is attached on to the splice site. The difference is that this promoter will respond to different hormones, e.g. FSH could stimulate it. This will then drive exons 2-10 to make aromatase. The aromatase is the same, but it is responding to different hormones. Similarly, in the breast, it is promoter 1.4 which will respond to different growth factors and substances to switch on aromatase in the breast to make oestrogen (responding to different stimulants compared to the ovary).
• When a woman goes into menopause, the menstrual cycle stops due to depletion of the follicles (eggs). The follicles make the steroid hormones, oestrogen and progesterone, which feed into the HPG axis. Women who are post-menopausal (have no ovarian function; not producing oestrogen from the ovaries) often get oestrogen dependent breast cancer due to abnormal activation of these various promoters in the breast that can drive the breast tissue to produce oestrogen (then drives the breast cancer).
• In post-menopausal women, the androgens come from the adrenal glands since the ovaries are not functioning.
• The same gene produces the same product, but it is behaving differently in different tissues because it can have different promoters. Each of those promoters in exon 1 would respond to different things.
• There are only ~20,500 genes in the body and yet they have to cover a vast amount of functions.

Question 6

How can one gene give rise to several products?

Answer 6

• Alternative splicing of exons. - One gene can give rise to one product in a tissue-specific manner that is controlled by different promoters. One gene can also give rise to separate products.
• One single gene can lead to the synthesis of multiple proteins through the different arrangements of exons produced by alternative splicing. These different products are called isoforms.
• Products are known as isoforms
• The protein can be modified once made
  1) Post-translational modification e.g. phosphorylation (groups added),
• Glycosylation i.e. adding on carbohydrates to protein, making protein more stable and soluble
• Can alter protein function, e.g. adding a phospho group can activate a protein.
• Often, hormones are secreted as “pro-hormones”, e.g. pre-proGnRH and proinsulin to insulin, and must be enzymatically processed (cleaved down) to form the active hormone.

Question 7

What is alternative splicing? Give a common example in repro.

Answer 7

• DNA is transcribed into RNA. Moves out of the nucleus and is spliced, so the introns are removed. Depending on how the exons join up, it can form different proteins. When translated, different isoforms of protein A are produced.
• In repro, a good example of this is in testes. Can find three alternatively spliced variants, so three isoforms, of FSH receptors in testicular tissue. It is thought that, depending on the proportion that you have, it could be associated with various defects in spermatogenesis and, hence, leading to fertility problems.

Question 8

How are LH and FSH modified?

Answer 8

• Glycosylation of FSH & LH
• Proteins are further modified in the endoplasmic reticulum. This is where additions can be made, e.g. glycosylation (adding on sugar residues (glycosyl groups)).
• FSH can be tetra glycosylated (four areas where glycosyl groups are added) or diglycosylated and, similarly, with LH, it can be tri glycosylated or di glycosylated. Each one of those is thought to behave a little bit differently. There is now research going on to look at how these proportions of variants by glycosylation with age, especially in women, can alter fertility or ability to conduct proper reproductive functions.

Question 9

What is glycosylation and where does it take place? Give an example.

Answer 9

• Proteins are further modified in the endoplasmic reticulum. This is where additions can be made, e.g. glycosylation (adding on sugar residues (glycosyl groups)).
• FSH can be tetra glycosylated (four areas where glycosyl groups are added) or diglycosylated and, similarly, with LH, it can be tri glycosylated or di glycosylated. Each one of those is thought to behave a little bit differently. There is now research going on to look at how these proportions of variants by glycosylation with age, especially in women, can alter fertility or ability to conduct proper reproductive functions.

Question 10

What are the DNA requirements for sexual reproduction?

Answer 10

• Fusion of haploid cells (gametes) to create unique diploid progeny
• This uniqueness brought about by crossing over and independent sorting of chromosomes.
• DNA needs to be able to be passed through generations
• Most cells and many organisms replicate by doubling DNA and dividing to give 2 identical progeny or clones = asexual reproduction
• Mitosis is the name given to the duplication of the DNA in this process
• Duplication occurs by mitosis, but it is different for sexual reproduction. It is essential for the number of chromosomes to be halved (so two parents can create a new organism with the right number of chromosomes). Haploid gametes allow a diploid progeny to be created (also introduces variation).

Question 11

How do somatic cells replicate?

Answer 11

• Somatic or diploid cells replicate by simple cell division
• give identical progeny, usually have limited number of divisions,
• eg hepatocytes, pancreas, skin cells
• Limited number of divisions before dying off. An example of an exception is in cancer where a patient can keep getting onward replication and overgrowth of the tissue, which should normally not happen because there are various checkpoints to stop that.

Question 12

What are the advantages of sexual reproduction?

Answer 12

• Prevents the accumulation of genetic mutations
• Increase in genetic diversity
• It is advantageous to be able to acquire genes from other organisms
• Maintenance occurs because of the advantage of genetic variability. With 2 copies of a gene, mutations in one may be advantageous. Maintenance can then occur.
• Variation in off-spring → may allow survival of the fittest. Better able to evolve and adapt to changing environment. Sexual reproduction allows us to adapt to the changing environment. The offspring in that population will then become the more dominant ones and survive (allows for survival of the fittest). Changing environment and adaptation is very slow; each generation faces different pressures, e.g. climate change currently, exposure to toxins, pollution from plastics, food processing. Eventually, these things will produce changes in our body and in our genes that will be passed on and will have an impact.
• With diploidy, there is an increased chance of obtaining a normal gene

Question 13

What are the differences between the X and Y chromosomes?

Answer 13

• Originally, X and Y chromosomes did not exist; it is thought that they came about from an identical pair of autosomes and developed this variation which meant one became Y and accumulated all of the traits for maleness while the other became X. Thought to have differentiated from a pair of identical chromosomes, termed autosomes, 300 million years ago.
• An ancestral mammal developed an allelic variation, a so-called ‘sex locus’, whereby simply possessing this allele caused the organism to be male. Gradually, the chromosome with this allele became the Y and the other the X.
• With evolution, genes advantageous to either sex became focussed on X or Y and those for ‘maleness’ close to SRY gene. Genes which were beneficial for males and harmful to (or had no effect on) females either developed on the Y chromosome, or were acquired through the process of translocation.
• X chromosome → 1000 working genes, Y chromosome → 86 working genes
• It was thought that the number of working genes on the Y chromosome was decreasing and it would eventually disappear, but this theory was disproved. Recent comparisons of human and chimpanzee (closest ancestral species humans have diverged from) Y chromosomes show that human Y chromosome has not lost any genes since divergence of human and chimpanzees 6-7 million years ago, so maybe it just became the efficient way (Hughes Jennifer et al, 2005, Nature 437).

Question 14

What is a gamete?

Answer 14

• A haploid cell specialised for sexual fusion (sperm and egg)
• They have 23 chromosomes in total. They have evolved for sexual fusion (reproduction).
• Unlike other cells gametes go through cycles of diploidy & haploidy. They go through cycles of both diploidy and haploidy; have to increase in number by mitosis and then undergo meiosis to become haploid.

Question 15

Gametes are highly specialised cells originating from which kind of cells?

Answer 15

• Gametes are formed from germ line cells: primordial germ cells that migrate into the gonad and then differentiate to either male or female gametes
• The process producing oocytes = oogenesis (incorporated as part of folliculogenesis). Oogenesis and folliculogenesis have to be thought of together.
• The process producing sperm = spermatogenesis
• Undergo cycles of mitosis to increase numbers
• Then undergo meiosis
• Then combine at fertilisation

Question 16

What are chromatids?

Answer 16

• Chromosomes replicate during S-phase of cell cycle
• Remain attached at the centromere
• When the chromosomes are being replicated during the S phase, the chromosome copies remain attached. The two copies, each an exact replicate of the original chromosome, stay attached to one another at a region called the centromere. As long as the replicate copies remain attached, each copy is called a chromatid. The two attached chromatids are genetically identical and are called sister chromatids.
• Before mitosis starts, the chromosome needs to replicate so produce two chromatids on each chromosome.
• Each copy known as a chromatid → the 2 copies are identical to each other → “sister” chromatids
• After replication, 1 chromosome has 2 chromatids. After the mitotic division, one chromosome only has one chromatid.
• Exact copy of original chromosomes
• Mitosis means thread

Question 17

What is interphase?

Answer 17

• Interphase is the period of the cell cycle between cell divisions. Interphase is not a “resting period,” as once thought. Instead, interphase is a time when the cell carries out its functions and grows. If the cell is going to divide, interphase is a time of intense preparation for cell division. During interphase, the DNA and organelles are duplicated. Throughout interphase, the genetic material is in the form of long, thin threads that are often called chromatin. Chromatin is an uncondensed form of DNA. They twist randomly around one another like tangled strands of yarn. In this state, DNA can be synthesized (replicated) and genes can be active. At the start of interphase, during G1, each chromosome consists of a DNA molecule and proteins.
• This all happens in interphase at the start of mitosis. The chromatin is not condensed (long, thread-like). They duplicate, form chromatids and condense to form duplicated chromatids (sister chromatids) making up one chromosome.
• Once mitosis begins, the DNA wraps around histones and loops around itself to produce condensed chromosomes.

Question 18

Summarise mitosis.

Answer 18

• For the purpose of discussion, mitosis is usually broadly divided into 4 stages = Prophase; Metaphase, Anaphase & Telophase
• DNA replication during interphase forms two sister chromatids, which are banded together to form a chromosome
• During mitosis, sister chromatids separate and move to opposite ends of the cell
• During cytokinesis, the parent cell divides, forming two daughter cells. Each daughter cell has two copies of each chromosome (homologous pairs)

Question 19

Describe prophase.

Answer 19

• In interphase, the DNA is duplicated to the sister chromatids. As well as the DNA, the centrioles in the cell are also duplicated (resulting in 2). The cell is preparing itself for cell division.
• As the centrioles duplicate, they form a spindle made up of microtubules. They migrate to opposite poles of the cell. The duplicated centrioles put out their spindles. The nuclear membrane around the nucleus will breakdown. This all happens in prophase.
• Mitosis begins with prophase, a time when changes occur in the nucleus as well as the cytoplasm. In the nucleus, the chromatin condenses and forms chromosomes as DNA wraps around histones. The DNA then loops and twists to form a tightly compacted structure. When DNA is in this condensed state, it cannot be replicated, and gene activity is shut down. In this condensed state, the sister chromatids are easier to separate without breaking. At about this time, the nuclear membrane also begins to break down.
• Outside the nucleus, in the cytoplasm, the mitotic spindle forms. The mitotic spindle is made of microtubules associated with the centrioles. During prophase, the centrioles, duplicated during interphase, move away from each other toward opposite ends of the cell.

Question 20

What happens in metaphase?

Answer 20

• Metaphase is the second stage of mitosis where the chromosomes attach to the mitotic spindles and form a line at the equator (centre) of the mitotic spindles. This alignment ensures each daughter cell receives one chromatid from each of the 46 chromosomes when the chromosomes separate at the centromere. The chromosomes containing chromatids line up on the spindle. In mitosis, the way they line up (which is different to meiosis) is very crucial.
• Centrosomes are microtubule-organising centres which are important for chromosome movement during mitosis. In many organisms, each centrosome contains a pair of centrioles.
• In prometaphase, the nuclear envelope breaks down and microtubules emanating from the centrosomes attach to the chromosomes. They attach to structures called kinetochores, which are found in the centromeres of the chromosomes. In metaphase, the centromere regions connecting paired chromatids become aligned in a plane at the cells’ equator.
• In metaphase, the chromosomes line up one after the other, i.e. chromosome 1 from mum is followed by the paternal chromosome 1 from dad and then the maternal chromosome 2 etc. all the way to chromosome 46 on the equatorial plane.

Question 21

What happens in anaphase?

Answer 21

• In metaphase, the centromere regions connecting paired chromatids become aligned in a plane at the cells’ equator. In anaphase, the centromere pairs from each chromosome separate and the single chromatids, which are now considered individual chromosomes, move towards the poles. As the cells enter the next interphase, the nuclear envelops and nucleoli reform and the chromatin becomes diffuse.
• Anaphase begins when the sister chromatids of each chromosome begin to separate, splitting at the centromere. Now separate entities, the sister chromatids are considered chromosomes in their own right. The spindle fibres pull the chromosomes toward opposite poles of the cell. By the end of anaphase, equivalent collections of chromosomes are located at the two poles of the cell.
• The microtubules making up the spindles shorten and pull apart the sister chromatids.

Question 22

What happens in telophase and cytokinesis?

Answer 22

• During telophase, a nuclear envelope forms around each group of chromosomes at each pole, and the mitotic spindle disassembles. The chromosomes also become more threadlike in appearance.
• After the sister chromatids have been pulled apart, they migrate to opposite ends which is followed by the cell membrane. The cytoplasm starts to divide and a new nucleus begins to form. Each new nucleus contains 46 chromosomes. The two daughter cells that are formed are the exact copy of the original parent cell.
• Cytokinesis is the division of the cytoplasm and it begins toward the end of mitosis, sometime during telophase. During this period, a band of microfilaments in the area where the chromosomes originally aligned contracts and forms a furrow. The furrow deepens, eventually pinching the cell in two. Thus, each daughter cell is a diploid cell that is genetically identical to the parent cell.

Question 23

Summarise the process of meiosis.

Answer 23

• Meiosis is greek for diminution = 2 divisions instead of one
• Allows for crossing over; happens between homologous chromosomes before the first division
• Meiosis and mitosis begin the same way. Both are preceded by the same event = the replication of chromosomes. Unlike mitosis, however, meiosis involves two divisions (stages) = meiosis 1 and meiosis 2.
• It starts off exactly the same as in mitosis; the DNA replicates, sister chromatids form, there is duplication of the centriole and formation of the spindle, but there is a crucial difference (the chromosomes align completely differently).
• In the first division, the chromosome number is reduced, because the two homologues of each pair of chromosomes (each replicated into two chromatids attached by a centromere) are separated into two cells so that each cell has one member of each homologous pair of chromosomes. In the second division, the replicated chromatids of each chromosome are separated.
• Meiosis begins with one diploid cell and, two divisions later, produces four haploid cells. The orderly movements of chromosomes during meiosis ensure that each haploid gamete produced contains one member of each homologous pair of chromosomes.
• Each of the two meiotic divisions has four stages similar to those in mitosis = prophase, metaphase, anaphase, and telophase.

Question 24

How is meiosis different in oogenesis compared to spermatogenesis?

Answer 24

• Beginning with 46 XY chromosomes. Through meiotic division I, haploid gametes are produced as chromatids. They divide in meiosis II to produce four daughter cells (spermatids in this case), all haploid.
• This is completely different from what happens in the egg. The egg, in oogenesis (as part of folliculogenesis), will have the initial division completed at ovulation. Instead of a secondary oocyte, like the secondary spermatocyte produced, a structure called a polar body is formed. When that divides again upon fertilisation, there will only be one egg being fertilised by one sperm. All the other structures containing those other chromosomes are all redundant structures, called polar bodies.
• It is important to understand this difference between spermatogenesis and oogenesis.

Question 25

What happens in meiosis I?

Answer 25

• The first meiotic division, meiosis I, produces two cells, each with 23 chromosomes. Note that the daughter cells do not contain a random assortment of any 23 chromosomes. Instead, each daughter cell contains one member of each homologous pair, with each chromosome consisting of two sister chromatids. It is important that each daughter cell receive one of each kind of chromosome during meiosis I. If one of the daughter cells had two of chromosome 3 and no chromosome 6, it would not survive.
• The separation of homologous chromosomes occurs reliably during meiosis I because, during prophase I (the I indicates this phase takes place during meiosis I), members of homologous pairs line up next to one another by a phenomenon called synapsis (“bringing together”).
• During metaphase I, matched homologous pairs become positioned at the midline of the cell and attach to spindle fibers. The pairing of homologous chromosomes helps ensure that the daughter cells will receive one member of each homologous pair. Consider the following analogy. By pairing your socks before putting them in a drawer, you are more likely to put matching socks on your feet than if you randomly pulled out two socks. This time, the chromosomes align completely differently. They are adjacent to each other; chromosome 1 from mum is next to chromosome 1 from dad, chromosome 2 from dad is next to chromosome 2 from mum etc.
• Next, during anaphase I, the members of each homologous pair of chromosomes separate, and each homologue moves to opposite ends of the cell. In anaphase I, when the microtubules shorten to pull them apart, each homologue will pull apart to different poles, form again, cytokinesis divides cytoplasm, new nuclear membrane forms. Each daughter cell now has 23 chromosomes (two chromatids).
• During telophase I, cytokinesis begins, resulting in two daughter cells, each with one member of each chromosome pair. Each chromosome in each daughter cell still consists of two replicated sister chromatids. Telophase I is followed by interkinesis, a brief interphase-like period. Interkinesis differs from mitotic interphase in that there is no replication of DNA during interkinesis.
• It is at this stage where important events occur that introduce genetic variation. The two sources of variation are recombination, due to crossing over, and independent assortment during metaphase I.
• There may be a brief rest period before moving onto metaphase II.

Question 26

What is synapsis?

Answer 26

• Pairing of homologous chromosomes to form a Tetrad in Prophase I
• The separation of homologous chromosomes occurs reliably during meiosis I because, during prophase I (the I indicates this phase takes place during meiosis I), members of homologous pairs line up next to one another by a phenomenon called synapsis (“bringing together”). For example, the chromosome 1 that was originally from your father would line up with the chromosome 1 originally from your mother. Paternal chromosome 2 would pair with maternal chromosome 2, and so on.
• As homologous chromosomes line up on the spindle so close to one another, they have the ability to exchange genetic material. They form structures, called tetrads, and at the chiasmata (points of contact), they can exchange genes and introduce variation.
• Genetic material from the homologous chromosomes is randomly swapped
• This creates 4 unique chromatids, hence increasing overall genetic diversity of the gametes.

Question 27

What happens in meiosis II?

Answer 27

• During the second meiotic division, meiosis II, each chromosome lines up in the center of the cell independently (as occurs in mitosis), and the sister chromatids (attached replicates) making up each chromosome separate. Separation of the sister chromatids occurs in both daughter cells that were produced in meiosis I. This event results in four cells, each containing one of each kind of chromosome. The events of meiosis II are similar to those of mitosis, except that only 23 chromosomes are lining up independently in meiosis II compared with the 46 chromosomes aligning independently in mitosis.
  1) Prophase II = chromosomes condense again, occurs in both daughter cells
  2) Metaphase II = chromosomes line up at the midline of the cell
  3) Anaphase II = centromeres of sister chromatids separate, chromatids of each pair are now called chromosomes, chromosomes move to opposite poles.
  4) Telophase II = one complete set of chromosomes is located at each pole, cytokinesis occurs in both daughter cells, forming four haploid daughter cells.
• There is no interphase or duplication, because the duplicated chromatids are still present. The same thing happens where the centrioles duplicate, the nuclear membrane breaks down and the chromosomes line up on the spindle.
• This time, the chromosomes line up similarly to mitosis; one after the other. When they pull apart, the chromatids now split apart from the centromeres to produced true haploid daughter cells with the 23 chromosomes.

Question 28

What are the main differences between mitosis and meiosis?

Answer 28

1) Mitosis: cell divides to produce 2 new ‘daughter’ cells that are identical to the original (parent) and diploid
Meiosis: similar to mitosis but more complex → results in production of ‘daughter’ cells that are non-identical and haploid
2) Mitosis involves one cell division, whereas meiosis involves two.
3) Mitosis produces two diploid cells, whereas meiosis produces up to four haploid cells
4) Mitosis occurs in somatic cells, whereas meiosis occurs only in ovaries and testes during the formation of gametes (egg and sperm)
5) Mitosis results in growth and repair, whereas meiosis results in gamete (egg and sperm) production
6) There is no exchange of genetic material in mitosis. Parts of chromosomes are exchanged in crossing over in meiosis.
7) Daughter cells are genetically similar in mitosis, but genetically dissimilar in meiosis.

Question 29

What are the functions of meiosis and why is it advantageous?

Answer 29

• Meiosis serves two important functions in sexual reproduction:
  1) Meiosis keeps the number of chromosomes in a body cell constant from generation to generation.
  2) Meiosis increases genetic variability in the population
• Meiosis is advantageous
  1) random distribution of male and female homologous chromosomes
  2) chromosomal crossing over occurs

Question 30

How is genetic variability achieved?

Answer 30

1) Independent assortment during metaphase I

2) Crossing over (recombination) during prophase I

Question 31

What is independent assortment?

Answer 31

• Random assortment of either male/female homologs. There are many different combinations of gametes that can be seen at the end of meiosis II depending on how the homologous chromosomes split up at metaphase I.
• Independent assortment is a random process that depends on how the homologous chromosomes line up on the spindle.
• Homologous pairs of chromosomes line up at the equator (midpoint) of the spindle during metaphase I.
• However, the orientation of the members of the pair is random with respect to which member is closer to which pole.
• The key to reproduction is the ability to pass genes on but also the ability to bring variability and variation to the next generation, so they can evolve and adapt to their circumstances. Crucial to this is independent assortment. In meiosis I, the chromosomes line up next to each other (but it is not maternal chromosomes on one side and paternal on the other) – when the homologous chromosomes are separated, this will already bring variation (the combinations are endless).

Question 32

What is crossing over (recombination)?

Answer 32

• Corresponding pieces of chromatids of maternal and paternal homologues (non-sister chromatids) are exchanged during synapsis in prophase I when the homologues are aligned side by side.
• When they are very close to each other on the spindle and form these tetrads, the homologues have crossing over points whereby they can mix up their genes and alter them at that stage.
• This exchange occurs between the different homologues, not between the sister chromatid on one chromosome, to bring in variation. The combinations can be endless, so the gametes that form will have a real mix of chromosomes and a mix of genes.
• Each of the affected chromatids has a mixture of maternal and paternal genetic information.

Question 33

Does crossing over occur in the sex chromosomes?

Answer 33

• Sex chromosomes align but crossing over does not usually occur in X and Y chromosomes apart from at the pseudoautsomal regions (PAR)
• This is because they are hemizygous to each other & so recombination proved harmful
• PAR allows the X & Y chromosomes to pair and properly segregate during meiosis in males
• X-inactivation occurs in which one of the copies of the X-chromosome is silenced to prevent females from having twice as many gene products as males.
• Choice of which is inactivated is random in placental mammals like humans.
• It resulted in males without necessary genes found on the X chromosome, and females with unnecessary or even harmful genes previously only found on the Y chromosome. As a result, genes beneficial to males accumulated near the sex-determining genes, and recombination in this region was suppressed in order to preserve this male specific region. Over time, the Y chromosome changed in such a way as to inhibit the areas around the sex determining genes from recombining at all with the X chromosome. As a result of this process, 95% of the human Y chromosome is unable to recombine.
• Crossing over probably did occur in the beginning so the advantageous genes would be lost and the individuals selected out. Those that accumulated and hung on to the advantageous genes out-bred them. Today, the human Y chromosome itself contains only 86 working genes, while there are close to 1000 working genes on the X chromosome. In some animals, Y degradation is even more severe.
• The sex chromosomes do align, but crossing over of the main genes would be lethal (no survival). Some degree of crossing over can occur in the pseudoautosomal regions (usually at the ends of the X and Y chromosomes). Females inherit two X chromosomes; duplication of genes/gene products could be harmful, so often one of the X chromosomes is inactivated to prevent having twice as many gene products. In humans and other placental animals (animals that form a placenta), this is random (may be fixed in some creatures).

Question 34

What are aneuploidies?

Answer 34

• A gain or loss of chromosomes from the normal 46 is called aneuploidy, affecting normal development and functioning. In humans, a lot of these abnormalities (called aneuploidies) occur. Humans produce a lot of aneuploid gametes.
• Since each chromosome contains hundreds of genes, the addition or loss of a single chromosome disrupts the existing equilibrium of the cell leading to profound phenotypes.
• Due to the concept of crossing over and random distribution, there can be phenotypic variations. Even with a very similar genetic disorder, there can be really big variations in how it presents (the phenotype of the genetic disorder).
• With assisted reproductive technology, cytogenetics/the ability to karyotype can be used to look for aneuploid gametes and prevent using them (so abnormal chromosomes/abnormal characteristics are not passed on).
• Using cytogenetics (including karyotyping) - aneuploid gametes produced at surprisingly high rates in humans.
• Majority of aneuploidies occur from an error in maternal meiosis I (Hassold & Hunt, 2001, Nat.Rev.Gen.2:280-291), because human oocytes are arrested in prophase I for decades
• Know it’s maternal because of presence of polymorphic DNA markers on individual chromosomes that all distinction between maternally and paternally derived chromosomes.
• Aneupolidy is present in 6% of sperm from ostensibly normal men (Egozcue J et al, 1997, Hum.Reprod.Update 3:441) and in 20% of oocytes (Kuliev A et al, 2011, Reprod.Biomed.Online 22:2-8)
• Majority are lethal, e.g. trisomies (47 chromosomes) account for 35% of spontaneous abortions/miscarriages
• Aneuploidy is not just all about the eggs; there is also aneuploidy occurring in sperm (even though spermatogenesis ongoing, “made fresh”)
• Often, if woman miscarry very early on in pregnancy, it is usually because they have got some kind of aneuploidy (often a trisomy).
• Before the pairs can separate, however, the crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sister chromatids. Failure to separate the pairs of chromosomes to different daughter cells is referred to as nondisjunction, and it is a major source of aneuploidy.
• 50% of recognized pregnancy loss result from chromosomal abnormality (other statistic was specifically about trisomies).

Question 35

What is non-disjunction?

Answer 35

• Failure of homologous chromosome to separate during MI or sister chromatids to separate during MII, resulting in extra or missing chromosomes
• Extra or missing chromosomes = Aneuploidy
• Most of the time, meiosis is a precise process that results in the chromosomes being distributed evenly to gametes. But meiosis is not fool-proof. A pair of chromosomes or sister chromatids may adhere so tightly to one another that they do not separate during anaphase. As a result, both go to the same daughter cell, and the other daughter cell receives none of this type of chromosome. The failure of homologous chromosomes to separate during meiosis I or of sister chromatids to separate during meiosis II is called nondisjunction.
• E.g. extra chromosome = Trisomy 21 (Down’s syndrome).
• E.g. missing chromosome = Turner’s (XO)
• The eggs are arrested in this phase; sit on the spindle for decades.

Question 36

What are common examples of aneuploidies?

Answer 36

• Most common aneuploidies in humans are trisomies (0.3% of live births).
• Viable ones are:
  1) Trisomy 21 (aka Down’s syndrome, 1:750 births)
  2) Trisomy 18 (Edwards syndrome)
  3) Trisomy 13 (Patau syndrome)
• 50% of patients with primary amenorrhea as a result of premature ovarian insufficiency (POI) have an abnormal karyotype
• Sex chromosome aneuploidy more viable, usually random event (not inherited):
  1) Turner syndrome (45, X monosomy) » caused by complete or partial absence of 2nd sex chromosome (occurrence 1:2000 female births) → phenotype=short stature, primary amenorrhea (classic Turners). Huge variations; classic = XO, some will get part of it
  2) Klinefelter syndrome (47,XXY trisomy) » caused by presence of two X and one Y chromosome (occurrence 1:500 male births) → variable phenotype=taller than average, small testes producing reduced testosterone, infertility. High variability; depends on how much of the X chromosome (sometimes only certain genes carry over)
• Nonmosaic triploidy (3n = 69) and tetraploidy (4n = 92) are common in abortuses. Triploid abortuses are usually 69,XXY or 69,XXX, resulting from dispermy.
• Primary ovarian insufficiency (POI), also known as premature ovarian failure, premature menopause, hypergonadotropic amenorrhea, hypergonadotropic hypogonadism, and ovarian insufficiency, refers to the loss of ovarian function before the age of 40 years.
• Classic Turners = 45,X monosomy with phenotype of short stature and primary amenorrhea. The ovaries develop normally at first, but egg cells (oocytes) usually die prematurely and most ovarian tissue degenerates before birth. Many affected girls do not undergo puberty unless they receive hormone therapy, and most are unable to conceive (infertile).

Question 37

How do patient’s with Turner’s syndrome clinically present?

Answer 37

• Turner’s = one X chromosome (classic type), short stature, webbing of the neck, characteristic notch of the aortic arch ( only identified with modern technology), often infertile (present with premature ovarian failure, reduced size of ovaries)
• Cardiac MRI revealing normal aortic arch in ‘candy cane’ configuration on the left, compared with a previously undiagnosed aortic coarctation, just after the origin of the left subclavian artery (arrow), detected by MRI in an adult woman with Turner’s syndrome with severe upper body hypertension
• Classic Turners = 45,X monosomy with phenotype of short stature and primary amenorrhea. The ovaries develop normally at first, but egg cells (oocytes) usually die prematurely and most ovarian tissue degenerates before birth. Many affected girls do not undergo puberty unless they receive hormone therapy, and most are unable to conceive (infertile).
• Huge variations; classic = XO. Caused by complete or partial absence of 2nd sex chromosome (occurrence 1:2000 female births).

Question 38

How do patient’s with Klinefelter’s syndrome clinically present?

Answer 38

• Klinefelter = XXY
• curved pinky fingers (fifth finger clinodactyly)
• 47,XXY trisomy » caused by presence of two X and one Y chromosome (occurrence 1:500 male births) → variable phenotype=taller than average, small testes producing reduced testosterone, infertility. High variability; depends on how much of the X chromosome (sometimes only certain genes carry over)

Question 39

How does maternal age affect the risk of trisomy?

Answer 39

• Multiple mechanisms contribute to the maternal age effect –
  1) Recombination failure
  2) Premature homologue separation (separate too early/incorrectly)
  3) Premature sister chromatid separation due to loss of cohesin between sister centromeres
• Intuitively, this observation makes sense when considering that human oocytes can be arrested in prophase I for several decades. The medical community is well aware that the risk of trisomy also increases sharply with maternal age, particularly as women near the end of their reproductive life span. Consequently, pregnant women over the age of 35 are routinely offered testing for foetal chromosome abnormalities.
• As you age, the incidence rate of these trisomies increases dramatically. There are multiple reasons for this.
  The graph shows where the abnormality typically occurs (whether the errors have occurred predominantly in meiosis I or meiosis II stage).
• When the sister chromatids sit next to each other, one of the proteins that is responsible for this is called cohesin. When lost, they can prematurely separate. Can also get issues with the microtubules and spindles, which causes a lagging microtubule that does not pull the homologue or chromatid apart very quickly.