Meiosis Problems

Question 1

Explain spermatogenesis

Answer 1

• Diploid germ cell (spermatocyte) → spermatocyte →four haploid daughter cells → spermatids→ sperm
• Happens once puberty has started and occurs continuously until death

Question 2

Explain oogenesis

Answer 2

• Diploid germ cell (oogonium) → primary oocyte→ (after Meiosis I) secondary oocyte (gets nearly all the cytoplasm) + first polar body (small) → (after Meiosis II) gamete (one cell that gets all the cytoplasm). Three polar bodies ultimately disintegrates
• Meiosis begins while the foetus is in the uterus, but hormones produced during puberty allows the process to continue

Question 3

When does meiosis occur in human females

Answer 3

• There are a number of arrested periods where meiosis will stop and then re-start at a later stage facilitated by a stimulus
• Meiosis I is initiated in the human fetal ovary at 11–12 weeks of gestation.
  Oocytes enter prophase I and homologous chromosomes undergo pairing, synapsis and the exchange of genetic material in a process known crossing over
• On completion of crossing over, the oocyte enters a protracted arrest stage known as dictyate

Question 4

What is the point of the polar bodies

Answer 4

Absorb different sets of chromosomes that have to be separated

Question 5

How long does it take an oocyte to grow

Answer 5

• completes growth each month and is ovulated in response to a mid-cycle surge of LH (Luteinizing hormone)
• Meiosis I resumes

Question 6

What happens before ovulation

Answer 6

• Before ovulation, the oocyte resumes meiosis I; chromosomes condense, and the homologs orient on the metaphase I plate and segregate from each other at anaphase I
• One group of chromosomes remains in the oocyte and the other is segregated to a small bleb of cytoplasm, the first polar body.
  – first division reduces the number of chromosomes in the oocyte by half

Question 7

What happens after meiosis one

Answer 7

• After meiosis I (MI)
  chromosome segregation, a second meiotic spindle forms immediately (prophase II), the remaining chromosomes align at the spindle equator and the cell arrests again
• The metaphase II arrested cell is known as an egg, and it remains in arrest until it is fertilized or degenerates
• Fusion of the sperm and egg plasma membranes at fertilization triggers the resumption and completion of MII

Question 8

Describe the karyotype of a female

Answer 8

• There are 23 pairs of chromosomes
• 22 pairs are autosome and one pair will be sex chromosomes
• Sex chromosomes have 2 X chromosomes

Question 9

Explain translocation

Answer 9

Exchange of genetic material between non-homologous chromosomes

Question 10

What is polyploid

Answer 10

Cells have one or more extra sets of chromosomes

Question 11

Explain non-disjunction

Answer 11

• Is the failure of chromosomes to separate in meiosis and it causes gametes to have incorrect chromosome numbers
• A sex chromosome abnormality is typically less severe than an incorrect number of autosomes
• Occurs in meiosis 1 or meiosis 2

Question 12

What are the other small-scale chromosome abnormalities

Answer 12

• Chromosomal rearrangements can delete or duplicate genes
• An inversion flips gene order possibly disrupting vital genes
• In a translocation two non-homologs exchange parts. Some translocations can cause cancer
• euploidy; aneuploidy; monosomic human zygotes; autosomal trisomies; trisomic

Question 13

What is euploidy

Answer 13

Corresponds to 22 pairs of autosomes and one pair of sex chromosomes

Question 14

Explain aneuploid

Answer 14

• An individual with an error in chromosome number
• Monosomy (loss of one chromosome) or
• Trisomy (gain of an extraneous chromosome)

Question 15

Explain trisomic

Answer 15

Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose

Question 16

Explain monosomic human zygotes

Answer 16

Missing any one copy of an autosome invariably fail to develop to birth because they have only one copy of essential genes

Question 17

Explain autosomal trisomies

Answer 17

Fail to develop to birth; however, duplications of some of the smaller chromosomes (13, 15, 18, 21, or 22) can result in offspring that survive for several weeks to many years

Question 18

Examples of polyploid animals

Answer 18

Flatworms; crustaceans; amphibians; fish; lizards

Question 19

Explain Turner syndrome briefly

Answer 19

• XO
• 1 in every 2000 females
• Result of inheriting too many or too few X or Y chromosomes
• Caused by nondisjunction during oogenesis or spermatogenesis

Question 20

Explain Turner syndrome in more detail

Answer 20

• Female with single X chromosome, other chromosome is partially or completely missing
• Short, with broad chest and widely spaced nipples
• Can be of normal intelligence and function with hormone therapy
• 99% of Turner-syndrome conceptions are thought to end in miscarriage or stillbirth

Question 21

Explain Klinefelter syndrome

Answer 21

• XXY
• 1 in every 500-1000 males
• Male with underdeveloped testes and prostate; some breast overdevelopment
• Long arms and legs; large hands
  Near normal intelligence unless XXXY, XXXXY, etc.
• No matter how many X chromosomes, presence of Y renders individual male

Question 22

Explain Poly - X females

Answer 22

• XXX
• 1 in every 1000-2000 females
• XXX simply taller & thinner than usual
• Some learning difficulties
• Many menstruate regularly and are fertile
• More than 3 Xs renders severe mental retardation

Question 23

Explain Jacob’s syndrome

Answer 23

• XXY
• 1 in every 2000 females
• Tall, persistent acne, speech & reading problems

Question 24

What are x-chromosomal abnormalities associated with

Answer 24

• Mild intellectual and physical disabilities, as well as sterility
• If the X chromosome is absent altogether, the individual will not develop

Question 25

Explain XX normal functioning

Answer 25

• X inactivation – shuts off one X chromosome in the cells of female mammals
• More genes on the X than the Y, Avoids over expression of proteins
• Normal state

Question 26

Explain XY normal functioning

Answer 26

• SRY gene on the Y chromosome controls other genes that stimulate development of the male structures and suppress development of female structures
• Produces the sex-determining region Y protein (TDF) - acts as a transcription factor,
  It attaches (binds) to specific regions of DNA and helps control the activity of particular genes.
• This protein starts processes that cause a foetus to develop male gonads (testes) and
  prevent the development of female reproductive structures (uterus and fallopian tubes)

Question 27

Explain disorders associated with SRY gene

Answer 27

• 46,XX testicular disorder of sex development,
  the condition results from an abnormal exchange of genetic material between chromosomes (translocation).
• This exchange occurs as a random event during the formation of sperm cells in the affected person’s father.
  TheSRYgene is misplaced in this disorder, almost always onto an X chromosome.
• A foetus with an X chromosome that carries theSRY gene will develop male characteristics despite not having a Y chromosome

Question 28

What will happen if a foetus’ cells do not produce functional sex-determining region Y protein

Answer 28

• Will not develop testes but will develop a uterus and fallopian tubes, despite having a typically male karyotype

Question 29

Explain SRY gene mutation

Answer 29

• Full mutation = complete shut off thus XY will be present but female characteristic available
• Partial = not completely eliminated thus there’s ambiguous genitalia ad won’t look clearly male or female

Question 30

What causes abnormal chromosome structure

Answer 30

• Deletion
• Translocation
• Duplication
• Inversion

Question 31

Explain Deletion and translocation for abnormal chromosome structure

Answer 31

• Missing segment of chromosome
  Lost during breakage
• Williams syndrome - Loss of segment of chromosome 7 – intellectual disability, distinctive behavioural and facial characteristics and cardiac problems
• Cri du chat syndrome (cat’s cry/Lejeune’s syndrome) - Loss of segment of chromosome 5 – characteristic cat-like sounds made by affected children due to problems with larynx and nervous system

Question 32

Explain translocation for abnormal chromosome structure

Answer 32

• A segment from one chromosome moves to a non-homologous chromosome
  Follows breakage of two nonhomologous chromosomes and improper re-assembly
• Alagille syndrome – affects liver, heart, kidneys and other body systems due to organ abnormalities
  Some cancers

Question 33

How can translocation lead to cancer formation

Answer 33

• Non homologous chromosome exchange
  Sometimes can break genes
• Known to cause leukemia or other cancers
• Chronic myelogenous leukemia – parts of chromosome 9 and 22 switch places
• This creates a combined gene on chromosome 22
• This gene encodes for a protein that speeds cell division and suppresses normal cell death (apoptosis)
• Leukemia – a form of cancer in which blood cells divide out of control

Question 34

Explain chromosome 18 inversion

Answer 34

• Contributed to the evolution of humans.
  Not present in our closest genetic relatives, the chimpanzees
• Believed to have occurred in early humans following their divergence from a common ancestor with chimpanzees approximately five million years ago
• Researchers have suggested that a long stretch of DNA was duplicated on chromosome 18 of an ancestor to humans, but that during the duplication it was inverted (inserted into the chromosome in reverse orientation).
• It is not known how this inversion contributed to hominid evolution, but it appears to be a significant factor in the divergence of humans from other primates