Meiosis Problems Flashcards
(34 cards)
1
Q
Explain spermatogenesis
A
- Diploid germ cell (spermatocyte) → spermatocyte →four haploid daughter cells → spermatids→ sperm
- Happens once puberty has started and occurs continuously until death
2
Q
Explain oogenesis
A
- 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
3
Q
When does meiosis occur in human females
A
- 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
4
Q
What is the point of the polar bodies
A
Absorb different sets of chromosomes that have to be separated
5
Q
How long does it take an oocyte to grow
A
- completes growth each month and is ovulated in response to a mid-cycle surge of LH (Luteinizing hormone)
- Meiosis I resumes
6
Q
What happens before ovulation
A
- 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
7
Q
What happens after meiosis one
A
- 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
8
Q
Describe the karyotype of a female
A
- There are 23 pairs of chromosomes
- 22 pairs are autosome and one pair will be sex chromosomes
- Sex chromosomes have 2 X chromosomes
9
Q
Explain translocation
A
Exchange of genetic material between non-homologous chromosomes
10
Q
What is polyploid
A
Cells have one or more extra sets of chromosomes
11
Q
Explain non-disjunction
A
- 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
12
Q
What are the other small-scale chromosome abnormalities
A
- 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
13
Q
What is euploidy
A
Corresponds to 22 pairs of autosomes and one pair of sex chromosomes
14
Q
Explain aneuploid
A
- An individual with an error in chromosome number
- Monosomy (loss of one chromosome) or
- Trisomy (gain of an extraneous chromosome)
15
Q
Explain trisomic
A
Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose
16
Q
Explain monosomic human zygotes
A
Missing any one copy of an autosome invariably fail to develop to birth because they have only one copy of essential genes
17
Q
Explain autosomal trisomies
A
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
18
Q
Examples of polyploid animals
A
Flatworms; crustaceans; amphibians; fish; lizards
19
Q
Explain Turner syndrome briefly
A
- 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
20
Q
Explain Turner syndrome in more detail
A
- 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
21
Q
Explain Klinefelter syndrome
A
- 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
22
Q
Explain Poly - X females
A
- 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
23
Q
Explain Jacob’s syndrome
A
- XXY
- 1 in every 2000 females
- Tall, persistent acne, speech & reading problems
24
Q
What are x-chromosomal abnormalities associated with
A
- Mild intellectual and physical disabilities, as well as sterility
- If the X chromosome is absent altogether, the individual will not develop
25
Explain XX normal functioning
- 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
26
Explain XY normal functioning
- 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)
27
Explain disorders associated with SRY gene
- 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.
The SRY gene is misplaced in this disorder, almost always onto an X chromosome.
- A foetus with an X chromosome that carries the SRY gene will develop male characteristics despite not having a Y chromosome
28
What will happen if a foetus' cells do not produce functional sex-determining region Y protein
- Will not develop testes but will develop a uterus and fallopian tubes, despite having a typically male karyotype
29
Explain SRY gene mutation
- 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
30
What causes abnormal chromosome structure
- Deletion
- Translocation
- Duplication
- Inversion
31
Explain Deletion and translocation for abnormal chromosome structure
- 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
32
Explain translocation for abnormal chromosome structure
- 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
33
How can translocation lead to cancer formation
- 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
34
Explain chromosome 18 inversion
- 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
