Meiosis Problems Flashcards
Explain spermatogenesis
- Diploid germ cell (spermatocyte) → spermatocyte →four haploid daughter cells → spermatids→ sperm
- Happens once puberty has started and occurs continuously until death
Explain oogenesis
- 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
When does meiosis occur in human females
- 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
What is the point of the polar bodies
Absorb different sets of chromosomes that have to be separated
How long does it take an oocyte to grow
- completes growth each month and is ovulated in response to a mid-cycle surge of LH (Luteinizing hormone)
- Meiosis I resumes
What happens before ovulation
- 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
What happens after meiosis one
- 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
Describe the karyotype of a female
- There are 23 pairs of chromosomes
- 22 pairs are autosome and one pair will be sex chromosomes
- Sex chromosomes have 2 X chromosomes
Explain translocation
Exchange of genetic material between non-homologous chromosomes
What is polyploid
Cells have one or more extra sets of chromosomes
Explain non-disjunction
- 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
What are the other small-scale chromosome abnormalities
- 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
What is euploidy
Corresponds to 22 pairs of autosomes and one pair of sex chromosomes
Explain aneuploid
- An individual with an error in chromosome number
- Monosomy (loss of one chromosome) or
- Trisomy (gain of an extraneous chromosome)
Explain trisomic
Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose
Explain monosomic human zygotes
Missing any one copy of an autosome invariably fail to develop to birth because they have only one copy of essential genes
Explain autosomal trisomies
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
Examples of polyploid animals
Flatworms; crustaceans; amphibians; fish; lizards
Explain Turner syndrome briefly
- 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
Explain Turner syndrome in more detail
- 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
Explain Klinefelter syndrome
- 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
Explain Poly - X females
- 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
Explain Jacob’s syndrome
- XXY
- 1 in every 2000 females
- Tall, persistent acne, speech & reading problems
What are x-chromosomal abnormalities associated with
- Mild intellectual and physical disabilities, as well as sterility
- If the X chromosome is absent altogether, the individual will not develop
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
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)
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.
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
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
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
What causes abnormal chromosome structure
- Deletion
- Translocation
- Duplication
- Inversion
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
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
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
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
