Meiosis Flashcards
Three characteristics used to determine chromosome:
- size
- centromere index
- G-bright
Centromere Index:
= P-arm length/(total chromosome length X 100)
G-bright areas:
- bright regions are euchromatic, early-replicating and GC rich
- rich in SINE and Alu sequences
- contain “house-keeping” genes
What is the goal of meiosis?
- to reduce the number of chromosomes in the parent cell (23 pairs; n = 46) by half and produce gamete cells (each with n = 23).
- which two homologs go into a single gamete and that an offspring inherits is completely random
Fertilization of an oocyte with a spermatocyte (each with 23 chromosomes) reconstitutes:
- a diploid zygote
- a cell with 23 pairs (46 individual) chromosomes
- these chromosomes will make up all of the descendant cells in the offspring via mitosis
Meiosis I separates:
homologue chromosomes
(sister chromatids remain attached at centromere)
Meiosis II separates:
sister chromatids
(sister chromatids separated at centromere)
The four stages of Meiosis I:
- Prophase I
- Metaphase I
- Anaphase I
- Interkinesis I
Prophase I:
- pairing of homologous chromosomes
- leptotene and zygotene (synapsis)
- chromosome condensation
- pachytene
- formation of sister chromatids
- diplotene
Metaphase I:
- homologous chromosomes lined up in middle of cell
Anaphase I:
- Chiasmata at the chromosome ends.
- Paired chromosomes migrate to opposite poles of the cell.
Interkinesis I:
- formation of nuclei and 2 daughter cells
Meiosis II:
Similar to mitosis:
- Prophase II (no DNA synthesis)
- Metaphase II
- Anaphase II
Final result = 4 haploid cells (in males)
Meiosis in males:
STARTS AT PUBERTY
- Meiosis I followed by meiosis II at puberty
- final result = 4 haploid spermatids
Meiosis in females:
- Meiosis I begins in utero.
- Primary oocytes arrested in prophase I until puberty.
- Meiosis I completed at time of ovulation. First polar body ejected.
- Meiosis II completed at fertilization. Second polar body ejected.
FINAL RESULT:
1 MATURE OOCYTE; 2 LOST POLAR BODIES
What stage of meiosis I are primary oocytes arrested in?
- dictyotene of prophase I before birth
- complete meiosis I at ovulation
The first polar body created by meiosis in females contains:
- one pair of sister chromatids
- ejected at time of ovulation when meiosis I is completed
The second polar body created by meiosis in females contains:
- a single chromatid
- ejected at time of fertilization when meiosis II is completed
Nondisjunction in meiosis I:
- failure of homologous chromosomes to separate
- will lead to abnormal gametes and aneuploidy in zygote
Nondisjunction in meiosis II:
- failure of sister chromatids to separate
- will lead to abnormal gametes and aneuploidy in zygotes
Klinefelter Syndrome:
47, XXY
- trisomy
- tall, hypogonadism, gynecomastia (breasts)
- more X, greater risk for mental retardation
Edward’s Syndrome:
47, XX, +18
- trisomy
- CNS, heart, renal, clenched hands
Turner Syndrome:
45, X
- monosomy, cystic hygroma, short stature, infertile
- 4% survive; 96% die in utero
Down Syndrome:
47, XX, +21
- trisomy
Patau Syndrome:
47, XY, +13
- trisomy
- CNS, renal, heart
How do you answer the question did the nondisjunction event occur in meiosis I or meiosis II?
- Studying polymorphic DNA markers (CA repeats) can help determine parental origin.
- Studying polymorphic DNA markers near the centromere will help determine whether meiosis I or meiosis II.
How do you know if a nondisjunction event occurred in meiosis I?
- heterozygosity for both alleles from one parent near the centromere
How do you know if a nondisjunction event occurred in meiosis II?
- homozygosity for one parental allele near centromere
Nullisomic gametes yield:
- monosomic zygotes
- lethal except for X chromosome
- “Turner’s Syndrome”
Disomic gametes yield:
- trisomic zygotes
- lethal except for X, Y, a few small autosomes (13, 18, 21)
Balanced translocations in a parent raise the risk of:
- partial trisomy/monosomy syndromes.
- increases the risk of extra/less genetic material being inherited.
- synapsis is driven by DNA homology, and translocations can lead to quadri-radial synapsis that does not separate properly.
Quadri-radial synapsis due to balanced translocations in a parent can lead to how many possible segregations?
16
- only 2/16 (12%) are balanced and will give rise to normal gametes
Reciprocal translocations:
- two non-homologous chromosomes break and exchange fragments.
- Individuals carrying such abnormalities still have a balanced complement of chromosomes and generally have a normal phenotype, but with varying degrees of subnormal fertility.
- quadri-radial synapsis must now form during meiosis, which leads to genetically unbalanced gametes 14/16 of the time
Recombination of homologous chromosomes when one has an inversion leads to:
- Formation of an inversion loop to maximize pairing.
- Recombination within the inversion loop that leads to abnormal chromatids.
- acentric/dicentric chromosomes with duplications/deficiencies of genetic material
Pericentric inversions:
- Inversions in which the rotated segment includes the centromere:
- ABC - cen - DEFGH
- AD - cen - CBEFGH
- recombinant gametes will have altered gene dosage (a duplicated region and a deleted region)
Paracentric inversions:
- Inversions in which the rotated segment is located completely on one chromosomal arm and do not include the centromere:
- cen - ABCDEFGH
- cen - ADCBEFGH
- recombinant gametes have altered gene dosage and centromere number (one acentric and one dicentric).
The two reasons why a couple may be having miscarriages:
- inversions
- balanced translocations
Barr bodies in 46, XY:
0
Barr bodies in 46, XX
one Barr body in each nucleus
Barr bodies in 47, XXX
2 Barr bodies in each nucleus
Barr bodies in 48, XXXX
3 Barr bodies in each nucleus
Lyon Hypothesis:
- states that during early development (blastocyst stage) one of the X chromosomes in a female gets turned off
- this is maintained in all descendant cells of the clone
A “darkly staining” Barr body is:
- the condensed, inactive X chromosome in females
During the blastocyst stage (roughly 100 cells), all females inactivate a X-chromosome in each of the 100 cells. Which X-chromosome is deactivated is completely random.
This means that:
- under normal conditions a female is a genetic mosaic in each tissue derived from somatic cells.
- some tissues will express the allele from the father and some will express the allel from the mother
If there is a translocation between an X-chromosome and an autosome, which X-chromosome in the nucleus is preferentially inactivated?
- the normal X-chromosome is inactivated.
- X-AUTOSOME TRANSLOCATIONS ARE PROTECTED/EXPRESSED.
If there is a deletion/insertion in an X-chromosome, which X-chromosome in the nucleus is preferentially inactivated?
- X-chromosomes with deletions/insertions are preferentially inactivated.
- Normal X-chromosomes expressed.
The three genes involved in X-inactivation in females:
Xic, Xist, and Tsix
Xic:
- X Inactivation Center
- contains genes for Xist and Tsix that control inactivation of X-chromosomes in females
Xist:
- a gene that encodes a large non-coding RNA that is responsible for mediating the specific silencing of the X chromosome from which it is transcribed.
- The inactive X chromosome is coated by Xist RNA.
Tsix:
- Antisense RNA strand to Xist
- inactivates Xist, activating X-chromosome expression
Uniparental disomy can occur via:
- a random event during the formation of egg or sperm cells.
- during trisomic rescue.
Epigenetic:
- changes in gene expression in response to the environment
Prader-Willis Syndrome is due to a deletion on what chromosome and where?
paternal chromosome 15q11-13
- Maternal disomy and Paternal deletion.
Angelmann Syndrome is due to a deletion on what chromosome and where?
maternal chromosome 15q11-13
- Paternal disomy and Maternal deletion
