Meiosis Lecture

Question 1

Explain chromosome segregation differences between Meiosis I and II

Answer 1

Meiosis I: Reductional division. Number of chromosomes(N) and chromatids(C) is cut in half. From 2N and 4C to 1N and 2C.

Meiosis II: Equational division. 1N and 2C to 1N and 1C. Similar to Mitosis.

Question 2

Discuss the distinguishing features and mechanisms of Meiosis I

Answer 2

Pairing and synapsis:

Attachment of clustering telomeres at the nuclear envelope (chromosome bouquet) associated with Rapid Chromosome Movements.

Synaptonemal Complex: SC is a tripartite structure that serves as the molecular glue holding homologous chromosomes together.Possible functions: organize loops of chromatin, mediate synapsis via protein-protein interactions, aid in assembly of recombination complexes.

Recombination:

Programmed formation of Double Strand Breaks at hotspots (DSBs)

Exonucleolytic resection of 5’ ends at breaks

Strand invasion into a chromatid of the homologous chromosome

Repair of DSBs by homologous recombination: Crossover (chiasmata, 23 meioses), non-crossover (chromosome pairing- most common, 200 meioses)

Failure of a chromosome pair to undergo at least one crossover event can lead to aneuploidy

Homologous chromosome segregation:

Homologous chromosomes are separated while sister chromatids stay held together by cohesions. The enzyme separase cleaves the cohesions holding arms together

Question 3

Explain the events that occur during each stage of Prophase I

Answer 3

Leptotene: Chromosomes Condense, bouquet formation, AE and SC begin to assemble, programmed DSBs at recombination nodules

Zygotene: Pairing extends, AE to LE, Synapsis begins

Pachytene: Completion of synapsis, maturation of meiotic recombination sites into crossovers

Diplotene: Chromosomes undergo desynapsis, Homologues are held together by crossovers (visible as chiasmata), further condensation occurs

Question 4

Discuss sex dimorphisms in meiosis

Answer 4

Males:

Meiosis begins at puberty

Sequential, synchronous, continuous

Each cell that enters meiosis will produce 4 haploid sperm

Spermatogenesis has more stringent checkpoints.

Females:

Meiosis begins during fetal development

Oocytes arrested in Prophase I (diplotene). Meiosis I resumes with ovulation and cells are then arrested in Metaphase II. Meiosis II will only go to completion when fertilization occurs.

Each cell that enters meiosis will produce 1 haploid egg. First and Second Polar bodies produced.

Most meiotic errors are maternal in origin and happen during Meiosis I.

Oogenesis is more robust

Question 5

Identify causes of meiotic errors

Answer 5

Advanced maternal age, environmental chemical exposure, parental exposure to radiation or chemotherapy, maternal diabetes or obesity, genetic mutations, oral contraceptives or fertility drugs, thyroid antibodies, alcohol consumption, smoking, seasonality, parity, consanguinity

Question 6

Describe the Maternal Age Effect

Answer 6

maternal age is linked to aneuploidy, exact relationship is chromosome dependent. Contributing factors: recombination errors, dna replication pathways compromised, low sensitivity to cell cycle checkpoints, spindle abnormalities, incorrect kinetochore-microtubule interactions, alterations in chromosome structure, deterioration of chromosome cohesion (chromosome segregation relying on old proteins that were put on chromosomes during S phase- as a fetus for females)

Question 7

clinical consequences of meiotic errors

Answer 7

Infertility, miscarriages, and birth defects

Question 8

Aneuploidy

Answer 8

Error in cell division that results in daughter cells lacking proper number of chromosomes.

Aneuploidy, for example, occurs in 10-30% of fertilized human eggs in a normal healthy population (upwards of 60% in females of advanced maternal age). Aneuploidy is the leading cause of pregnancy loss and the genetic cause of developmental disabilities and mental retardation.

Question 9

Trisomy (16,18,21)

Answer 9

Trisomy: 3 of the same chromosome

Trisomy 16: leads to miscarriage-most common

Trisomy 18: Edwards Syndrome- usually fatal before birth or first year of life

Trisomy 21: Down Syndrome

Question 10

Homologous Chromosome

Answer 10

Chromosomes that differ in parental origin

Question 11

Describe the process of Recombination

Answer 11

1. Programmed formation of Double Strand Breaks at hotspots (DSBs)
2. Exonucleolytic resection of 5’ ends at breaks
3. Strand invasion into a chromatid of the homologous chromosome
4. Repair of DSBs by homologous recombination: Crossover (chiasmata, 23 meioses), non-crossover (chromosome pairing- most common, 200 meioses)

Failure of a chromosome pair to undergo at least one crossover event can lead to aneuploidy

Question 12

Sister Chromatid

Answer 12

Two copies of a single chromosome produced by DNA replication

Question 13

Bivalent

Answer 13

Structure formed from association of homologous chromosomes

Question 14

Centromere

Answer 14

Region on chromosomes upon which kinetochores assemble

Question 15

Kinetochore

Answer 15

Protein based structure that allows attachment to the spindle

Question 16

Chiasmata

Answer 16

Physical manifestations of genetic recombination that tether homologous chromosomes. 1-2 chiasmata per pair.

Question 17

Cohesion

Answer 17

Established during pre-meiotic S phase. Protein complex established at centromere and at chromosome arms. Stabilizes the physical linkages (chiasmata) established geometry for kinetochore-microtubule interactions. Holds sister chromatids together.

Question 18

Goals of Meiosis

Answer 18

1. To create haploid gametes that upon fertilization will produce a diploid zygote.
2. To generate genetic diversity

Question 19

Ways to generate genetic diversity

Answer 19

1. Independent Assortment 2. Crossing-over

Question 20

Female Meiosis

Answer 20

Oocytes arrested in Prophase I (diplotene) as a fetus.

Meiosis I resumes with ovulation and cells are then arrested in Metaphase II.

Meiosis II will only go to completion when fertilization occurs.

Each cell that enters meiosis will produce 1 haploid egg. First and Second Polar bodies produced.