Session 11 - Mitosis and Meiosis Flashcards
How many chromosomes are in a human somatic cell?
46 Chromosomes, 23 pairs
- 22 autosomes and X and Y (pair of sex chromosomes)
What are homologous chromosomes, sister chromatids and non-sister chomatids?
- Homologous Chromosomes - A pair of chromosomes, one paternal one maternal, that carry the same genes (but not necessarily the same alleles)
- Sister Chromatids - Identical copies formed by DNA replication with both copies joined together by a common centromere
- Non-Sister Chromatids - Chromatids from homologous chromosomes
Describe the 4 different structures of chromosomes with relation to their p and q arms.
Metacentric - P and Q are equal
Submetacentric - P is shorter than Q
Acrocentric - P is almost none existent
Telocentric - No p arms (not in humans)
What is a centromere?
A centromere consists of repetitive sequences and links the chromatids and p and q arms
What are telomeres and what is their role?
Telomeres are repeated sequences (TTAGGG) at each end of a chromosome and act as a sort of cap. They maintain genomic integrity by preventing end fusion and degradation during DNA replication
What is a metaphase spread and karyotyping?
An image of chromosomes arrested (growth stopped) in metaphase and prepared for imaging using stains.
This can then be arranged to pair chromosomes to form a karyotype
Where are chromosomes during interphase?
Chromosomes are in highly organised positions within the nucleus called chromosome territories.
Name and describe the 6 stages of mitosis
Prophase - Spindle fibres appear, chromosomes condense, Nuclear envelope disappears
Prometaphase - Spindle fibres attach to chromosomes
Metaphase - Chromosomes line up at metaphase plate
Anaphase - Centromeres divide - sister chromatids move to opposite poles
Telophase - Nuclear membrane reforms, Chromosomes decondense and spindle fibres disappear
Cytokinesis - Cytoplasm divides, parent becomes 2 daughter cells
What is aneuploidy and how can it occur in mitosis?
Aneuploidy is when there is an unusual number of chromosomes. This can occur if there is mis-segregation/non-disjunction of chromosomes in mitosis
How does non-mosaic and mosaic aneuploidy occur during early development via mitosis?
If non-disjunction occurs during the very first post zygotic division the this leads to a non-mosaic phenotype as the cell line that follows monosomy is usually lost (it is not feasible with life) leaving only the trisomy cell line
However, if the mitotic non-disjunction occurs during later post zygotic division then this leads to mosaicism (the presence of two or more cell lines in an individual)
Name and describe all the stages in meiosis
DNA replication (occurs in S phase)
Meiosis I:
- Prophase I - Chromosomes condense and homologous chromosomes pair up through complementary base attraction. Close proximity results in the formation of chiasmas and recombination occurs
- Metaphase I - Spindle fibres associate with centromeres and tetrads (pairs of homologous chromosomes) line up. The random assortment of chromosomes occurs
- Anaphase I - Homologous pairs are separated to either poles
- Telophase I - Nuclear envelope reforms and each daughter cell has 23 chromosomes (each chromosome is still replicated and has 2 sister chromatids)
Meiosis II:
- Prophase II - Nuclear envelope disappears and spindle fibres appear
- Metaphase II - Chromosomes line up at metaphase plate and attach to spindle fibres
- Anaphase II - Sister chromatids move to opposite poles and centromere divides
- Telophase II - Nuclear envelope reappears and there are now 4 haploid daughter cells
How is genetic diversity generated in meiosis?
1) recombination of sister chromatids in prophase 1
2) Random assortment of chromosomes, in Metaphase 1
Describe the process of oogenesis
1) Before birth the Oogonium undergoes mitotic division to form a bank of primary oocytes which are then arrested at prophase 1 until puberty
2) After puberty 1 primary oocyte in each ovary restarts meiosis. The first meiotic division produces 1 secondary oocyte, which continues to meiosis 2, the other is terminated and forms 2 “waste” polar bodies
3) The secondary oocyte then undergoes the second meiotic division and forms 1 mature ovum (egg) and another polar body
How can aneuploidy occur in meiotic division?
1- can occur during meiosis 1 when a pair of homologous chromosomes may go to the same pole (all gametes affected)
2- May occur in meiosis 2 if both sister chromatids go to the same pole (half of the gametes are affected).
