MCBG Session 11 - Mitosis & Meiosis Flashcards
Describe the structure of a replicated chromosome
A replicated chromosome consists of 2 sister chromatids (identical), which consist of solenoid loops, which are bundles of ‘beads on a string’ structures, held together by a centromere. Each chromatid has a short (p) arm and a long (q) arm. At the end of chromosomes are telomeres which are repeated TTAGGG sequences.
How are metaphase spreads used in chromosome analysis?
Metaphase spreads are when growth is arrested during metaphase, and stained which results in banding patterns. Dark bands = AT base rich, Light bands = GC base rich.
- Can be turned into karyotyping, where chromosomes are lined up.
- Can be turned into an ideogram, where computer shows banding pattern and analysis of each chromosome.
Why makes chromosomes homologous?
What is the name given to different variants of genes on homologous chromosomes?
- They have the same DNA sequence/genes located on them
- Alleles - non-sister chromatids may have the same genes, but different alleles (variants of genes)
- Sister chromatids = same genes, same alleles
- Non-sister chromatids = same genes, different alleles
Where are chromosomes during interphase?
In chromosome territories, specific positions within the cell nucleus that are highly organised and specific.
What is mitosis?
What are the 2 divisions that occur?
- Cell division for somatic cells. Chromosome number stays the same.
- Nuclear division (mitosis) and cellular division (cytokinesis)
What is produced by mitosis and why is it so important
2 identical daughter cells with same (diploid) chromosome content as parent cell. Very important during early development and for tissues that need to be continuously be made, e.g.: epidermis and bone marrow.
Describe the events in the 5 stages of mitosis.
1) Prophase - Centrosomes separate and spindle fibres are created, nuclear membrane is disintergrated.
2) Pro-metaphase = Kinetochore and spindles are connected together, centrioles move to poles of the cell.
3) Metaphase = Chromosomes line up on metaphase plate
4) Anaphase = Chromatids separate, now referred to as (daughter) chromosomes
5) Telophase = Cleavage of daughter cells and nuclear envelope develops again.
What is nondisjunction and what does it lead to?
Non-disjunction is when sister chromatids do not separate to each daughter cell equally. This leads to aneuploidy (incorrect amount of chromosomes in a cell)
- Monosomy is where there is only 1 chromosome, usually leads to loss of cell lines
- Trisomy is where there is 3 chromosomes, e.g.: trisomy 21. Non-mosaic if occurs in first post-zygotic division.
- This is in the first division, if it occurs later in zygotic division, non-disjunction leads to mosaicism (presence of two or more cell lines).
What is meiosis and what does it produce?
Cell division for germ line cells, produces 4 non-identical daughter cells w/ half the chromosome content (haploid) of parents cell. Used for the production of gametes (sperm and eggs).
What occurs during meiosis 1 and meiosis 2?
Meiosis 1 = homologous pairs of chromosomes line up and separate (same way as mitosis)
Meiosis 2 = chromosomes line up and chromatids separate again.
What are the consequences of meiosis?
1) Generation of genetic diversity via
- Crossing over of genetic material in prophase 1
- Random assortment of chromosomes in metaphase 1 (2^23 possibilities - the possibilities of random assortment/the way in which chromosomes line up on the metaphase plate).
2) Constant chromosome number is maintained from generation to generation (but not from mother to daughter cell).
What is produced in spermatogenesis and oogenesis and how long does this take?
- Spermatogenesis is the production of 4 mature sperm cells from 1 spermatocyte which takes approx 60 days
- Oogenesis is the production of 1 egg (ovum) and 3 polar bodies from a primary oocyte. Each menstrual cycle a primary oocyte is released from arrest to produce the egg, process takes 12-50 years.
How often does meiotic nondisjunction occur in humans and what are the consequences of it?
- Same at mitotic non-disjunction (also leads to monosomy/trisomy (aneuploidy))
- every 30 in 100 meiotic events (a-lot compared to other organisms)
- causes 1/3 of all identified miss-carriages, causes infertility and leading cause of mental retardation.
What are the consequences of non-disjunction in meiosis 1 and meiosis 2?
Meiosis 1 = 2 daughter cells will display trisomy, 2 will display monosomy
Meiosis 2 = 2 daughter cells will be okay, 1 will display trisomy, 1 will display monosomy
