Meiosis Flashcards
What is the biochemical cause of Down’s syndrome
Aneuploidy
Define homologous chromosomes, sister chromatids, bivalents, centromeres and kinetochore
Homologous chromosomes are chromosomes that are similar in structure but differ in parental orgins. Sister chromatids are identical chromosomes that have been made by DNA replication Bivalents are structures formed by the association of homologous chromosomes Centromere is a structure at the center of the chromosome which has kinetochore attached to it whereas the kinetochore is the protein structure that attaches the centromere to the spindles
What is N and C
N is number of chromosomes, C is number of chromatids
What happens to N and C after Meiosis 1
Initially we have 2N, we go down to 1N and for C we have 4C at the beginning, after meiosis 1 we go down to 2C. The 4C form bivalents
What happens in meiosis II
Sister chromatids separate, very similar to mitosis
3 processes that happens during Prophase 1
- Homologous chromosomes pairing 2. Synaptonemal complex formation 3. Crossing over, also called recombination
What are the different stages of prophase 1
LZPD 1. Leptotene 2. Zygotene 3. Pachytene 4. Diplotene
The DNA is extremely long. How does the chromosome identify their homologous pairs and adjacent positions
The telomeres cluster around the nuclear envelope forming a chromosome bouquet. It is associated with rapid chromosome movement. Know that there are chromosomal attachment proteins that bind to the dyenins and help with this movement of chromosomes so that they can align themselves
What happens after the chromsomes have aligned themselves
There is the formation of synaptonemal complex. It is a tripartite structure that functions as a glue to hold the chromsomes together. It consists of several different proteins
What are the functions of synaptonemal complex
- It organizes the chromtids 2. Mediates synapses via protein-protein interactins 3. It aids in the assembly of recombination complexes
How does the synaptonemal complex look like
What happens during homologous recombination
- There is programmed double stranded breaks along the length of the DNA at locations called ‘hot spots’
- Exonucleic resection of the strand at 5’ end - this basically mean that the small length of one chromatid breaks off
- Strand invasion
- repair of the double stranded breaks by Homologous recombination
Cross over leads to the formation of chiasmata
What happens if there is not even 1 cross over event in the chromsomes during Meiosis
If there is not even a single cross over event then it will lead to aneuploidy
How many cross over events usually occur in 1 meiotic division
Around 23
How does meiosis introduce genetic diversity
There are 2 ways:
- Independent assortment of the chromsomes - this basically means that the matenal and paternal homologs are aligned such that they are mixed up
- Crossing over
What happens at the first stage of the prophase 1
At the liptotene stage we have:
- Chromosome bouquet formation
- The synaptonemal complex proteins start to polymerize to align the chromsomes
- There are programmed double stranded breaks at the hot spots