Meiosis Flashcards
What are the stages and substages of meiosis?
1) Interphase
2) (Meiosis 1)
3) Prophase 1
- Leptotene
- Zygotene
- Pachytene
- Diplotene
- Diakenesis
4) Metaphase 1
5) Anaphase 1
6) Telophase 1
7) Interphase 1
8) (Meiosis 2)
9) Prophase 2
10) Metaphase 2
11) Anaphase 2
12) Telophase 2
What happens in interphase?
The cell is carrying out
its normal activities. It is
not clear if the cell is in
G1, S-phase or G2.
Cells in S-phase are
replicating their DNA
Interphase is the […] stage
Interphase is the longest stage
Interphase has 3 stages:
G1, S, G2
What is G1 phase?
- housekeeping
- before S phase each chromosome is present as one DNA molecule (chromatid).
What is S phase?
- synthesis of DNA
- after S phase each chromosome is present as two DNA molecules
What is G2 phase?
- preparation for cell division
Substages of prophase 1?
Leptotene
Zygotene
Pachytene
Diplotene
Diakenesis
What happens in prophase 1?
- Zygotene
- Pairing takes place between homologous chromosomes.
What does Homologous Chromosomes mean?
- This term means ‘the same’
- Here it refers to chromosomes that generally have the same genes and similar DNA sequences but these DNA sequences are not identical.
- In the case of human chromosomes each ‘pair’ of chromosomes is homologous e.g. the two chromosome 21’s
In prophase 1, a protein structure synaptonemal complex) forms between …
…the homologous chromosomes holding them together
In prophase 1, the pairing begins at …
…a few sites along the chromosomes and brings corresponding parts together
Describe Prophase 1 Pachytene
The homologous chromosomes are fully paired and contracted
Whats the name of the protein structure which forms in prophase 1 between the homologous chromosomes holding them together?
synaptonemal complex
What occurs in pachytene?
- Crossing over / recombination occurs.
- For each crossing over event the DNA breaks in each of two non- sister chromatids followed by crosswise rejoining of the broken ends.
Define Chiasmata
- Sites of crossing over, occur in synapsis.
- Exchange of genetic material between non-sister chromatids.
Crossing over produces…
… recombinant chromosomes.
What happens in Diplotene?
- The synaptonemal complex breaks down
- The homologous chromosomes are held together by the crossovers.
- (Each chromosome at this stage comprises two sister chromatids)
What happens in Chiasma?
- The homologous chromosomes now ‘repel’ each other and reorganise themselves in three dimensional space.
- The result is often a cross shape called a chiasma
There is at least one and usually more crossovers for …
…‘pair’ of homologous chromosomes
Crossovers have two roles:
- to hold the homologous chromosomes together after the synaptonemal complex has broken down; this ensures that the homologous chromosomes separate into the new cells correctly (segregate)
- to generate new arrangements of DNA (genetic variation)
Homologous chromosomes usually carry the same gene but …
…different versions (alleles) e.g. A and a
After crossovers, the different alleles in homologous chromosomes are …
…physically recombined into new orders
- Resulting in genetic variation
What happens in Diakinesis?
- The chromosome ‘pairs’ continue to condense
- If the chromosomes are held as a pair they are called bivalents
- If the chromosomes are held in threes they are called a trivalent.
What happens in Metaphase 1?
1) The chromosome ‘pair’ is attached to the spindle fibres by their centromeres
2) The chromosome ‘pairs’ move towards the equator