4. Meiosis Flashcards
Meiosis definition
Division of a diploid nucleus to form 4 non-identical haploid nuclei
What is a haploid cell?
A cell containing one member/a single copy of each pair of chromosomes
Why are haploid cells important?
When two haploid cells fuse during fertilisation they form a diploid cell, which will have pairs of chromosomes (but in each pair there will be one maternal and one paternal chromosome).
Variation in offspring, mix of characteristics from each parent.
No doubling of chromosome number in each generation.
What happens before meiosis?
The chromosomes are copied and chromatids form.
Prophase 1
- Chromosomes condense & become visible
- Chromosomes come to together in homologous pairs (bivalents) via synapsis
- Nuclear envelope breaks down
- Centrioles move to opposite poles of the nucleus and spindle fibres form
Metaphase 1
- Chromosomes line up across the equator in their homologous pairs/bivalents
- The orientation of the pair is random (independent segregation)
- Chromosomes attach to the spindle by their centromeres
Anaphase 1
- Spindle fibres pull homologous pairs apart
- Sister chromatids are still attached to each other: centromeres don’t divide yet!
Telophase 1
Two HAPLOID cells form (because there is one member from each homologous pair) and nuclear envelope may reform
Prophase 2
- Chromosomes condense
- Nuclear envelope breaks down
- Centrioles replicate & new spindle fibres start to form
Metaphase 2
Chromosomes line up across the equator and their centromeres attach to the spindle fibres
Anaphase 2
Spindle fibres pull the centromeres (i.e. the SISTER CHROMATIDS) apart
Telophase 2
Nuclear envelope reforms
How does meiosis produce non-identical gametes?
Independent segregation
Crossing over
What is independent segregation?
In metaphase 1, when the chromosomes line up across the cell’s equator, each pair’s orientation is random and independent of any other pair.
How to calculate the number of possible different orientations in independent segregation?
2^n
n = number of pairs of chromosomes (e.g. in humans, n = 23)
What is crossing over?
This takes place during prophase 1, when the chromosomes come together in their homologous pairs and exchange genetic material. This leads to the formation of recombinants, and later on new combinations of alleles in the haploid cells formed.
What is the chiasma?
The point at which the alleles cross over during crossing over.
Chromosomes break and reform here.
Plural of chiasma
Chiasmata
Chromosomes vs chromatids
We count chromosomes by HOW MANY CENTROMERES THERE ARE!
Chromatids are STRANDS of replicated chromosomes
DNA replication in humans: how many chromosomes & chromatids?
Still 46 chromosomes because there are 46 centromeres present!
92 chromatids
Comparing meiosis I and II
Meiosis I separates homologous chromosomes
Meiosis II separates sister chromatids
What is random fertilisation?
Any gamete from one parent could potentially fuse with any gamete from the other parent
What is an advantage of random fertilisation?
Increases genetic variation
Gene mutation
Change in the sequence of bases of a gene (nucleotides)
When do gene mutations occur?
DNA replication/S phase
Substitution mutation
When one base is replaced by another
Deletion mutation
When one base is removed and the bases after that are shifted one position backwards
Somatic vs germ line mutation
Somatic mutations affect body cells, germ line mutations are exclusive to gametes
Chromosome mutation
A change in the entire chromosome rather than individual bases/nucleotides
When do chromosome mutations happen?
During meiosis!
Chromosome mutations are caused by…
Non disjunction
What happens during non disjunction?
Homologous pairs fail to separate during anaphase I or II
