Meiosis Flashcards
How many cells form in gametogenesis? What is noticeable about the chromosomes that end up in gametes? What does this ensure?
4 cells form from one
They’re a NEW combination of mother and father chormatin
This ensures the chromosomes passed onto the 4 gametes are unique and genetically diverse.
In gametogenesis what DIPLOID cells form the haploid gametes (eggs and sperm)?
What is the process to form these haploid cells? What happens when the gametes fertilise with each other?
in gametogenesis:
Diploid germ line cells (which have two sets of homologous chromosomes) make haploid eggs
Diploid somatic cells make haploid sperm gametes.
The process of these diploid cells becoming the haploid cells is meiosis
The haploid gametes then fertilise into a diploid zygote.
What does haploid and diploid mean?
Haploid means cells have only one set of chromosomes
Diploid means cells have two sets of homologous chromosomes
Once an embryo which is diploid is made what occurs to make the cell expand?
This is when mitosis occurs.
Where does meiosis occur?
Remember this process separates homologous chromosomes in stage one of meiosis 1. In stage 2 of meiosis these individual chromosomses into chromatids. In this way diploid germ line cells and somatic cells become haploid egg and sperm cells.
Meiosis occurs in the testes and the ovaries
Where are the two stages of meiosis?
Remember this process separates homologous chromosomes in stage one of meiosis 1.
I.e. the diploid germ line cells have their homologous chromosomes pulled apart
In stage 2 of meiosis these individual chromosomses into chromatids. In this way diploid germ line cells and somatic cells become haploid egg and sperm cells.
I.e. the cells which have individual chromosomes in have their chromosomes pulled apart so the gametes only have chromatin in.
What does the end of meiosis cause?
Mature egg and sperm cells to be made.
In reference to both germ line cells and somatic cells
What do you get after meiosis 1? And meiosis 2?
For germ line cells in oocytes
After meiosis 1 test diploid germ line cells become polar bodies
After meiosis 2 the polar bodies become eggs
For somatic cells?
- after meiosis 1 you get spermatocytes
- after meiosis 2 you get spermatids
After meiosis one for both cells:
- you go from 2n to n
What happens in meiosis one? What happens when the homologous chromosomes are brought together?
(Remember the stages of meiosis is basically mitosis twice, the first time separating homologous chromosomes and the second separating individual chromosomes into chromatin)
Homologous chromosomes are brought together
When the Homologous chromosomes come together homologous recombination then sees each chromosome swapping genes with each other
This allows the homologous chromosomes to become unique
These homologous chromosomes then move to the oppsoite poles of the cell and separate.
What happens in meiosis 2?
This is when sister chromatids separate (the individual chromosome splits into two)
When does homologous recombination occur and what happens?
This occurs in meiosis one
When the homologous chromosomes line up in the centre of the cell
The chromosomes are brought in close proximity to each other
They then form bridges called a chiasma.
This causes new strands of chromosome to form.
What are synaptonemal complex? What does this do?
These complexes form between homologous chromosomes. They occurs thus in meiosis one.
These complexes allow homologous chromosomes to be held together.
They happen along the whole of the chromosome
They allow for homologous recombination to occur
The points at which homologous recombination occurs is called the chiasma
What are the structures of homologous chromosomes that are next to each other? What connects the two homolgous chromosomes together together? How big is the gap?
Each homologous chromosomes has a a loop structure.
Each homologous loop structure is attached to a cohesin ring.
These cohesin rings bind to axial cores.
Transverse filaments link the axial cores of each homologous chromosome to the other adjacent homologous chromosome
The gap is about 100nm between each homologous chromosome .
What is homologous segregation and when does it happen?
This is when homologous chromosomes move to separate poles of a cell and split
Homologous suggests this is in meiosis one
What happens in meiosis 1 to make homologous chromosomes separate?
The cohesin bridges which hold the chromosomes to the axial core dissolve
This causes the homologous chromosomes to separate
Kinetochore microtubules then cause the homologous chromosomes to be pulled to opposite side of the cell
What is the key difference in meiosis and mitosis?
Meiosis sees 4 daughter haploid cells made
Mitosis sees 2 DIPLOID daughter cells formed
Key note about crossing over in homologous chromosomes?
It happens no more than at 4 sites at a time
There are mechanisms which stop crossing over happening too much
What can go wrong with meiosis?
The wrong numbers of chromosomes end up in cells
What is aneuploidy? What is monosomy? What is trisomy? What is polyploidy?
Aneuploidy is where you have the wrong number of chromosomes in the cell
Monosomy?
- you get one copy of chromosomes in the cell
Trisomy?
- you have three copies of chromosomes
Polyploidy?
- this is when you get a whole extra set of chromosomes in a cell
How do you get aneuploidy gametes?
This is caused by non disjunction ( incorrect separation) of homologous chromosomes or sister chromatids either in meiosis one or two
What causes Non disjunction in meiosis one? Why is non disjunction in meiosis one considered more serve than meiosis in meiosis two?
Note in meiosis one remember homologous chromosomes have lined up in the centre of the cell and are supposed to separate to opposite poles
You can get one set of homologous chromosomes going to the same pole of the cell instead of separating into two individual chromosomes and going to opposite poles.
It is considered more severe to have non disjunction happening in meiosis one as this affects all four daughter cells
Whereas in meiosis two non disjunction only affects 2 daughter cells
What can trisomy (having one extra chromosome) and polyploidy (having a whole other set of chromosomes) cause?
Trisomy 21 can cause downsyndrome
Polyploidy can create triploid chromosomes such as XXY
Additional sex chromosome conditions?
This is when you can lack a chromosome for example
This results in turners syndrome as you miss a 45 x chromosome
Most of the time the embryo is aborted. Or if born individuals are infertile.
Describe the nondisjunction disorder trisomy 22:
You get a undeveloped midface
You get malformed ears
Wide spaced eyes (hypertelorism)
Microcephaly
Congenital heart disease
This is usually found in miscarried foetuses. Rare in live born.
