Meiosis

Question 1

How many cells form in gametogenesis? What is noticeable about the chromosomes that end up in gametes? What does this ensure?

Answer 1

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.

Question 2

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?

Answer 2

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.

Question 3

What does haploid and diploid mean?

Answer 3

Haploid means cells have only one set of chromosomes

Diploid means cells have two sets of homologous chromosomes

Question 4

Once an embryo which is diploid is made what occurs to make the cell expand?

Answer 4

This is when mitosis occurs.

Question 5

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.

Answer 5

Meiosis occurs in the testes and the ovaries

Question 6

Where are the two stages of meiosis?

Answer 6

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.

Question 7

What does the end of meiosis cause?

Answer 7

Mature egg and sperm cells to be made.

Question 8

In reference to both germ line cells and somatic cells

What do you get after meiosis 1? And meiosis 2?

Answer 8

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

Question 9

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)

Answer 9

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.

Question 10

What happens in meiosis 2?

Answer 10

This is when sister chromatids separate (the individual chromosome splits into two)

Question 11

When does homologous recombination occur and what happens?

Answer 11

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.

Question 12

What are synaptonemal complex? What does this do?

Answer 12

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

Question 13

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?

Answer 13

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 .

Question 14

What is homologous segregation and when does it happen?

Answer 14

This is when homologous chromosomes move to separate poles of a cell and split

Homologous suggests this is in meiosis one

Question 15

What happens in meiosis 1 to make homologous chromosomes separate?

Answer 15

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

Question 16

What is the key difference in meiosis and mitosis?

Answer 16

Meiosis sees 4 daughter haploid cells made

Mitosis sees 2 DIPLOID daughter cells formed

Question 17

Key note about crossing over in homologous chromosomes?

Answer 17

It happens no more than at 4 sites at a time

There are mechanisms which stop crossing over happening too much

Question 18

What can go wrong with meiosis?

Answer 18

The wrong numbers of chromosomes end up in cells

Question 19

What is aneuploidy? What is monosomy? What is trisomy? What is polyploidy?

Answer 19

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

Question 20

How do you get aneuploidy gametes?

Answer 20

This is caused by non disjunction ( incorrect separation) of homologous chromosomes or sister chromatids either in meiosis one or two

Question 21

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

Answer 21

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

Question 22

What can trisomy (having one extra chromosome) and polyploidy (having a whole other set of chromosomes) cause?

Answer 22

Trisomy 21 can cause downsyndrome

Polyploidy can create triploid chromosomes such as XXY

Question 23

Additional sex chromosome conditions?

Answer 23

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.

Question 24

Describe the nondisjunction disorder trisomy 22:

Answer 24

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.

Question 25

Describe the nondisjunction disorder trisomy 18:

Edwards syndrome

Answer 25

Symptoms include

Servere intellectual disability

Low birth weight

Small shaped head

Clenched fists

Congenital heart defects

Various abnormalties of other organs

Usually early death.

Question 26

Turner syndrome common symptoms?

This is when there is a missing chromosome

Affecting chromosome 45 x

Answer 26

You see poor growth

Short stature

Absent pubertal development

Congenital heat defects

Skeletal abnormalities

Only 1 % survive

Question 27

So remember turner syndrome sees complete or partial loss of a second sex chromosome in phenotypic females

What is two reasons as to why this is bad?

Answer 27

Because it affects haploinsufficiency

• on sex chromosomes theres pseudoautosomal genes
• if there is a change in the amount of these psdeudoautosomal genes (due to a chromosome being lost)
• it means there isnt enough of that gene for coding a embryo and the embryos dies.

Because it affects imprinted genes on the X chromosome

• so if you lose a X sex chromosome due to turner syndrome, you may lose a sex chromosome which has an important imprinted gene on it
• imprinted genes on an individual X chromosome is important in making both X chromosomes active
• Remember these imprinted genes are found on one of the X chromosomes, not both
• in most cases of turner syndrome you lose the X chromosome with the imprinted gene (which would make both X sex chromosomes active)
• in rare cases you don’t have a imprinted gene lost hence why there is 1% survivability

Question 28

What is X inactivation?

Answer 28

During meiosis certain genes are inactivated on both X chromosomes

This is called X inactivation

However not all genes are turned off in X inactivation

One example of genes which arent turned off in pseudoautosomal genes

Question 29

Why X inactivation isnt a reason for turner syndrome?

Answer 29

Pseduoautosomal genes which are directly related to turner syndrome, aren’t turned off by turner syndrome

So turner syndrome must be caused by chromosome non disjunction during meiosis instead.

And remember this is the incorrect separation of homologous chromosomes and chromatids causing the loss of X chromosomes from an embryo

This is the true reason you don’t have the full number of pseudoautosomal genes in turner syndrome.

Question 30

Another reason of why X inactivation isnt the cause of turner syndrome?

Answer 30

X inactivation can’t turn off imprinted genes

Thus it must be faulty meiosis which causes incorrect chromosome numbers in cells and thus turner syndrome

Question 31

What is Escape from X inactivation and is it true?

Answer 31

Yes its true

This means the x inactivation isnt the true cause of turner syndrome

We have escaped x inactivation

Thus incorrect meiosis is the reason for X inactivation

Question 32

How many genes on the X chromosome can escape X inactivation ?

Answer 32

3 out of 21

These are ARSE, STS, TBLx