Meiosis Flashcards
Where does meiosis take place and why
Mitosis - happens in almost all cell types
Cell division by meiosis only takes place in sex organs
That is because cell division by meiosis is only ever used to produce gametes (sperm and eggs in mammals and pollen and eggs in plants)
What generally happens in meiosis
In cell division by meiosis we start with a diploid cell and at the end we have four haploid gametes
These cells contain individual chromosomes not pairs
Explain why it is important that gametes are haploid cells
It’s important that gametes are haploid cells
This is because during fertilisation gametes fuse together to produce a fertilised egg or zygote
Because gametes are haploid, the zygote has the diploid number of chromosomes
State a feature of the gametes made by meiosis
Haploid
All the gametes made by meiosis are genetically different
Describe homologous chromosomes
Diploid cells contain chromosomes in pairs
Chromosome pairs referred to as homologous chromosomes
One chromosome in a homologous pair comes from your mother and the other chromosome in the homologous pair comes from your father
What do both chromosomes in a homologous pair contain
Both chromosomes in a homologous pair have exactly the same genes
E.g. both copies of chromosome 9 have the gene which determines blood group
However you can inherit different alleles from each parent
E.g. you could inherit the A allele from your mother and the B allele from your father
Have the same genes but the alleles can be different
Describe how meiosis can lead to genetic variation
Before the nucleus divides by meiosis all the chromosomes are copied in interphase
The sister chromatids remain joined at the centromere
At an early stage in meiosis, the two chromosomes in a homologous pair come together
The chromatids of the two chromosomes wrap around each other
This process is called crossing-over
The attached pair of chromosomes are called a bivalent
The points where the chromatids are joined are called chiasmata
Parts of the chromatids can break off and exchange between the homologous chromosomes
The maternal and paternal chromosomes have now exchanged DNA
These are called recombinant chromosomes
Because the chromosomes have exchanged DNA, that means they can exchange alleles
E.g. after crossing over, both the maternal and paternal chromosomes now contain one chromatid with the A allele and one chromatid with the B allele
Since chiasmata can form at multiple points, so a large number of alleles can be exchanged
This exchange of alleles is a major source of genetic variation in meiosis
Each gamete made by meiosis is genetically different to the others
Meiosis increases genetic variation in another way
When homologous chromosome pairs line up on the spindle, we cannot predict whether the paternal or maternal chromosome will end up in which gamete
This is called independent assortment
Most organisms produce a vast number of genetically different gametes
During fertilization, male and female gametes fuse randomly with each other
We can predict which male gamete will fuse with which female gamete
This random fusion of gametes, introduces a whole extra level of genetic variation in the offspring
Factors leading to genetic variation in meiosis
Ways to produce genetic variation in the gametes
Crossing over
Independent assortment of Chromosomes
Describe the stages of meiosis 2
Now the cells enter meiosis 2
In prophase 2, the chromosomes condense and become visible again
Again, the nuclear membrane breaks down and spindle fibres begin to develop
In metaphase 2, the chromosomes are lined up, on the equator of the spindle apparatus
In anaphase 2, the centromere of each chromosome divides
The spindle fibres shorten
The chromatids are now pulled towards the opposite poles of the cell
In telophase 2, the chromatids have reached the poles of the cell
We now call the chromosomes
The nuclear membranes reform and the chromosomes uncoil back to their chromatin state
Finally each cell undergoes cytokinesis, to produce 2 haploid cells
Since the chromosome number halves
It is said that meiosis is reduction division
Describe the stages of meiosis 1
Meiosis involves two rounds of nuclear
division
In meiosis 1, homologous chromosomes are separated from each other
In meiosis 2, sister chromatids are separated from each other
Before meiosis starts , the cell will have been through interphase
During interphase, the cell copies chromosomes and the organelles
The chromosomes are not visible as distinct structures during interphase
First the cell enters meiosis 1
The first stage of this is prophase 1
During prophase 1, the chromosomes condense and become visible
Homologous chromosomes link together forming chiasmata
When the homologous chromosomes are paired like this, this is a bivalent
At this point, crossing over can take place, exchanging alleles between the homologous chromosomes
During prophase 1, the nuclear membrane also breaks down
The centrioles move to opposite poles of the cell
Spindle fibres also start to assemble into the spindle apparatus
In metaphase 1, the pairs of homologous chromosomes are now lined up on the equator of the spindle apparatus
During anaphase 1, the spindle fibres shorten and the homologous chromosomes move towards the opposite poles
For this to happen, the chiasmata between the homologous chromosomes break
The last stage in meiosis 1 is telophase 1
In telophase 1, the chromosomes have now reached the poles of the cell
At this point the nuclear membranes reform, and the chromosomes uncoil back to their chromatin state
At this point the cell undergoes cytokinesis, dividing into two cells
These cells are haploid since they no longer contain pairs of homologous chromosomes
how to calculate the number of genetically different gametes produced by independent assortment
2^n
where n is the number of homologous chromosome pairs
