Meiosis 6.3 Flashcards
How many copies of each chromosome do we have in each cell?
We have two copies of each chromosome, one from both parents and our cells are called diploids.
Sex cells
They are called gametes and must each have only one chromosome or children would have double the chromosomes that we actually have. They are called haploids.
Meiosis
the formation of gametes that have half the chromosomes of a normal cell. Meiosis is known as reduction division
How do our cells store chromosomes
In a cell, we have two chromosomes of each type called homologous chromosomes and within these, we also have two copies of each gene, one from each parent. Each type of gene will code for a specific characteristic.
Alleles
These are different versions of the same gene. An example may be eye colour. The gene will be for the colour of your eyes, but the different alleles may be the gene for blue eyes and the gene for brown eyes.
Meiosis 1
This is when a cell will be divided and each cell will only get one of the homologous chromosomes. This means it will only have one full set of genes so will be a haploid cell instead of a diploid
Meiosis 2
This is when the two haploid cells with one chromosome inside will separate into 4 haploid cells that contain 1 chromatid in each one. This is now a zygote.
Meiosis 1 - Prophase 1
The chromosomes will condense, the nuclear envelope disappears and the spindle fibres will assemble.
The chromosomes will also pair up into their homologous chromosomes which are bivalents. As they are moving through the cytoplasm, the chromosomes will entangle and exchange chunks of DNA, this is called crossing over. Crossing over doesn’t finish until Anaphase.
What do you see in a microscope during meiosis 1 prophase?
Little chromosome shapes paired up in the middle of the cell.
Meiosis 1 - Metaphase 1
The bivalents will now align on the metaphase plate, in the centre which is the same as mitosis. However, any one of the homologous chromosomes can face either direction, it is random which results in genetic variation. This is called independent assortment.
What do you see in a microscope during meiosis 1 metaphase
Bivalents all lined up along the centre which should be slightly longer than in mitosis.
Meiosis 1 - Anaphase 1
The homologous chromosomes are being pulled to opposite ends of the cell, they stay joint to each other but the chromatids that became entangled during prophase will break off at the chiasmata and rejoin which means some DNA is exchanged between the two chromatids in each of the chromosomes. This results in genetic variation which forms recombinant chromatids as they could now have exchanged alleles. The sister chromatids are no longer identical.
What do you see in a microscope during meiosis 1 anaphase?
The little v shapes being moved to either end of the cell.
Meiosis 1 - Telophase 1
Each chromosome has now moved to either end of the cell and unwinded. The nuclear membrane has reformed and the cell undergoes cytokenesis into two haploid cells that contain 1 chromosome in it
What do you see in a microscope during meiosis 1 telophase?
Two big masses at either end of the cell and a line forming through them.