Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Meiosis
Homologous= a pair of chromosomes, one maternal and one paternal, with the same gene loci and
therefore determine the same features
...

Meiosis occurs in the reproductive organs of plants and animals, forming haploid gametes
...
Cells produced are genetically different
...

The zygote (fertilised egg) divides by mitosis forming an embryo
...
The first division may be called a reduction division
as the chromosome number is halved, crossing over may occur in this
...
It results in genetic variation in offspring, due to:



Independent assortment of homologous chromosomes
Crossing over between the chromatids of homologous chromosomes

During meiosis I, homologous chromosomes pair up and chromatids wrap around each other,
equivalent portions of these chromatids may be exchanged by crossing over
...
This forms two haploid daughter nuclei
...
This forms four haploid nuclei
...

Meiosis I
Prophase 1








Chromosomes become shorter and thicker and split into two chromatids
In cells with centrioles (animals and lower plants), centrioles move to the poles of the cells
and microtubules begin to radiate from them, forming asters, forming the spindle
...

Each bivalent consists of four strands, made up of two chromosomes each split into two
chromatids
...

Here, chromatids may break and recombine with a different but equivalent chromatid;
crossing over
At the end of prophase, the nuclear membrane disintegrates and the nucleolus disappears
...

Thus each pole receives only one of each homologous pair of chromosomes and because the
arrangement is random at metaphase, these will be a random mixture of maternal and
paternal chromosomes
...
The diploid number of chromosomes is reduced to the haploid
...
Meiosis more than halves the number of chromosomes in a cell and introduced
genetic variation into the gametes and thus the zygotes that are produced
...

Sources of variation are essential for species to survive in a constantly changing environment and to
colonise new environments
...
Each of the chromosomes in a homologous pair carries different genetic material
...

2
...
When they then separate, they do so independently of each other
so daughter cells contain different combinations of maternal and paternal chromosomes
...
As the gametes come from different parents this creates greater variety
...
Crossing over during chiasmata formation during prophase 1 of meiosis
...
This is recombination
...




---