Cell Division -meiosis Flashcards
Somatic cells
body cells that have specific no. of chromosomes according
How many pairs of chromosomes does a human have?
23
How many types of chromosomes are there?
There are 22 different pairs of chromosomes (autosomes) and a pair of sex chromosomes (gonosomes). Autosomes are numbered as chromosome number 1, 2, 3, up to 22 and there are 2 of each. The 2 chromosomes making up each pair are identical to each other.
Chromosomes in somatic cells and sex cells for females
In females, the 2 sex chromosomes are also identical and are referred to as XX chromosomes.
Chromosomes in somatic cells and sex cells for males
In males, one of these is an X chromosome and the other is a Y chromosome.
How many sets of chromosomes are in each somatic cell?
There are 2 sets of 23 different kinds of chromosomes in each somatic cell ∴ the cells have a diploid (2n) number of chromosomes.
e.g. Humans’ diploid (2n) number is 46.
Describe a pair of chromosomes?
The 2 chromosomes that make up each pair of autosomes are identical, one is of maternal origin and the other is of paternal origin.
homologous pair of chromosomes
a pair of identical chromosomes
Definition of Meiosis
the division of one cell into 4 cells each of which has half the chromosome number as the parent cell and each of the 4 cells are genetically different from each other. / division of 1 diploid cell into 4 haploid cells that are genetically different from each other and from the parent cell.
When does Meiosis occur?
occurs during gametogenesis.
Where does Meiosis occur in plants?
In plants: meiosis occurs in the anther to produce pollen grains and in the ovary to produce an ovule.
Where does Meiosis occur in humans?
In humans: meiosis occurs in the testes to produce sperm and in the ovaries to produce an ovum.
Meiosis during gamete formation (i.e. before fertilisation)
Gametes have half the chromosome number (1 set of chromosomes) compared to the adult
Fusion of gametes 🡪 the chromosome number being restored in the zygote.
Meiosis after fertilisation
In some algae and fungi, the adults are haploid giving rise to haploid gametes which on fertilisation results in a diploid zygote.
Meiosis occurs in the zygote or zygospore to halve the chromosome number so the new offspring also have haploid chromosomes.
The place of meiosis in the moss and fern:
Mosses and ferns show alternation of generations whereby a haploid gametophyte (gamete producing generation) alternates with a diploid sporophyte (spore producing generation)
The haploid gametophyte produces haploid gametes by mitosis
The diploid sporophyte produces haploid spores by meiosis
It is the halving effect of meiosis that allows for alternation of generations.
