4B - Diversity, Classification And Variation Flashcards
What are gametes?
- Sex cells with a haploid number of chromosomes (n)
* Sperm cells in makes and egg cells in females
What is fertilisation?
When a haploid sperm and haploid egg fuse to give a zygote with a diploid number of chromosomes.
What does haploid mean?
Having only one chromosome from each pair.
What does diploid mean?
Having two of each chromosome.
What is the symbol for haploid?
n
What is the symbol for diploid?
2n
What is formed when two gametes join?
Zygote
Are gametes haploid or diploid?
Haploid (n)
Are normal body cells haploid or diploid?
Diploid (2n)
In a normal body cell, where does each of the two chromosomes in a pair come from?
One from the mother and one from the father.
How does fertilisation produce genetic diversity?
- Any sperm can fertilise any egg
* So random fertilisation can produce zygotes with different combinations of chromosomes to both parents.
How is fertilisation random?
Any sperm can fertilise any egg.
How are gametes formed?
By meiosis.
Where does meiosis take place?
In the reproductive organs.
What are chromatids?
- Identical copies of a chromosome made by semi-conservative replication.
- i.e. In a double-armed chromosome, each arm is called a chromatid.
What is a centromere?
The point where the chromatids are joined in a double-armed chromosome.
Describe the process of meiosis.
1) Before meiosis, DNA unravels and replicates so there are two copies of each chromosome, called chromatids (i.e. 2 copies of mother’s chromatid 1 and 2 copies of father’s chromatid 1)
2) DNA condense to form double-armed chromosomes made from two sister chromatids and joined by a centromere.
MEIOSIS I
3) Chromosomes arrange themselves into homologous pairs (two double-armed chromosomes make up a homologous pair)
4) Chromatids twist around and sections of the chromosomes swap over -> Have the same genes, but different alleles.
5) The homologous pairs are separated randomly, so that it is random which chromosome from each pair ends up in which daughter cell.
MEIOSIS II
6) Pairs of sister chromatids in each chromosome are separated -> Centromere is divided.
7) Four haploid cells (gametes) that are genetically different are produced.
Describe the chromosome number before meiosis.
2n
Describe the chromosome number after DNA is replicated before meiosis I.
2 x 2n
Describe the chromosome number after meiosis I.
2 x n
Describe the chromosome number after meiosis II.
n
What are autosomes?
Chromosomes other than sex ones.
What are homologous chromosomes?
- Corresponding chromosomes from a pair, one from the father and one from the mother.
- Of the same size and with the same genes.
- e.g. Chromosome 1 from your mother and chromosome 1 from your father.
Remember to practise writing and drawing out the process of meiosis.
Pg 88 of revision guide.