4B- Meosis And Classification Flashcards
What are gametes?
Sex cells.
They join and form zygotes at fertilisation.
Normal body cells have…
The diploid number (2n) of chromosomes.
Gametes have…
A haploid (n) number of chromosomes. There’s only one copy.
Fertilisation is random. This produces zygotes with…
Different combinations of chromosomes.
This mixing increases genetic diversity within a species.
Gametes are formed by…
Meiosis
Meiosis
Occurs in the reproductive organs.
Cells start as diploid but the cells formed are haploid. The chromosome number halves.
It forms gametes
The process of meiosis
The DNA unravels and replicated so there’s two copies of each chromosomes. These are called chromatids.
The DNA condensed to form double armed chromosomes, each made of sister chromatids that are joined by a centromere.
Meiosis I (first division) the chromosomes are in homologous pairs.
These homologous pairs separate. Halving the chromosome number.
Meiosis II (Second division) the sister chromatids are separated. The centromere is divided.
Four haploid cells that are genetically different are produced.
What is cross over in meiosis I?
Homologous pairs of chromosomes come together.
They twist around each other and bits of chromatids swap over.
They contain the same genes still but have different alleles.
What does crossing over result in?
Each four daughter cells contain chromatids from different alleles.
What is independent segregation?
Each homologous pairs is made up of one paternal and one maternal chromosome.
When they are separated in meiosis I it’s completely random which one from each pair ends up in the cell.
The four daughter cells have a unique combination of maternal and paternal cells.
This leads to genetic variation.
What are mitosis’ outcomes?
Produces cells with the same number of chromosomes as parent cells.
Daughter cells are identical.
Produce two daughter cells.
What is meiosis’ outcome?
Produces cells with half the number of chromosomes as parent cells.
Daughter cells are genetically different.
Produces four daughter cells.
Mutations are caused by what?
Errors in cell division.
What might chromosome mutation cause?
Inherited conditions because the errors are present in gametes.
What is non disjunction chromosome mutation?
Failure of chromosomes to separate properly.
Eg. Down’s syndrome is caused by an extra copy of chromosome 21.
