Meiosis and Genetic Variation 4B Flashcards
Define a gamete
Sex cells (sperm and egg cell in humans)
How many chromosomes do gametes have
Haploid number
How many chromosomes do normal body cells have
Diploid number
What occurs at fertilisation
sperm and egg cell fuse making a cell with a diploid number of chromosomes
How are gametes formed
By meiosis
What happens before meiosis occurs
The DNA unravels and replicates so that their are two copies of each chromosome, called chromatids, the DNA condenses to form double-armed chromatids each made from two sister chromatids which are joined in the middle by a centromere
What happens during meiosis I (first division)
The chromosomes arrange themselves into homologous pairs, which are then separated, halving the chromosome number
What happens during meiosis II (second division)
the pairs of sister chromatids that make up each chromosome are separated - the centromere divides
What type of cells are formed at the end of meiosis
4 genetically different haploid cells (gametes)
Give two ways in which meiosis leads to genetic variation
Crossing over of chromatids and independent assortment/segregation of chromosomes
Describe how crossing over of chromatids leads to genetic variation
This means that the four daughter cells formed contain chromosomes with different alleles
Define an allele
all the different forms of the same gene
Describe how independent assortment/segregation leads to genetic variation
Each homologous pair of chromosomes is made up of one paternal chromosome and one maternal chromosome
When homologous pairs are separated in meiosis I it’s completely random which chromosome from each pair ends up in which daughter cell
So the four daughter cells produced by meiosis have completely different combinations of maternal and paternal chromosomes
The shuffling of these chromosomes is what leads to genetic variation in any potential offspring
Define maternal chromosomes
Chromosomes inherited from the mother
Define paternal chromosomes
Chromosomes inherited from the father
Explain the differences between meiosis and mitosis
Mitosis:
Produces cells with the same number of chromosomes as the parent cell
Daughter cells are genetically identical to the parent cell
Produces 2 daughter cells
Meiosis:
Produces cells with half the number of chromosomes as the parent cell
Daughter cells are genetically different to the parent cell
Produces 4 daughter cells
Define chromosome mutation
when cells do not have the correct number of chromosomes due to errors during meiosis
Explain one type of chromosome mutation
non-disjunction - a failure of the chromosomes to separate properly in humans non disjunction of chromosome 21 in meiosis can lead to Down’s Syndrome
One zygote will receive three copies of chromosome 21
