A-level Biology Notes

Meiosis

What does meiosis produce?

- 4 genetically different cells, haploid [half the amount of chromosomes/DNA]

Benefits of meiosis?

- produces gametes which will be used in sexual reproduction in animals and plants [two gametes fuse to form a zygote, zygote develops into organisms]

Stages of meiosis?

- interphase/ meiosis 1/ meiosis 2/ cytokinesis

Interphase? 

G1: protein synthesis

S: DNA replication [doubles set of DNA]

G2: organelles synthesis

Meiosis 1?

-prophase 1: DNA coils to form chromosomes, nucleus breakdown, spindle fibers form, crossing over occurs.

-Metaphase: homologous pair of chromosomes line up in middle of cell and attach to spindle fibers via centromere.

-Anaphase 1: spindle fibers pull, homologous pair of chromosomes separate to opposite sides by independent assortment.

-Telophase 1: chromosomes uncoil, nucleus reforms (left with 2 nuclei)

Meiosis 2?

-Prophase 2: DNA coils to form chromosomes, nucleus breaks down, spindle fibers form.

-Metaphase 2: chromosomes line up in the middle of cell and attach to spindle fibre via centromere

-Anaphase 2: spindle fibers pull, centromere splits, sister chromatids move to opposite sides by independent assortment

-Telophase 2: Chromatids uncoil, nucleus reforms [left with 4 genetically different nuclei]

Cytokinesis?

-separating cells into 4 [each receives a nucleus and organelles/cytoplasm]

How does Meiosis produce variation?

-Crossing over and independent assortment

What is crossing over?

-Occurs in prophase 1 of meiosis 1- homologous pairs of chromosomes wrap around each other and swap equivalent section of chromatids- produces new combination of alleles

What is independent assortment?

In anaphase 1 of meiosis 1- the homologous pairs of chromosomes separate

In anaphase 2 of meiosis 2- the chromatids separate

Independent assortment produces a mix of alleles from paternal and maternal chromosomes in gamete

What happens to DNA mass in meiosis?

Quarters

What happens to chromosome number in meiosis?

Halves (haploid)

What is mutation?

Change in DNA

2 types: chromosomes mutation and gene mutation

What causes mutation?

Random or due to mutagens (e.g., chemical, radiation)

What are chromosome mutations?

In plants, inherits more than one diploid set of chromosomes- called polyploidy

In animals, homologous pair of chromosomes do not separate in meiosis, so either inherit one extra or one less chromosome- called non-disjunction

What is a gene mutation?

A change in base sequence of DNA

2 types= substitution and insertion/deletion

Substitution= replace one base for another, changes one triplet code

Insertion= adding a base, deletion= removing a base, both insertion/deletion causes frameshift, all the triplet codes after the mutation changes, so normal polypeptide chain/protein not produced.