Eduqas A Level Biology; Component 2: "Cell Division"

What is chromatin?

a complex of DNA and histone protein; what DNA exists as when the cell isnt dividing

Describe the Prophase stage.

Chromatin condenses to chromosomes

Centrioles move to opposite poles and develop spindle fibres

The nuclear envelope snd nucleolus disintegrate

Outline the cell cycle.

Mitotic phase > first growth > synthesis phase > second growth

What occurs in Interphase?

Organelle production, protein synthesis and DNA replication

What do chromosomes consist of?

DNA, protein and a little RNA

What do chromosomes become visible as after DNA replication?

two identical sister chromatids joined at the centromere

How many pairs of homologous chromosomes do humans have?

23 pairs

How is mitosis vital in the growth of an organism?

it repairs damaged tissues and replaces dead cell

Explain the effect of mitosis on genetic stability

Two daughter cells are clones and identical to the parent cell so the organism itself is stable

How does mitosis have a role in asexual reproduction?

it produces genetically identical offspring

What are oncogenes?

Cells that have been altered, previously existing as protoncogenes, and cause cause cancer5

What is the function of the tumour suppressor gene and what is its significance?

Controls the cell cycle to stop it repeating continually

Correct embryonic development and replacement of tissue

What happens if the genes that control the cell cycle are damaged?

The cycle is disrupted causing rapid division, and a tumour forms

Name the stages of mitosis in order

prophase, metaphase, anaphase, telophase

What are the spindle fibres made from?

Protein microtubules

Describe the mitotic metaphase stage

Chromosomes move to the equator of the cell

Spindles attach to the centromeres and contract slightly

Describe the mitotic anaphase stage

Spindles contract and shorten, splitting the centromere and sister chromatids are pulled apart

The chromatids are pulled apart centromere first and have become sister chromosomes

What are the longest and shortest stages of mitosis

Longest; prophase

Shortest; anaphase

Describe the mitotic telophase stage.

Chromosomes uncoil and lengthen

Spindle fibres break down

Nucleoli and nuclear membranes reform

Cleavage furrow starts

Describe cytokinesis in animals and plants

Constriction in the centre of the cell separating the two cells in animals

The cell plate forming from the centre out for a new cell wall in plants

List some differences of mitosis between animals and plants

Cells become more rounded in animals before mitosis but no change in plants

Centrioles arent present in plants

Spindles dont disintegrate in plants cells until after cytokinesis

Cleavage furrow in animals, cell plate in plants

Give some benefits of asexual reproduction

Populations can rapidly increase

Offspring survive in stable conditions

Less time and energy wasted as no reproductive partner needed

Give a disadvantage of asexual reproduction

Lack of genetic variation means offspring cant adapt to environment change

What is the purpose of meiosis?

Produces gametes in sexually reproducing organisms

Describe prophase 1.

Centrioles move to opposite poles

Nuclear envelope and nucleolus disappears

Chromosomes condense further into 2 chromatids

Homologous chromosomes pair up in a bivalent

What is the name of the area where bivalents are joined in meiosis 1?

Chiasma

What is crossing over and when does it occur?

Prophase 1

segments of the DNA at the chiasma can be exchanged from one chromatid to the other

Describe metaphase 1.

Microtubules attach to centromere

Bivalents align randomly along the spindle and across the equator

Independent Assortment occurs

What is independent assortment?

Independent assortment is a source of variation since the paternal and maternal chromosomes are aligned on either side of the equator, irrespective of each other.

With x pairs of chromosomes, what is the number of possible combinations of independent assortment?

2^(x)

Describe Anaphase 1

Spindle fibres contract and shorten

Chromosomes in bivalent are pulles to opposite poles

Describe Telophase 1.

Chromosomes stay in condensed form and cytokinesis occurs, forming two haploid cells

What does meiosis result in?

Four haploid genetically different daughter cells

Describe Prophase 2.

Centrioles migrate to the poles at right angles from Prophase 1.

Describe Metaphase 2.

Chromosomes align on the equator and spindle fibres attach to the centromere

Independent Assortment occurs

Describe Anaphase 2

Spindle fibres contract and shorteb

Centromeres divide and the chromatids are pulled to opposite poles

Describe Telophase 2.

Chromatids uncoil, spindle breaks down, nuclear membrane and nucleolus reform

Why should genetic variation occur in meiosis?

So that newborns are adapted to changing environments and can survive by natural selection

What are the sources of genetic variation in offspring?

Crossing over

Independent assortment

Random fusion of haploid gametes