OCR Biology A-level - Cell Division

Interphase

The stage in which normal growth and working take place e.g. producing enzymes or hormones and preparing for division, no active division takes place.

During interphase

DNA replicated + checked. Protein synthesis. Mitochondria grow + divide )in plant and algal cells). Chloroplasts grow + divide. Normal metabolic processes occur

Mitotic phase

The period of cell division

Stages of cell division

1. Mitosis 2. Cytokinesis

Mitosis

The nucleus divides

Cytokinesis

The cytoplasm divides and two cells are produced

Stages of interphase

G1 = 1st growth = proteins are synthesised, organelles replicate = cell increases in size.

S = synthesis = DNA replicates

G2 = 2nd growth = cell grows more, energy stores increase, duplicated DNA is checked for errors

G 0

The phase when the cell leaves the cycle, either temporarily or permanently.

Differentiation

A cell that becomes specialised to carry out a particular function is no longer able to divide. It will carry out the function indefinitely and not enter the cell cycle

Why might a cell leave a cycle (temporarily or permanently)

Differentiation, damaged DNA = cell arrest, get older = more cells

How is the cell cycle controlled?

Checkpoints = monitor and verify whether each stage has been accurately completed before the cell goes in to the next phase.

Checkpoints

Monitor and verify whether each stage has been accurately completed before the cell goes in to the next phase. G1, G2 and metaphase checkpoints

G1 checkpoint

After the G1 phase, before the S phase.

If the requirements are met then the checkpoint triggers the DNA replication, if not it enters resting state

G2 checkpoint

At the end of G2 phase, before the mitotic phase. DNA is checked. If it passes the cell initiates the molecular process that signifies the beginning of mitosis

What does mitosis produce?

The nuclear division, producing two, genetically identical daughter cells = exact copy of the parents DNA and same number of chromosomes.

When is mitosis used?

In multicellular organisms. Growth, replacement and repair of tissue. Also necessary fro asexual reproduction

Chromosomes

Structures of condensed and coiled DNA in the form of chromatin.

Chromatids

Two identical copies of DNA held together at the centromere

Centromere

A region where two chromatids are held together

Chromatin

Uncondensed DNA in a complex with histones

Stages of mitosis

Prophase, metaphase, anaphase, telophase

Prophase

Chromatin fibres start to coil and condense --> chromosomes. Nucleolus disappears. Nuclear envelope begins to break down.

Protein microtubules form spindle fibres = link poles = move chromosomes into correct place before division.

Centrioles migrate to poles.

Spindle fibres attach to specific areas on centromeres = start to move the chromosomes to the centre of the cell.

Nuclear envelope has disappeared

Metaphase

Chromosomes moved by spindle fibres = form plane in centre of cell (metaphase plate)

Anaphase

Centromeres in each chromosome divide and are pulled to poles. 'V' shape chromatids.

Telophase

Chromatids have reached poles and are now called chromosomes. Nuclear envelope reforms around the groups of chromosomes at each end. Chromosomes start to uncoil and nucleolus is formed. Cell division begins

Cytokinesis in animal cells

A cleavage furrow forms in each side of the cell. The membrane is pulled in by the cytoskeleton until it is close enough to fuse = forming 2 cells

Cytokinesis in plant cells

Vesicles from the Golgi begin to assemble in the same place as the metaphase plate. The vesicles fuse with each other and the membrane, dividing into 2 cells. New sections of cell wall then form and the cell spilts

Meiosis

A form of cell division where the nucleus divides twice, resulting in half the number of chromosomes and producing 4 haploid cells from one diploid cell.

Gametes

Haploid sex cells produced by meiosis in organisms that reproduce sexually

Zygote

The initial diploid cell formed when 2 gametes are joined by the means of sexual reproduction

Haploid

Half the number of normal chromosome number; one chromosome of each type

What is produced during meiosis?

4 daughter cells - the gametes - that contain half the number of chromosomes of the parent

Homologous chromosomes

Matching pair of chromosomes, one from each parent

Alleles

Different versions of the same gene

Stages of meiosis

Meiosis 1 = Prophase 1, Metaphase 1, Anaphase 1, Telophase 1

Meiosis 11 = Prophase 11, Metaphase 11, Anaphase 11, Telophase 11

Meiosis 1

The first division, the reduction division. The pairs of homologous chromosomes are separated into 2 cells. Each cell only contain one full set of genes instead of 2 = haploid

Meiosis 11

The second division (similar to mitosis), pairs of chromosomes in each daughter cell are separate, forming 2 more cells. 4 haploid cells are produced in total.

Prophase 1

Chromosomes condense, nuclear envelope disintegrates, nucleolus disappears and spindle formation begins, as in prophase of mitosis.

Difference = homologous chromosomes pair up --> bivalents. Chromatids entangle = crossing over.

Sections of DNA break off and re-join - sometimes resulting in the exchange of DNA. Exchange forms recombinant chromatids, with genes being exchanged between chromatids. Genetic variation occurs from the new combination of alleles = not identical

Metaphase 1

Homologous pairs assemble along the metaphase plate independently and randomly = genetic variation

Anaphase 1

Homologous chromosomes are pulled to opposite poles and chromatids sat joined to each other

Telophase 1

Chromosomes assemble at each pole, nuclear envelope reforms, chromosomes uncoil.

Cell undergoes cytokinesis and divides in two = haploid.

Prophase 11

The chromosomes, which still consist of two chromatids, condense and become visible again. Nuclear envelope breaks down and spindles formation begins.

Metaphase 11

Individual chromosomes assemble on the metaphase plate.

Due to crossing over, the chromatids are not identical, there is independent assortment and more genetic variation is produced.

Anaphase 11

Results in the chromatids of the individual chromosomes being pulled to opposite poles after division of the centromere

Telophase 11

Chromatids assemble at the poles. The chromosomes uncoil and form chromatin again. Nuclear envelope reforms and the nucleolus becomes visible.

Cytokinesis results in the division of cells forming 4 daughter cells. Cells are haploid because of the reduction division. They will be genetically different from each other and the parent cell due to crossing over and independent assortment

Genetic variation

A variety of different combinations of alleles in a population.

Crossing over

Sections of DNA which become entangled, breaks and re-joins during prophase 1 of meiosis = genetic variation

Independent assortment

The arrangement of each homologous chromosome pair in metaphase 1 and 2 of meiosis is independent of each other and results in genetic variation