Mitosis

Aims:1. Be able to name and explain the events occurring

during each stage of mitosis.

2. Recall that during mitosis, the parent cell divides to produce two genetically identical daughter cells.

3. Recognise that mitosis increases the cell number in this way in growth and tissue repair.

4. Be able to recognise the stages of mitosis from drawings and photographs

Chromosomes

• Each chromosome is a single very long strand of DNA.

• The DNA is supported and neatly packaged by a histone protein bound to it.

• At different times, these proteins cause the DNA to be spread out or tightly condensed into the X-shaped chromosomes we can see in the microscope.

• Each chromosome has a central constricted region called a CENTROMERE that serves as an attachment point for the machinery of mitosis.

More Chromosomes

• Chromosomes exist in 2 different states– Before replication, chromosomes have

one chromatid. – After replication, chromosomes have 2

sister chromatids, held together at the centromere.

• Somatic cells have 46 chromosomes and are said to be diploid (2n).

• Sex cells have 23 chromosomes and are said to be haploid (n).

Somatic Cells

• Form the majority of cells in the body.

• They are characterised as having:• 46 chromosomes• 23 homologous pairs (50% from each parent)

• They are said to be diploid cells.

Diploid Cells

• This is a cell containing two sets of chromosomes, one set inherited from each parent.

• Each chromosome has a partner with exactly the same:• Length• Genes

• Diploid cells are formed in a process known as Mitosis.

• In humans the diploid number is 46, which is abbreviated to 2n.

2n = 46

Sex Cells (Gametes)

• These cells are distinct from somatic cells.

• They are characterised as having:• 23 chromosomes• 0 homologous pairs

• They are said to be haploid cells.

Haploid Cells

• A cell containing only one set of chromosomes.

• Each chromosome is unique with respect to its:• Length• The genes it contains

• Haploid cells are formed in a process known as meiosis.

• In humans the haploid number is 23, which is abbreviated to n.

n = 23

What do you remember about mitosis?

Mitosis• The process whereby a mother cell divides to give rise to TWO

daughter cells that are GENETICALLY IDENTICAL to the original mother cell.

• In mitosis, the 2 chromatids of each chromosome separate, with each chromatid going into a daughter cell.

• Remember that diploid cells have two copies of each chromosome, one from each parent. These pairs of chromosomes are NOT attached together.

• Diploid (2n) 2 X Diploid (2n)

Functions of Mitosis

1. Growth

2. Repair

3. Cloning/Asexual Reproduction

Interphase

• The ‘living’ phase of the cell

• Cell obtains its nutrients, grows, and replicates it’s DNA

• Prepares the cell for mitosis or meiosis

• The cell spends the majority of time in this phase

Prophase1. The chromosomes condense

becoming more tightly coiled. They appear to shorten and thicken.

2. Pairs of sister chromatids become visible.

3. The nuclear envelope disappears.

4. The Microtubule Organising Centre (MTOC) and their centrioles move to opposite poles.

5. The spindle starts to form, growing out of the MTOC towards the chromosomes.

6. Some microtubules make contact with the centromere , others with the other pole of the cell

Metaphase1. Replicated chromosomes

line up at the equator of the cell (middle) and divide.

2. The spindle fibre is fully formed

3. One sister chromatid from each pair is attached by microtubules to one pole; the other is attached to the other, by the centromere

Anaphase1. The centromeres divide.

2. At this point, each individual chromosome goes from:– 1 chromosome with 2

chromatids – to:– 2 chromosomes with 1

chromatid each.

3. The spindle fibers contract

4. The sister chromatids are pulled to opposite poles.

EACH POLE RECEIVES ALL OF THE GENETIC INFORMATION

REQUIRED

Telophase1. The two sets of chromosomes

reach the poles of the cell

2. The spindle disintegrates

3. The nuclear envelope re-forms around the two sets of chromosomes.

4. The cytoplasm is divided into 2 separate cells, the process of cytokinesis.

Cytokinesis

To Do

• You have been given pictures of the different stages of mitosis.

• You should:– Order the pictures and diagrams– Name the stage– Explain what is happening in terms of

chromosome behaviour

Exam Practice

• Q1 Jan 03• Q1 Jan 04• Q2 Jan 05• Q7b Jan 06• Q1 Jun 03• Q2 Jun 04• Q2 Jun 05