Mitosis

• Mitosis is the process of reproduction where two identical cells are produced. Asexual reproduction.

• During mitosis each chromosome splits and the genetically identical chromatid move to opposite poles to become chromosomes.

What triggers mitosis?• External signals,

hormones, internal factors, growth factors (proteins able to trigger mitosis): these proteins have a receptor on the cell membrane. When the protein is attached to the receptor, it triggers several events in the cell that end with the division of the cell.

The process of mitosis is only part of a continuous cell cycle where most of the cell's 'lifetime' is spent carrying out its prescribed role; a phase in the cycle called interphase.

Interphase is itself divided up into three stages:

G1 First Gap

S Synthesis

G2 Second Gap

Thecell

cycle

The Cell Cycle

MitosisM

Second gap as

cell grows and

ensures DNA

replication is

completeG2

G1

First gap as cell monitors its

surroundings, growing and determining

whether to replicate DNA

Synthesis of

DNA to

replicate

chromosomes S

What are the phases of the cell cycle and what are the main events during each phase?

Interphase Prophase Metaphase Anaphase Telophase

What are the 4 stages of mitosis?

INTERPHASE• Before we start describing the different phases

of this cell division, we need to remember what a cell looks like in its normal state, which is known as the interphase. A normal eukaryotic cell has 2 pairs of chromosomes (2n chromosomes). During this phase, DNA is being replicated. At the end of interphase, the amount of DNA will be twice the normal amount. For a transient moment the cell will have 4n chromosomes.

PROPHASE

• The first stage is prophase, when the chromosomes begin to condense (shorten and thicken). They become visible under the microscope.

• At this stage, each chromosome is double-stranded. This is because during the S phase of the cell cycle the DNA has replicated, resulting in two identical copies of the chromosomes. These two copies, called chromatids, are held together by a constricted region called the centromere.

• Late in prophase, the nuclear membrane disappears and a network of microtubules assemble. This is called the spindle or spindle fibres, which extends between the two poles of the cell.

What happens at prophase?

METAPHASE

• During metaphase (meaning middle phase), the chromosomes gather in the central region (equator) of the spindle.

• At the equator, the centromere of each chromosome attaches to the spindle fibres.

What happens at metaphase?

ANAPHASE• In anaphase the spindle fibres

contract (pull tight).

• The centromere of each chromosome is pulled in two directions and splits.

• The chromatids separate and are drawn to opposite poles.

• The separated chromatids, which are single-stranded, are now called chromosomes.

What happens at anaphase?

ANAPHASE

• The two sister chromatids separate. Each one migrates to opposite ends of the cell. So each daughter cell has an identical complement of chromosomes .

• The nuclear membrane has disappeared at this stage.

• The cell membrane expands as the cell itself elongates. The diameter of the cell decreases at the equator.

TELOPHASE• In the last stage of mitosis,

telophase, nuclear membranes form around each group of chromosomes.

What is cytokinesis?

CYTOKINESIS• This is the last stage of mitosis. It is

the process of splitting the daughter cells apart.

• A furrow forms and the cell is pinched in two. Each daughter cell contains the same number and same quality of chromosomes.

Mitosis MicrographsCell division for somatic growth and repair.

1. Interphase 2. Prophase

4. Anaphase5. Late Anaphase

3. Metaphase

6. Telophase

Why do we need meiosis?

• Meiosis is necessary to halve the number of chromosomes going into the sex cells

• Why halve the chromosomes in gametes?

At fertilisation the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents

Meiosis

• Meiosis involves one duplication of the chromosomes but two divisions.

• In the first meiotic division homologous chromosomes align and crossing over occurs. A spindle forms but centromeres do not divide.

• For each pair of homologous chromosomes one chromosome consisting of two chromatids goes to each pole. Therefore there is a haploid set of chromsomes at each pole

Meiosis

• Cellular division that reduces the number of chromosomes in the parent cell by half to produce four haploid daughter cells.

• The product of meiosis is gamates or spores.

Non-Conservative Multiplication :

• the daughter cells are different

Meiosis I

The first division of meiosis is

called a ‘reduction’ division

because it reduces (halves) the

number of chromosomes.

One chromosome from each

homologous pair is donated to

each intermediate cell. Anaphase 1Telophase 1

1N

Intermediate cellIntermediate cell

2N

Metaphase 1 Chromosomes

line up on the

equator

2N

Prophase 1

Synapsis

and

crossing

over

2N

Interphase DNA replication

Interphase

• The step preceding the first cell division is the 'interphase'. The DNA is replicated into two identical copies, just as in mitosis. During interphase, the DNA is not visible.

• During 'prophase', which is quite different to the mitotic prophase, the chromosomes become visible. They look very long, as they are not yet totally condensed. Their ends are linked to one pole of the nucleus. The chromosomes that belong to the same pair come next to each other. The chromosomes become thicker and shorter, as they condense more and more. They are linked to each other in pairs. At this stage, the 2 chromatids of each chromosome become visible. The chromosomes then start to move away from each other, but remain linked at points called chiasmata. At this stage, some genetic material can be exchanged between chromosomes : it is now that crossing-over may take place.

Crossing over

• During prophase 1, with homologous pairs, they may touch at points called chiasma. At chiasmata the pairs may exchange chromosome segments. This results in RECOMBINATION.

Metaphase I

• the pairs of chromosomes are situated at the equator of the cell. At this stage, the nuclear membrane has disappeared. The centromeres are orientated toward the poles of the cell, and the chromosomes are fully condensed.

Anaphase I

• the chromosomes (two chromatids each) migrate toward opposite poles. There is a separation of chromosomes, and not of chromatids as in mitosis. Each pole will receive one set of chromosomes of 2 chromatids.

Anaphase 1

Telophase• The telophase is very short, and

often mistaken with prophase II. There is no time for the formation of a new cell membrane, nor for duplication of DNA. The next step starts straight away.

Telophase 1

Meiosis II

The second division of

meiosis is called a ‘mitotic’

division, because it is

similar to mitosis.

Sister chomatids of each

chromosome are pulled

apart and are donated to

each gamete cell.

1N

Intermediate cell

NOTE: Half the full chromosome

complement shown

1N

Prophase 2

Metaphase 2

Anaphase 2

1N

Gamete (egg or sperm) Gamete (egg or sperm)

Telophase 2

Cell division II

• The second division conserves the number of chromosomes but divides the chromatids.

Prophase 2

• it is very short. Everything is ready. The two centrioles migrate away from each other, and a network of microtubules forms in each daughter cell.

Metaphase II

• the chromosomes are situated on the equator. We therefore see two equators

Metaphase 2

Anaphase II :

• the chromatids, although present at the first cell division, only separate now. They migrate toward the two opposite poles of the cell.

Anaphase 2

Telophase II:

• the reconstitution of four daughter cells starts and finishes

Telophase 2

Meiosis

First Division • Interphase• Prophase I • Metaphase I • Anaphase I • Telophase

Second Division • Prophase II • Metaphase II • Anaphase II • Telophase II

Meiosis• In the testis of a mammal , or the anthers of a

flower, or any organ containing germ cells, you can observe a type of cell division called meiosis.

• During meiosis, DNA replication (which makes the cell have twice its usual amount of DNA) is followed by two cell divisions.

• At the end, one parent cell gives rise to four haploid gametes (each with one set of chromosomes).

• The haploid cell contains only one chromosome of each homologous pair (which is only half of the normal amount).

• There is no replication on the DNA during the short interphase between the two cell divisions. Meiosis is a succession of events:

Meiosis I - Stages

Prophase I Metaphase I Anaphase I Telophase I

Figure 10.4 Page 167

Effect of Crossing Over

• After crossing over, each

chromosome contains both maternal

and paternal segments

• Creates new allele combinations in

offspring

Meiosis

Parent cell – chromosome pair

Chromosomes copied

1st division - pairs split

2nd division – produces 4 gamete cells with ½ the original no. of chromosomes

Comparing meiosis and mitosis

Mitosis and Meiosis

• These two processes are very important.

• Mitosis because it allows replication within the body

• Meiosis because this is how genes are passed on from parents to children.

COMPARING MITOSIS & MEIOSIS

MITOSIS• Occurs in somatic cells• One division during the

process• IPMAT

• Homologous chromosomes don’t pairdon’t pair

• Two daughter cells formed which are DIPLOID

• Daughter cells are genetically identical

MEIOSIS Occurs in sex organs of

plants and animals Two divisions during the

process IPMAT/PMAT

Homologous chromosomes pairpair

Four daughter cells formed which are HAPLOID

Daughter cells are genetically different

When Meiosis goes wrong

• Meiosis is usually an exact process, but sometimes errors occur.

• Errors result in gametes with defects, resulting in serious consequences for offspring.

Nondisjunction: When Meiosis Goes Wrong

n + 1

n + 1

n - 1

n - 1chromosome alignments at metaphase I

nondisjunction at anaphase I

alignments at metaphase II

anaphase II

Non-disjunction in Meiosis IThe meiotic spindle normally distributes chromosomes to daughter cells without error.

Sometimes, homologous chromosomes or sister chromatids fail to separate.

In these cases, one gamete receives two of the same type of chromosome and the other gamete receives no copy.

This mishap, called non-disjunction,results in abnormal numbers of chromosomes in the gametes.

This example shows non-disjunction in meiosis I; homologous chromosomes fail to separate properly at anaphaseduring meiosis I.

Non-disjunction in Meiosis I

n+1 n+1 n–1 n–1

Meiosis I

Meiosis II

Meiosis II

Non-disjunction in Meiosis IINon-disjunction can also occur in meiosis II, when sister chromatids fail to separate during anaphase of meiosis II.

In cases of non-disjunction, if either of the aberrant gametes unites with a normal one at fertilization, the offspring will have an abnormal chromosome number(e.g. 2N+1 or 2N–1).

This is known as aneuploidy and it accounts for chromosomal defects such as Down syndrome (trisomy 21) and Edward syndrome (trisomy 18).

Non-disjunction in Meiosis II

n+1 n–1 n n

Meiosis I

Meiosis error - fertilisation

Should the gamete with the chromosome pair be fertilised then the offspring will not be ‘normal’.

In humans this often occurs with the 21st pair – producing a child with Downs Syndrome

21 trisomy – Downs Syndrome

Can you see the extra 21st chromosome?

Is this person male or female?

Polyploidy

• Having more than two sets of chromosomes (for example 3N, 4N, 6N) in a genome is called polyploidy.

• Polyploidy can come about through errors in meiosis.

• In humans, polyploid zygotes do not survive.

• Polyploidy is more common in plants than animals because many plants can survive by asexual reproduction.

Meiosis – division error

Chromosome pair

Meiosis error - fertilisation

Should the gamete with the chromosome pair be fertilised then the offspring will not be ‘normal’.

In humans this often occurs with the 21st pair – producing a child with Downs Syndrome