Cell ReplicationMitosis & Meiosis

Femaleembryo2N

Maleembryo2N

Meiosis

Meiosis

A single set of chromosomes

Egg1N

Sperm1N

Several mitotic

divisions

Somatic cell production

A double set of chromosomes

Embryo2N

Gamete production

Fertilization

Zygote2N

Many mitotic

divisions

Somatic cell production

Adult

2N

Somatic cell production

Many mitosis

divisions

Many mitosis

divisions

Maleadult

2N

Femaleadult

2N

Cell Division

If the cells of a growing root tip are examined, a proportion of them are in mitosis. Cells in different stages of division can be seen.

Mitosis is the process bywhich the cell producestwo new daughter cellsfrom the original parent cell.

Anaphase

Late anaphase

Prophase

Telophase

Mitosis in Onion Cells

Cell division for somatic growth and repair.

1. Interphase 2. Prophase

4. Anaphase

3. Metaphase

5. Telophase

Mitosis Micrographs

Interphase is the first step of the process

Cells prepare for Mitosis by accumulating materials required

At this point, individual chromosomes cannot be seen

Interphase

1. Interphase

This is the longest phase in which the DNA replicates

Chromosomes need to be duplicated into double stranded chromosomes, so each daughter cell can receive an identical copy

Interphase

During Prophase, the chromosomes begin to condense, and become visible under a microscope

Eventually they can be seen as individual chromosomes

Centrioles appear at opposite ends of the cell

The nuclear membrane breaks down

Prophase

2. Prophase

During Metaphase, the chromosomes line up through the middle of the cell

Spindle fibres extend from the centrioles, and meet up with the chromosomes in the middle

The ends of the spindle fibres attach to the chromosomes

Metaphase

3. Metaphase

During Anaphase, the spindle fibres begin to contract, pulling the chromosomes. They split apart at the centromere, separating them into 2 chromatids

The spindle fibres draw the chromosomes to either end of the cell, each side having the exact copies

These chromatids are now single stranded chromosomes

Anaphase

4. Anaphase

During Telophase, the opposite occurs to prophase. The chromosomes begin to expand.

The chromosomes can no longer be seen individually

The spindle dismantles and disappears

A nuclear membrane begins to form around each set of chromosomes

Telophase

5. Telophase

Cytokinesis is not part of Mitosis, although it starts occurring during Anaphase or Telophase

It is the separation of the two daughter cells

The membrane is formed and 2 daughter cells are born!!

The membrane pinches inwards and a cleavage furrow appears

Cytokinisis

Femaleembryo2N

Maleembryo2N

Meiosis

Meiosis

A single set of chromosomes

Egg1N

Sperm1N

Several mitotic

divisions

Somatic cell production

A double set of chromosomes

Embryo2N

Gamete production

Fertilization

Zygote2N

Many mitotic

divisions

Somatic cell production

Adult

2N

Somatic cell production

Many mitosis

divisions

Many mitosis

divisions

Maleadult

2N

Femaleadult

2N

Cell Division

haploid (1N)

diploid (2N)

The purpose of meiosis is to produce haploid sex cells.Haploid sex cells have only one copy of each homologous pair of autosomes plus one sex chromosome.

Gametes(eggs or sperm)

1N

2N

Intermediate cell Intermediate cell

1N

First Division(Reduction division)

Second Division('Mitotic' division) 1N

Crossing over may occur at this stage in meiosis

2N

2N

Meiosis

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 Bivalents line up on the equator

2N

Prophase 1 Synapsis and crossing over

2N

Interphase DNA replicationMeiosis I

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

Meiosis II

Cell division

1N 2N

Cell division

Mei

osis

II‘M

itotic

’ div

isio

n

Cell division

Homologous chromosomes pair up at the equatorial plate

Homologous chromosomes do not pair up at the equatorial plate

Mei

osis

IR

educ

tion

divi

sion

MITOSISMEIOSIS

2N 2N

Mitosis and Meiosis Compared

The 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 gamete 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

Non-disjunction in Meiosis I

Meiosis II

Non-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

Non-disjunction in Meiosis II

Mitosis Cells undergo mitotic division

during which time the genetic material is doubled and divided into two cells.

Meiosis Meiosis is a reduction division that

results in the formation of haploid (N) cells from diploid (2N) ones.

Its purpose is to produce gametes for sexual reproduction.

During meiosis, genetic material is exchanged between chromosomes;this introduces genetic variation into the offspring.

Overview

1. Cell division takes place for the growth, development, repair and replacement of tissues

2. Mitosis replaces cells and meiosis produces gametes for reproduction

3. Males gamete = sperm formed in the testes and female gamete = ovum formed in the ovaries

4. Haploid the daughter cells have half the number of chromosomes of the parent cell.

5. Diploid have double, in other words the daughter and parent cells have the same number of chromosomes

6. Each human body cell has 23 pairs, or 46 chromosomes. Sperm and ova have only 23 chromosomes

Helpful Hints to remember

Helpful Hints to rememberMITOSIS

I Inter = Prepare/replicate DNAP Pro = CondenseM Meta = Line upA Ana = SplitT Telo = expand/2 new cells/diploid