MITOSIMITOSISSThe nature of replicating cellsThe nature of replicating cells

Learning Objectives:

(a) state the importance of mitosis in growth, repair and asexual reproduction.

(b) explain the need for the production of genetically identical cells and fine control of replication.

(c) identify, with the aid of diagrams, the main stages of mitosis.

(d) state what is meant by homologous pairs of chromosomes.

(e) identify, with the aid of diagrams, the main stages of meiosis.

(f) define the terms haploid and diploid, and explain the need for a reduction division process prior to fertilisation in sexual reproduction.

(g) state how meiosis and fertilisation can lead to variation.

Human ChromosomesHuman Chromosomes

• Each cell in the human body contains two sets of 23 chromosomes.

• Cell divides by mitosis to produce the same number of chromosomes as the parent nucleus. Each cell therefore has the same genetic material.

• Reproductive cells (sex gametes) only have one set of chromosomes. These combine to make a new

person with different genetic material to both parents.

Difference between function of Mitosis and Meiosis.

Mitosis• A cell divides to produce two daughter cells, each

having the same number of chromosomes as the parent cell. (diploid number, 2n)

• Daughter cells have the same hereditary materials or genes as the parent cell.

• Young or offspring produced asexually will therefore be genetically identical to the parents.

• Mitosis is mainly responsible for (a)cell growth in both plants and animals.(b)repair for worn-out cells(c)asexual reproduction in plants (vegetative

propagation to produce new plants)

parent cell

16.2

2 genetically identical

daughter cells

DNA replicates

DNA = deoxyribonucleic acid

chromosomes

mitosis

What is mitosis?

REMEMBER!REMEMBER!

IInterphase

PProphase

MMetaphase

AAnaphaseTTelophase

IPMATIPMAT

INTERPHASE ( INTERPHASE ( ‘Resting’ Stage‘Resting’ Stage))

• The cell is replicating its DNA content before the cell divides.

• DNA replication is a process that faithfully copies all the information stored within the chromosomes to ensures that daughter cells produced by mitosis contain all the DNA needed for subsequent cell division and differentiation.

• The resulting daughter cells can be said to be genetically stable.

• In adult human, mistakes made in DNA replication or mitosis may cause the uncontrolled division of cells, which lead to cancer. Cancer can be fatal in most cases because the cells lose the ability to control the way they divide.

INTERPHASE ( INTERPHASE ( ‘Resting’ Stage‘Resting’ Stage))

Chromatin thread (contains one DNA molecule)

sister chromatids (contain two identical DNA molecule

Chromatin threads coil and shorten

One chromosome

DNA replication

• Each chromosome consist of two identical DNA molecules known as sister chromatids.

• The sister chromatids are joined at a point known as the centromere.

DNA replication

sister chromatids

centromere

PROPHASEPROPHASE• The chromatin (unravelled DNA) in the nucleus, condenses to

form pairs of chromosomes.

• The centrioles move to opposite ends of the nucleus.

• As this is happening the nucleolus begins to break

down

• Nuclear membrane begins to break down

METAPHASEMETAPHASE

• The spindle becomes fully developed.

• The nuclear membrane has completely gone

• The chromatid pairs place themselves onto individual fibres and are aligned along the centre of the spindle.

ANAPHASEANAPHASE

• The chromatid pairs are split into two (This is done by movement of the spindle fibres)

• The pairs then travel to opposite ends of the spindle.

• The halved chromatids are now called chromosomes

TELOPHASETELOPHASETwo new nuclei are formed when the chromosomes reach the opposite poles of the cell

The nuclear membrane is formed- the nucleolus reappears

The chromosomes disperse in the nucleus

CYTOKINESISCYTOKINESIS

Literally means, division of the

cytoplasm.

Mitosis is the splitting of the nucleus.

Cytokinesis is the splitting of cytoplasmIt usually begins during ANAPHASE.

Common Misconception• X During interphase, a chromosome

replicates to form two new chromosomes.• √ During interphase, DNA replicates and two

chromatids are formed, but both are still considered as one chromosome.

A B C

D E

Quick Review

What’s the order of stages in mitosis?

Quick Review: Identify What happens in each phase of Mitosis:

Prophase

Metaphase

Anaphase

Telophase

Interphase

Quick review:What’s the importance of mitosis?

• Mitosis is mainly responsible for (a)cell growth in both plants and animals.(b)repair for worn-out cells(c)asexual reproduction in plants

(vegetative propagation to produce new plants)

Why We Are Who We Are

MeiosisMeiosis

Meiosis• A type of cell division that results in the nucleus

of each daughter cell containing half the number of chromosomes as the nucleus of the parent cell.

• This choromosome number in the sex gamete is the haploid number.

• Twice the haploid number of chromosomes make up the diploid number.

• In man, normal cell has diploid number of 46. • Sperm/egg has only have half the number of

chromosomes, which is 23 (haploid number).

16.3

1 diploid parent cell with 2 pairs

of chromosomes

Meiosis I

Each daughter cell contains 2 chromosomes, each consisting of 2 chromatids

4 haploid gametes, each

containing 2 chromosomes

Meiosis II

Overview of Meiosis

Replication of chromosomes

During Meiosis gamete (sex) cells undergo a “double division”, maintaining the DNA, but reducing the chromosomal count to 23

+ =

Sperm (23) + Egg (23) = Fertilized Cell (46)

Importance of Sexual Reproduction

• Sexual reproduction by meiosis shuffles the genes to produce genetically unique offspring.

• Variation in offspring provided by sexual reproduction confers a large evolutionary advantage

Common Misconception

• X Meiosis II is the same as mitosis.• √ The stages in meiosis II are the same as

mitosis, but while mitosis begins with a diploid number of chromosomes, meiosis begins with a haploid number of choromosomes.

Stages of Meiosis• Meiosis consists of meiosis I and II.• Meiosis I and II consists of the prophase,

metaphase, anaphase and telophase stages.• During prophase I, homologous pair up and crossing

over occurs.• Crossing over is the process during which

chromatids of homologous chromosomes cross each other and exchange parts. The points where homologous chromosomes cross over are called chiasmata.

• Anaphase I separates homologous choromosomes.• Telophase I produces 2 daughter cells that have

haploid number of chromosomes.

Stages of Meiosis

• Meiosis consists of meiosis I and II.

• Meiosis I and II consists of the prophase, metaphase, anaphase and telophase stages.

What is ‘Homologous Chromosomes’?

• In diploid organism (2n), one chromosome in the pair comes from the male parent and the other from the female parent during Meiosis I.

A pair of homologous chromosomes

(same shape, genes and length)

Importance of Meiosis

• Meiosis results in variations in the gametes produced.

• Variations occurs during

(a) Crossing over at Prophase I;

(b) Independent assortment of chromosomes at Metaphase I and II

(c) Random fertilisation of gametes

(a) Crossing over:

• During prophase I, homologous chromosomes pair up and crossing over occurs.

• Crossing over is the process during which chromatids of homologous chromosomes cross each other and exchange parts.

(a) Independent Assortment

• During metaphase I (pairs of homologous chromosomes) and metaphase II (chromosomes) arrange themselves in the equator of the cell.

Comparison of Mitosis and Meiosis

Mitosis MeiosisParent cell(before chromosome replication)

Chromosome replication

Chromosome replication

2n = 4

Prophase

DuplicatedChromosome

(2 sister chromatids)

Metaphase

AnaphaseTelophase

2n 2n

Daughter cells of mitosis

Chromosomes alignat the metaphase plate

Sister chromatidsseparate during

anaphase

Prophase I

Metaphase I

Anaphase ITelophase I

Meiosis I

Meiosis II

Tetrads alignat the metaphase plate

Daughter cells of meiosis IIn n n n

Sister chromatidsRemain together

Homologouschromosomesseparate

Sister chromatidsseparate

duringanaphase II

Tetrads formed bysynapsis of homologouschromosomes

5112109

Cell division Mitosis MeiosisNo. of chromosomes produced in daughter cells

Diploid (2n) number (same as parent cells)

Haploid (n) number (half the number of choromosomes as parent cells)

Function Growth and repair Formation of gametes

Variation No genetic variation Recombination of chromosomes gives rise to variation.

Location All plant and animal cells

In sexual reproductive cells