Cell Reproduction

Need For Reproduction

Reproduction is the life process in which living things produce other living things of the same species.

Reproduction is not necessary for the life of one individual organism.

It is necessary for the continued survival of a particular group of organisms.

Cell Division All cells arise from other cells by cell division. When cells grow to a certain size they must

either divide or die. If a cell continued to grow without dividing,

the surface area of the cell would become too small to hold the cell’s contents. (Problem of Surface Area/Volume Ratio)

Mitosis Cell Division

Cell division is a complex series of changes in the nucleus of a cell that leads to the production of two new cells.

The new cells are called daughter cells. The nuclei of the daughter cells are identical to

each other and to that of the parent cell. The daughter cells grow and increase in size until

they divide and produce two more daughter cells. This process continues, resulting in organism

growth and reproduction.

Mitosis All cells in the body (except sex cells) are produced

by the process of mitotic cell division. Mitosis involves a complex series of changes in

the nuclei of body cells that produce identical (same) daughter cells.

They have the same number and type of chromosomes as the parent cells. Primary purpose is to increase the number of cells

Daughter cells are genetically identical to the parents

Occurs during growth and asexual reproduction.

The Cell Cycle – Stages of Mitosis

Although the events of mitosis are an ongoing process, they are generally described in terms of separate phases, or stages. Interphase Prophase Metaphase Anaphase Telophase

Mitosis

Interphase Interphase is the period

between cell divisions. During interphase, the

single-stranded chromosomes replicate (make an extra copy).

The chromosomes can not be seen during interphase.

Nucleus

Chromosomes

Interphase

Interphase Sometimes called the

“resting stage” between divisions

BUT cells are metabolically very active

The amount of DNA in the nucleus doubles

New organelles such as mitochondria are made.

Prophase In prophase, the double-

stranded chromosomes become visible, and the nuclear membrane disappears.

A spindle apparatus, consisting of fibers, forms between opposite poles of the cell.

Chromosomes

Prophase

Prophase Protein microtubules

develop from each centriole forming spindle fibres.

(In plant cells there are no centrioles and the spindle forms independently)

Prophase

Prophase Towards the end of

prophase each chromosome can be seen to consist of two chromatids held together by a centromere.

Prophase

Prophase At the end of prophase

the nucleolus disappears and the nuclear envelope breaks down.

Metaphase During metaphase, the chromosomes move toward

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

Chromosomes

Centrioles

Metaphase

Metaphase Chromosomes line up

on the equator of the spindle.

They attach themselves to the spindle by their centromere

Anaphase During anaphase, the double-stranded chromosome

separates and move to opposite poles of the cell. There is a complete set of chromosomes at each pole

of the cell.

ChromosomesCentrioles

MITOSIS

Anaphase This movement results

from the contraction of the spindle fibres.

As they shorten they pull the chromatids apart.

Anaphase

Anaphase The centromeres divide The free chromatids

move to the poles

Telophase and Cytoplasmic Division

The last stage of mitosis is telophase. In telophase, a nuclear membrane forms around each

set of chromosomes, forming two identical nuclei. At the end of mitosis,

the cytoplasm divides, forming two new identical daughter cells.

Telophase

Telophase The chromatids have

reached the poles and are now regarded as distinct chromosomes again.

A nuclear envelope forms around each group of chromosomes

Telophase

Telophase The chromosomes

uncoil returning to chromatin

The cytoplasm divides by cytokinesis.

CYTOKINESIS

Animal cells The centre of the cell

‘pinches in’ to form a division furrow.

As the division deepens, the cell surface membrane on each side joins up.

Two separate cells result.

CYTOKINESIS

Plant cells Vesicles produced by the Golgi body collect

on equator of cell These vesicles fuse to form a cell plate. The cell plate eventually stretches right

across the cell forming the middle lamella. Cellulose builds up on lamella to form cell

walls.

CYTOKINESIS

The Cell Cycle

Meiosis The process of meiosis involves two cell

divisions and produces cells that are different from the parent cell. Meiosis produces cells that have one-half the

number of chromosomes as the parent cells. If meiosis did not take place, the fertilized egg would

have double the amount of chromosomes it needs. Meiosis takes place during sexual reproduction

when sex cells, called the egg and sperm, are produced in sex organs.

Meiosis One-half the number of chromosomes is called

the hapolid, (or n) number. In humans, the haploid number (n) is 23.

When the egg and sperm unite during fertilization, the species normal chromosome number called the diploid or (2n) number is restored (brought back). The diploid number in humans is 46. n (sperm) + n (egg) = 2n

Meiosis

Meiosis

Cell division that is vital for sexual reproduction

Takes place in reproductive organs

Results in formation of haploid gametes

The four daughter cells are not genetically identical

Meiosis and genetic variation

There are two main ways in which genetic

variation occurs at meiosis:

1. Random segregation of chromosomes

2. Crossing over

Meiosis and genetic variation

Random segregation During metaphase 1

homologous chromosomes arrive at the equator

They arrange themselves in a random order on the equator

Meiosis and genetic variation

Random segregation In a cell with just two

pairs of chromosomes there are four different types of outcomes for the way the chromosomes can separate.

Meiosis and genetic variation

Random segregation In human cells with 23

chromosomes there are 223 different possible ways the chromosomes can segregate!

That is over 8 000 000 different types of gamete

Meiosis and genetic variation

During fertilisation any male gamete can join with any female gametes.

So thousands of millions of new genetic combinations are possible.

Meiosis and genetic variation

Crossing over During prophase 1 of

meiosis homologous chromosomes come together in pairs

Meiosis and genetic variation

Crossing over Each chromosome is divided into two

chromatids The homologous chromosomes twist around

each other This creates tension, which may cause

breaks to occur along the length of the chromatids

Meiosis and genetic variation

Crossing over During cross over, corresponding fragments

may get swapped over. This “cutting and sticking” means that genetic

material is exchanged. This creates new genetic combinations and

variation in the gametes is increased.

Differences between mitosis and meiosis

Mitosis Meiosis

one division

Differences between mitosis and meiosis

Mitosis Meiosis

one division two divisions

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisions

the number of chromosomes

remains the same

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisions

the number of chromosomes the number of

remains the same chromosomes is halved

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisions

the number of chromosomes the number of

remains the same chromosomes is halved

homologous chromosomes

do not pair up

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisions

the number of chromosomes the number of

remains the same chromosomes is halved

homologous chromosomes homologous chromosomes

do not pair up pair up to form bivalents

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisions

the number of chromosomes the number of

remains the same chromosomes is halved

homologous chromosomes homologous chromosomes

do not pair up pair up to form bivalents

chiasmata do not form and

cross over never occurs

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisions

the number of chromosomes the number of

remains the same chromosomes is halved

homologous chromosomes homologous chromosomes

do not pair up pair up to form bivalents

chiasmata do not form and chiasmata form and cross over

cross over never occurs occurs

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisions

the number of chromosomes the number of

remains the same chromosomes is halved

homologous chromosomes homologous chromosomes

do not pair up pair up to form bivalents

chiasmata do not form and chiasmata form and cross over

cross over never occurs occurs

daughter cells are genetically

identical

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisions

the number of chromosomes the number of

remains the same chromosomes is halved

homologous chromosomes homologous chromosomes

do not pair up pair up to form bivalents

chiasmata do not form and chiasmata form and cross over

cross over never occurs occurs

daughter cells are genetically daughter cells are genetically

identical different from the parent cells

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisionsthe number of chromosomes the number ofremains the same chromosomes is halvedhomologous chromosomes homologous chromosomesdo not pair up pair up to form bivalentschiasmata do not form and chiasmata form and cross overcross over never occurs occursdaughter cells are genetically daughter cells are geneticallyidentical different from the parent cellstwo daughter cells are formed

Differences between mitosis and meiosis

Mitosis Meiosisone division two divisionsthe number of chromosomes the number ofremains the same chromosomes is halvedhomologous chromosomes homologous chromosomesdo not pair up pair up to form bivalentschiasmata do not form and chiasmata form and cross overcross over never occurs occursdaughter cells are genetically daughter cells are geneticallyidentical different from the parent cellstwo daughter cells are formed four daughter cells are

formed

Comparison of Mitosis and Meiosis

Characteristic Mitosis MeiosisNumber of daughter cells 2 4

Number of cell division 1 2

Daughter cells are diploid or haploid

Diploid Haploid

Daughter cells are identical or different

Identical Different

Parent cell is diploid or haploid

Diploid Diploid