15 Mitosis and Meiosis

7/27/2019 15 Mitosis and Meiosis

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- GCSE Bitesize - Mitosis and meiosis

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Science

Mitosis and meiosis

There are two types of cell division. Mitosis is used for

growth and repair and produces diploid cells identical to

each other and the parent cell.

Meiosis is used for sexual reproduction and produces

haploid cells dif ferent to each other and the parent cell.

Growth

Humans are made of millions of cells. This has a number of

benefits:

Cells can be specialised to do particular tasksGroups of cells can function as organs making a more

efficient but complex organism

The organism can grow very large

Cell division

New cells are needed throughout life. These are for growth, to

replace damaged cells and repair worn out tissues. Normal

human body cells are diploid they have two of each

chromosome. When new cells are made, these 46

chromosomes (in other organisms the number is different) are

copied exactly in a process called mitosis.

Mitosis

Mitosis is the type ofcell division used for growth, repair and

asexual reproduction. Mitosis occurs wherever new cells are

needed. It produces two cells that are identical to each other,

and the parent cell.

n mitosis each chromosome is copied exactly. The new

chromosomes are moved to opposite sides of the cell, before

the cell divides leaving one complete set of 46 chromosomes in

each of the two new cells.


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Constant cell division ensures that cells never become too large.

The larger the cell becomes, the smaller its surface-area-to-

volume ratio. Objects with this small ratio find it difficult tomaintain exchange of materials with their environment. Large

cells could run out of oxygen, and accumulate too much waste,

such as carbon dioxide. For this reason it is more efficient for

arge organisms to be multicellular, rather than single-celled.

Formation of sex cells

Fertilisation

n humans all reproduction is sexual . It involves joining together

haploid gamete cells from each parent with halfthe normal

number of chromosomes to make a new cell containing both

parents' genetic material. This is a diploid zygote.

The cells from each parent that combine to form the zygote are

called gametes . In humans, the male gamete is called sperm ,


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and the female gamete is called an egg . When the gametes join

they form a cell called a zygote. Human sperm and eggs

contain 23 chromosomes. Human zygotes contain 46

chromosomes.

The type of cell division that produces gametes with half the

normal chromosome number is called meiosis. It is used toproduce cells for repair and asexual reproduction.

Gametes contain different genetic information to each other and

to the parent cell.

Meiosis is responsible for causing genetic variation .

These are helpful terms to learn:

Gamete cell with half the normal number of

chromosomes, and only used for sexual reproduction

Zygote cell formed when two gametes combine

Fertilisation term to describe the joining of two

gametes

Haploid having half the normal number ofchromosomes

Diploid having the normal number of chromosomes

Gametes

All gametes are haploid. They also have other adaptations to

ncrease the chances of fertilisation and successful development

Process of fertilisation


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of the embryo.

Sperm cells need to move to the egg cell. They have a tail to

propel them, and many mitochondria to provide energy. The

front of the sperm contains enzymes to digest the egg

membrane.

The egg contains a large food store to support the developingzygote until it can feed from the placenta.

Meiosis

Meiosis is a different kind of cell division. It is used to produce

male and female gametes. A human body cell contains 46

chromosomes arranged in 23 pairs . The gametes are sperm

or eggs , and only contain halfas many chromosomes (23).

This is why meiosis is sometimes called reduction division.

At fertilisation, the nuclei of the sperm and an eggjoin to form


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// bb k/ h l / bit i / i / dd d l/ ll / it i i t ht l[10/21/2013 8 17 43 PM]

the zygote. The zygote contains 23 pairs of chromosomes - 23

single chromosomes from the sperm, and 23 single

chromosomes from the egg , thereby creating the correct

number of 46 chromosomes for all body cells. It also means the

zygote contains a complete set of chromosomes from each

parent.

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15 Mitosis and Meiosis