Cell Division and Inheritance

Mitosis is the division of body cells to produce new cells.

Mitosis occurs ....

• for growth
• for repair
• in asexual reproduction (cells produced by asexual reproduction contain the same alleles as the parents)

During Mitosis

• a copy of each chromosome is made
• the cell then divides once to produce two body cells
• the new cells contain exactly the same genetic information as the parent cell, i.e the same number of chromosomes and the same genes.

Meiosis occurs in the testes and ovaries to produce the gametes (sperm and egg) for sexual reproduction. Remeber, gametes only contain one set of 23 chromosomes.

When a cell divides to form gametes:

• Copies of the genetic material are made.
• The cell then divides twice to form four gametes, each with a single set of chromosomes.

Of the 23 pairs of chromosomes in the human body, one pair carries the gene that determine sex:

• In females, the sex chromosomes are identical and are called the X chromosomes
• In males, one sex chromosme (Y) is much shorter than the other (X).

As with all chromosomes, offspring inherit...

• one sex chromosome from the mother (always an X chromosme)
• one sex chromosme from the father (either an X or Y chromosome)

So, the sex of the offspring is decided by whether the ovum is fertilised by an X-carrying sperm or a Y-carrying sperm.

Some characteristics are controlled by a single gene. Genes may have different forms, or variations, called alleles. For example...

• the gene that controls tongue-rolling ability has two alleles- either you can or you can't.
• the gene that controls eye colour has two alleles

Sexual reproduction gives rise to variation beacues when gametes join during fertilsation...

• one allele for each gene comes from the mother
• one allele for each gene comes from the father

In a pair of chromosomes, the alleles for a gene can be the same or diffrerent. If they are different...

• one allele will be dominant
• one allele will be recessive

A dominant allele will always control the characteristic; it will express itself even if present on only one chromosome in a pair.

A recessive allele will only control the characteristic if it is present on both chromosomes in a pair (i.e no dominant allele is present)

When a characteristic is determined by just one pair of alleles it is referred to as monohybrid inheritance. A simple genetic cross diagram is a biological model and can be used to predict the outcome of crosses in monohybrid inheritance.

Gregor Mendel is known as the father of modern genetics. Mendel...

• was an Austrian monk
• studied monhybrid inheritance using pea plants
• proposed the idea of separately inheritance factors.

The importance of Mendel's research wasn't recognised until after his death, when other scientists discovered chromosomes and linked then to his ideas.

Most types of animal cells differentiate and become specialed at an early stage. Many plant cells retain the ability to differentiate throughout their life.

Stem cells have the ability to develop into any kind of cell because they have not yet differentiated. Stem cells in humans are found in...

• human embryos
• adult bone marrow.

Human stem cells can be made to differentiate into any time of human cell, e.g. nerve cells. These cells could potentially be used to help treat people with conditions like paralysis.

There are some ethical issues surrounding the use of stem cells for scientific research:

• Against - some people believe that human embryos shouldn't be used because they are a potential human life.
• For - some people believe it is more important to help people already living.

Chromosomes are made up of DNA. A DNA molecule consists of two long strands that are coiled to form a double helix.

Each person has unique DNA ( apart from identical twins). So, DNA can be used for identification. A gene is a smalle section of DNA.

Genes code for a particular characteristic by providing a code for a combination of amino acids that make up a specific protein.

Some disorders are inherited, such as ...

• polydactyly
• cystic fibrosis

People with polydactyly have extra fingers or toes. It is caused by a dominant allele, so it can be passed on even if only one parent has the disorder.

Cystic fibrosis is a disorder of the cell membranes. It's caused by a recessive allele, so both parents msut carry the allele. Because the allele is recessive, the parents can be carriers without have the disorder themselves.

Embryos can be screened for the alleles that cause genetic disorders. This has economic, social and ethical issues