1.9 Genetic variation 2

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Chromosomes are made up of

DNA

The chromosomes carry genes

along them

Genes code for proteins

DNA is a triplet code; 3

bases = one amino acid

Some bases are stop/start,

otherwise nonsense (don't code for anything we know)

Sequence of bases determines

the protein produced - which gives the characteristic/trait

Small differences in the DNA

sequence of a gene = alleles result by mutation

DNA is a

complex chemical that carries genetic information. DNA Is contained in

chromosomes, which are found in the nucleus of most cells. The gene is the unit

of inheritance and different forms of the same genes are called alleles.

Chromosomes

Chromosomes

are x-shaped objects found in the nucleus of most cells. They consist of long

strands of deoxyribonucleic acid, or DNA for short. A section of DNA that has

the genetic code for making a particular protein is called a gene.

The gene

is the unit of inheritance, and each chromosome has several thousand

genes. We inherit particular chromosomes

through the egg of our mother and sperm from our father. The genes on those chromosomes carry the code

that determines our physical characteristics, which are a combination of our

two parents.

The bases

in the DNA molecule carry the different codes needed for different amino acids.

The code for a particular amino acid is made from three bases in a particular

order.

The

backbone of the DNA molecule consists of two strands of alternating chemical

strands of alternating sugar molecules and phosphate groups, the two strands

twisted to form a double helix. Each sugar molecule is attached to one of the 4

bases called adenine, guanine, thymine and cytosine (A, G, T, C for short) The

bases are paired up on opposite strands. A is always paired with T, and G is

always paired up with C. This is called complementary base pairing - the order

of bases in one strand determine the order of bases in another. A nucleotide is

a basic building block for DNA made up of phosphate, sugar and base. The two

strands are held together by weak hydrogen bonds. A gene consists of hundreds

or thousands of bases. A gene codes for a particular protein by its particular

base sequence.

DNA must

copy or replicate itself before any cell division can take place. The DNA Double Helix is perfectly suited for

replication because each strand can serve as a template to produce a strand

opposite to itself.

First it

is "unzipped" - the two strands are separated. New nucleotides in the

cell line up alongside the unpaired bases, A pairing with T and G pairing with

C. The new nucleotides are joined together using an enzyme called DNA

polymerase.

DNA

replication is semi conservative i.e. each strand in the original DNA molecule

is used as a template o make a new strand of DNA. Each new DNA molecule

contains an original strand and a newly made strand.

Crossing over - bits of

homologous chromosomes are exchanged/swapped during meiosis.

Independent assortment - it

is random which one of a pair of homologous chromosomes goes into a gamete

after they line up in pairs during meiosis.

Each

parent cell has a pair of homologous chromosomes, one homolog from the father,

and one from the mother. In meiosis the maternal and paternal chromosomes can

be shuffled into the daughter cells in many different combinations. This

ensures genetic variation in sexually reproducing organisms. Further genetic

variation comes from crossing over during prophase I.

In

prophase I of meiosis, the replicated homologous pair of chromosomes comes

together in the process called synapsis, and sections of the chromosomes are

exchanged. You can see that after crossing over, the resultant chromosomes are

neither maternal or paternal, but contain genes from both parents.

When

homologous pairs of chromosomes line up during meiosis, they do so randomly.

This means it is completely random which combination of alleles end up in a

particular gamete. This process is called independent assortment.

Permanent change in DNA

sequence or number of chromosomes

Difference in base sequence

-> Different amino acid -> different protein

Change in order = slightly

altered gene

Alternate for of same gene is

called an allele

Mutations can only be passed

on if they are in a gamete / sex cell

Changes

to genes are called mutations. Mutations can be spontaneous (they just happen).

They can also happen because of:

Radiation

Chemicals such as tar from

cigarette smoke

Mutations

may have no effect. For example, the protein that a mutated gene produces may

work just as well as the protein from the non mutated gene.

Mutations

may sometimes be helpful but they re often harmful. For example, haemophilia is

an inherited disorder that stops blood from clotting properly. It is caused by

a mutated gene.

hh

Genes can

be switched on and off. In any one cell, only some of the full set of available

genes are used. Different types of cells produce different ranges of proteins.

This affects the functions they can carry out. For example, only pancreas cells

switch on the gene for making insulin.

The big picture

Crossing over and independant assortment

Mutation