Chromosomes are thread like structures composed of genes found inside the nucleus of plant and animal cells.

A human cell nucleus contains 23 pairs of chromosomes or 46 chromosomes in each of their cells. Other organisms are different e.g. Chickens have 78 chromosomes, dogs have 78 chromosomes and cats have 38 chromosomes. Half of your chromosomes are inherited from your father and the other half are from your mother.

Almost everyone’s DNA is unique. The only organisms which share identical DNA are identical twins and clones, organisms that are identical to their parents e.g. Dolly the sheep.

DNA is arranged into sections. Short sections of a chromosome or DNA that code for a characteristic, for example eye colour, are called genes. The code that a gene contains causes specific proteins to be made. The particular proteins that are made will determine a cell’s function.

the protein Haemoglobin is found in red blood cells and is needed to bind oxygen so it can be transported around the body.

The combination of genes in an organism will control how an organism functions, and what it looks like e.g. your genes determine you blood type and whether you have freckles or dimples…

What does the structure of DNA look like ?

DNA is made of two strands. The two strands are joined together by bases. The strands are then twisted together forming a double helix – looks like a twisted ladder.

DNA is made of lots of small units called nucleotides, which are joined together. This means that DNA is a polymer.

Each nucleotide is made of a sugar (called deoxyribose), a phosphate group and a base.

There are four different types of nucleotide base in DNA. The four bases are:

•Adenine = A

•Thymine = T

•Guanine = G

Cytosine = C

To hold the two strands of DNA together a base from strand will bond with a base on the other strand. This forms a base pair. The base pair always bond together in the same formation – this is known as complimentary base pairing.

•adenine always bonds with thymine (A-T).

•cytosine always bonds with guanine (C-G).

Protein Synthesis.

DNA is present in the nucleus of the cell and contains the genetic code to make proteins. However, DNA is too big to leave the nucleus to travel to the ribosomes in the cytoplasm where the proteins are made. Instead a copy of the DNA is made called mRNA (messenger RNA). This is like a single strand of DNA and so is small enough to move out of the nucleus.

Transcription.

Transcription is the first process in protein synthesis and it is where the messenger RNA is made.

1.DNA around the gene unzips so that both strands are separated.

2.One of the DNA strands the acts as a template.

3.Complimentary bases attach to the strand being copied (A – U, U – A, C – G and G – C). This forms a strand of messenger RNA.

4. When complete, the strand of mRNA detaches itself from the DNA template.

5.The DNA then zips back up.

6.The small messenger RNA strand can now travel out of the nucleus. It travels to subcellular structures called ribosomes in the cytoplasm where the protein is made.

Proteins are made from amino acids. Different amino acids join together to form different proteins. The order of the nucleotides bases in the DNA determines the type and order of the amino acids and so which proteins are produced.

Proteins are made by the process called translation.

Proteins are made by a process called translation. The mRNA attaches to a ribosome. The nucleotide sequence is interpreted and a new protein is made.

•The ribosome “reads” the nucleotides on the mRNA in groups of three. These groups are called base triplets or codons. Each triplet codes for a different amino acid  e.g. GCU codes for a different amino acid to ACG.

•The ribosome continues to “read” the triplet code, adding more and more amino acids.

•The amino acids join together in a chain. This is a protein.

The sequence of amino acids will also determine how the protein will fold. Each type of protein has a different shape. This is important for the protein’s function. Many types of protein can be produced, including enzymes and hormones The sequence of amino acids will also determine how the protein will fold. Each type of protein has a different shape. This is important for the protein’s function. Many types of protein can be produced, including enzymes and hormones

Amino Acid-changing molcues which make protein