[genome] DNA, protein synthesis & mutation

[ BBC Bitesize 'Inheritance, variation, and evolution ⇢ Variation' page 5-10 ]

DNA

Deoxyribonucleic acid - the material inside the nucleus of cells, carrying the genetic information of a living being.

DNA Structure

• DNA is a polymer, a large and complex molecule

• made up of 4 different nucleotides

• made up of two strands forming a twisted ladder structure called a double helix

Chromosomes

Thread-like structures made of DNA molecules, containing many different genes. Normal body cells have 46 of them, arranged in 23 pairs.

Gene

The basic unit of genetic material inherited from parent cells. A section of a chromosome that contains the code for the production of a particular protein, thus a particular trait.

Genome

The complete set of DNA found in an organism. The whole human genome has now been studied and this will have great importance for medicine in the future

Studying the Human Genome Enables us to:

• search for genes linked to different types of disease

• understand inherited disorders and their treatment

• trace human migration patterns from the past

Nucleotide

The units or molecules of which DNA or RNA is composed.

Nucleotide Structure:

• alternating sugar (pentagon)

• phosphate group (circle)

• one of the four different bases attached to the sugar (rectangle)

DNA Contain 4 Bases:

• T → thymine

• A → adenine

• G → guanine

• C → cytosine

T - A

Thymine pairs with adenine.

G - C

Guanine pairs with cytosine.

Triplet / Triplet Code

A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.

Protein

Organic compound made up of amino acid molecules.

Protein Synthesis

The production of proteins from amino acids, which happens in the ribosomes of the cell.

1st Stage of Protein Synthesis:

DNA code for protein remains in nucleus, but a copy of it (mRNA) moves from the nucleus to the ribosomes in the cytoplasm, through a nuclear pore, where the protein will be synthesised.

2nd Stage of Protein Synthesis:

Carrier molecules (tRNA) attach to specific amino acids and bring them to the mRNA. The tRNA binds to a specific part of the mRNA, in a specific order to form a protein molecule. There is a particular version of tRNA for each type of amino acid.

3rd Stage of Protein Synthesis:

More carrier molecules will bring amino acids to the mRNA until a triplet is formed, where enzymes will link the amino acids together and tRNA is released.

4th Stage of Protein Synthesis:

This leaves a protein (polypeptide), which folds into a unique shape so it is able to carry out it’s function.

Not all parts of DNA code for proteins. Non-coding parts of DNA can…

… switch genes on and off, so variations in these areas of DNA may affect how genes are expressed.

Mutation

A random and spontaneous change in the structure of a gene, chromosome or number of chromosomes.

Causes of Mutations:

• can occur continuously and can be spontaneous

• ionising radiation

• chemical mutagens - such as tar from cigarette smoke

Mutations can switch genes on or off, leading to…

… synthesis of different or faulty proteins. Most do not alter the protein, or only alter it slightly so that its appearance or function is not changed.

An enzyme may no longer…

… fit the substrate binding site or a structural protein may lose its strength.