Biology notes- DNA
Structure of DNA
Individual nucleotides of DNA are made up of three components:
A sugar called deoxyribose
A phosphate group
An organic base belonging to one of two different groups:
A single ring bases- cytosine and thymine
Double ring bases- adenine and guanine
The sugar, phosphate group and organic base are combined as a result of condensation reactions, to give a single nucleotide.
DNA is made up of two strands of nucleotides. Each of the two strands is extremely long, and they are joined together by hydrogen bonds formed between certain bases.
In its simplified form, DNA can be thought of as a ladder, in which the ...view middle of the document...
The triplet code
What is a gene
Genes are sections of DNA that contain the coded information for making polypeptides.
The coded information is in the form of a specific sequence of bases along the DNA molecule.
Polypeptides combine to make proteins and so genes determine the proteins of an organism
A gene is a sequence of DNA bases that determines a polypeptide and a polypeptide is a sequence of amino acids.
The triplet code
Only 20 amino acids regularly occur in proteins
Each amino acid must have its own code of bases on the DNA
Only four different bases are present in DNA
As there are 64 possible codes and only 20 amino acid it follows that some amino acids have more than one code.
In eukaryotes much of the nuclear DNA does not code for amino acids.
These sections are called introns and can occur within genes and as multiple repeats between genes.
Features of the triplet code
A few amino acid have only a single triplet code
The remaining amino acid have between two and six triplet codes each
The code is known as the degenerate code because most amino acids have more than one triplet code
The triplet code is always read in one particular direction along the DNA strand
The start of the sequence is always the same triplet code, this codes fro the amino acid methionine. If this methionine molecule does not form part of the final polypeptide it is later removed.
Three triplet codes do not code for any amino acid. These are called stop codes and mark the end of a polypeptide chain
The code is non overlapping i.e. Each base in the sequence is only read once
The code is universal
DNA and chromosomes
In prokaryotic cells, e.g. Bacteria the DNA molecules are smaller, and form a circle and are not associated with protein...