Topic 4: Genetic Information: Genes and Genetic code

DNA molecules are incredibly long and must be tightly packed up to fit within the nucleus of a eukaryotic cell

DNA molecules are wound around proteins known as histones to form a DNA histone complex. These complexes coil further to form chromatin, which helps pack the DNA into chromosomes. Each chromosome contains just a single molecule of DNA.

Eukaryotic cells

• Stores DNA in nucleus

• Contain long, linear DNA molecules

• Associate the DNA with histones

• DNA contains introns

Prokaryotic cells

• Stores DNA in the cytoplasm

• Contain shorter, circular DNA molecules

• Do not associate DNA with histones

• DNA doesn’t contain introns

Eukaryotic cells also contain DNA in their mitochondria and chloroplasts

This DNA like in prokaryotes is short, circular and not associated with histones

Gene

Short section of DNA that codes for a polypeptide (protein)

DNA contains genes

Each gene is located at a specific position along a chromosome known as a locus

In eukaryotes, genes consist of regions known as introns and exons

• Exons - sections of DNA that do code for amino acid

• Introns - sections of DNA that do not code for amino acids

The complete set of genes within a cell is known as the genome.

The full range of proteins that cell is capable of producing is known as the proteome

The genetic code refers to the sequence of bases that code for amino acids.

Scientist found that each amino acid is coded for by a sequence of three DNA bases, known as a triplet

Genetic code has the following features

Universal, non-overlapping, degenerate

Universal

Each DNA triplet codes for the same amino acid in all organisms (with a few minor exceptions)

Non-overlapping

Each base in the DNA sequence is only read once (e.g. CGTATC is read as CGT and ATC)

Degenerate

Most amino acids are coded for by more than one triplet (e.g. ACA and ACG both code for cysteine)

Genetic code can be summarised into a table. The code is often shown as mRNA basis rather than DNA.

Each amino acid is coded for by a sequence of three mRNA bases (codon). Many amino acids are coded for by multiple codons e.g. UUU and UUC code for phe amino acid)

mRNA

Messenger RNA is a type of RNA synthesised during the process of transcription. Its role is to carry genetic information from the DNA to the ribosome (where proteins are made).

Features of mRNA

• Single-stranded, linear molecule

• Contains a base sequence complementary to a DNA sequence

• Contains codons, which are set of three bases that code for an amino acid

• Small enough to leave the nucleus

tRNA

Transfer RNA is a type of RNA used in the process of translation. Its role is to transport amino acids to ribosomes to build up a polypeptide chain.

Features of tRNA

• Single stranded molecule folded into a clover leaf shape

• Uses hydrogen bonds between complementary base pairs to hold its shape

• Contains a specific sequence of three bases at one end, known as the anticodon

• Contains an amino acid binding site at opposite end

• Shorter than mRNA