DNA, RNA AND CHROMOSOMES

eukaryotic cells DNA

In the nucleus of eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones. Together a DNA molecule and its associated proteins form a chromosome.

prokaryotic cells DNA

In prokaryotic cells, DNA molecules are short, circular and not associated with proteins.

mitochondria and chloroplast DNA

The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with protein

GENES

A gene is a section of DNA that contains sequence of DNA bases that codes for either a polypeptide or functional RNA.

what does a gene occupy

A gene occupies a specific fixed position, called a locus, on a particular DNA molecule.

triplet

The code is in the form of a specific sequence of bases along the DNA molecule.

Different polypeptides have different number and order of amino acids.

It’s the order of bases in a gene that determines the order of amino acids in a particular polypeptide.

Each specific amino acid is coded for by a sequence of three DNA bases in a gene called a triplet or codon.

Functional RNA

Genes that don’t code for a polypeptide code for functional RNA instead.

Functional RNA is RNA molecules other than mRNA, which perform special tasks during protein synthesis

ALLELES

An allele is an alternative form of a gene. 

A gene can exist in more than one form. These forms are called alleles. One of two (usually one dominant and one recessive) or more DNA sequences occurring at a particular gene locus.

how is an allele coded ?

Each individual inherits one allele from each of its parents, they can be the same allele or 

different and when they are different they code for a different polypeptide.

The order of bases in each allele is slightly different, so they code for different amino acids and 

therefore a different polypeptide and different protein. 

GENOME

The complete set of DNA (genetic material) in an organism. The full genome is present within every cell of an organism, but not every gene is expressed in every cell. Which genes are expressed depends on the cell type.

PROTEOME

The full range of proteins that a cell is able to produce / DNA is able to code for/ made by an organism. The proteome is usually larger than the genome of an organism.

This due to the large amount of post-translational modification of proteins (often in the Golgi apparatus) and each gene is also capable of producing multiple different proteins via alternative splicing.

THE GENETIC CODE

The genetic code is the sequence of base triplets (codons) in mRNA which code for specific amino acids. 

FEATURES OF THE GENETIC CODE

NON-OVERLAPPING - Base triplets don’t share their bases. Each triplet is read in sequence and is separate from the triplet before and after.

DEGENERATE - There are more possible combinations of triplets than there are amino acids (20 amino acids but 64 possible triplets as 4³ is 64 and there are 4 bases each read as a triplet).

UNIVERSAL - The same specific base triplets code for the same amino acids in all living things. E.g. UAU codes for tyrosine in all organisms.

NON-CODING DNA 

Between genes in eukaryotes, there are non-coding sequences made up of multiple repeats of base sequences.

A lot of the nuclear DNA doesn’t code for polypeptides. Some genes code for functional RNA.

introns and exons

INTRONS - section of genes in DNA that do not code for amino acids. 

EXONS - triplet/base/nucleotide  sequence of a gene that do code for amino acids. 

Prokaryotic DNA doesn’t have introns. 

Eukaryotic DNA also contains regions of multiple repeats outside of genes.

These are DNA sequences that repeat over and over. 

CHROMOSOME STRUCTURE

• Chromosomes are only visible when a cell is dividing. For the rest of the time they are widely dispersed throughout the cell. 

• When visible they appear as 2 threads joined at a point - a chromatid. DNA has already replicated to give two identical DNA molecules. 

• The DNA is held in position by histone proteins. 

• DNA is around 2m in every human cell and is tightly coiled and folded.

the order of the chromosome structure

1. DNA double helix molecule 

2. Combined with histone proteins, DNA wound around it to fix it in position 

3. DNA-protein complex is coiled to form loops 

4. Loops coil and pack together, condensing into a single chromosome

HOMOLOGOUS CHROMOSOMES

A pair of matching chromosomes, one maternal and one paternal