3.4.1 DNA, genes and chromosomes

The formation of a chromosome

-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.

Comparisons between the way DNA is stored in prokaryotic cells vs eukaryotic cells:

Eukaryotic cells:

• Store DNA in nucleus

• Linear DNA

• Longer DNA

• Associated with histones

• contains introns

Prokaryotic cells:

• DNA is stored in the cytoplasm

• Circular DNA

• Shorter DNA

• not associated with histones

• does not contain introns

Gene:

A gene is a base sequence of DNA which codes for:

• the amino acid sequence in a polypeptide

• a functional RNA (including ribosomal RNA and tRNAs)

-A gene occupies a fixed position, called a locus, on a particular DNA molecules

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

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

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

• Exons: A base sequence of a gene that codes for amino acid sequences in a polypeptide.

• Introns: does not code for amino acid sequences (non-protein-coding regions)

In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are, for example, non-coding multiple repeats of base sequences between genes.

Alleles:

-An allele is a different version or variant of the same gene, differing slightly in their DNA base sequence.

-Homologous chromosomes are a pair of chromosomes with genes for the same features at the same loci.

The genetic code:

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

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

The genetic code has the following features: 

• Universal - Each DNA triplet codes for the same amino acid in all organisms

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

• Degenerate - Multiple codons can code for the same amino acid (e.g. ACA and ACG both code for cysteine).

Interpretation

-The genetic code can be summarised into a table as shown below. The code is often shown as mRNA bases rather than DNA, so the base uracil (U) is shown instead of thymine (T).

-Each amino acid is coded for by a sequence of three mRNA bases known as a codon (similar to a DNA triplet).

-We can see from the table above, that many amino acids are coded for by multiple codons. For example, the amino acid phe (phenylalanine) is coded for by codons UUU and UUC.