Genes and Genetic Code

Nucleus (Structure):

Spherical between 10 and 20 µm across

Nuclear Envelope

• Double membrane, controlling the movement of materials in and out

Nuclear Pores

• Allow passage of large molecules in and out (e.g. mRNA) doesn’t allow Chromosomal DNA out of the nucleus

Nucleoplasm

• Jelly like material, makes up the bulk of the Nucleus

Chromosomes

• Protein bound DNA

Nucleolus

• Small spherical region

• Manufactures rRNA (ribosomal RNA) and assembles Ribosomes

Nucleus (Function):

Controls the activity of the cell:

Production of:

• tRNA,

• mRNA,

• rRNA

Stores the DNA

Genes:

• The section of DNA where a gene is found is called a locus

• Section of DNA (varying in length) that codes for a chain of amino acids

• Only 4 bases present in DNA

• 3 bases code for an amino Acid (Triplet Code)

  • 43 bases allows for 64 possible combinations

Amino Acids:

Amino Acids

• Only 20 different amino acids regularly occur in proteins 

  • Every organism on Earth is a combination the same 20 amino acids

• Every amino acid is coded for by 3 bases (triplet code)

• 64 possible means most amino acids are coded for by more than one triplet. This is known as a ‘Degenerative code’

Codons:

• The mRNA triplet code that will code for an amino acid (or the stop codon)

• All protein coding sequences Exons begin with the triplet code AUG (start codon) 

  • This means that there is a large amount of DNA that does not code for any amino acids (Introns)

• There are 3 triplet codes that will code for the Stop codon, which will signal the end of a peptide chain. These are UAA, UGA and UAG

99% of DNA does not code for proteins

Amino Acids:

• Only 20 different amino acids regularly occur in proteins

  • Every organism on Earth is a combination the same 20 amino acids

• Every amino acid is coded for by 3 bases (triplet code)

• 64 possible means most amino acids are coded for by more than one triplet. This is known as a ‘Degenerative code’

The code is ‘non-overlapping’, in other words each of the base in the sequence is read only once. e.g. read 123, 456 not 123, 234