Genes, Transcription & Translation

A Gene - A length of DNA that codes for one or more polypeptides. It is located at a specific loci on the chromosome

DNA - Deoxyribose sugar, Nitrogenous base and phsophate groups

Triple code - 3 bases = a codon

Degenerate Code - amino acids coded for by more than 1 combination - some for stop or start

Codes are very similar but not identical in all organisms

... a copy of DNA is made using one strand of DNA as a template

1) DNA dips into the nucleus

2)Hydrogen bonds between base pairs break so the DNA unwinds and unzips

3) Activated RNA nucleotide bond to the exposed complementary base pairs on the template strand using RNa polymerase (emzyme is a catalyst)

4) The mRNA strand produced is complementary to the template strand of DNA and so is a copy of the coding strand of DNA

5) The mRNA is released fromt he DNA and passes through the nuclear pore into the cytoplasm, into a ribosome

RNA nucleotides are activated by the two extra phosphoryl groups they contain which release energy for bonding adjacent nucletides (4 types ATP GTP CTP and UTP)

...the assembly of polypeptides at the ribosome into the sequence dictated by the seqence of codons on the mRNA - the genetic code is translated into a sequence of amino acids

• Ribosomes are assembled in the nucleous from rRNA and proteins. It is made of two subunits with a groove into which the length of mRNA fits.
• The ribosome can then move along the mRNA, which slides through the ribosomal groove, reading the code and assembling the amino acid in the correct order to make a functioning protein

The sequence is critical because:

• It forms the primary structure of a protein
• This determines the tertiary structure, which is what allows proteins to function
• If altered, the protein can no longer function so effectively

tRNA:- made in the nucleus and passes into the cytoplasm. these are lengths of RNA that fold into hairpin shapes with 3 exposed bases at one end where a specific amino acid can bind. On the other end are 3 unpaired nucleotide bases, an anticodon, which can bind temporarily with its complementary codon on the strand of mRNA

1) mRN binds to a ribosome - two codons attach to the small subunit and are exposed to the large subunit. The first mRNA codon is always AUG. Using ATP energy, a tRNA with methionine and the anticodon UAC forms hydrgen bonds with this codon

2) A 2nd tRNA with a different amino acid binds to the second exposed codon with a complementary codon. Peptide bonds form between adjacent amino acids - catalysed by an enzyme in the small subunit

3) Ribosomes move along mRNA, reading the next codon. A 3rd tRNA brings an amino acid, and a peptide bond forms, The first tRNA leanves and is able to collect another amino acid

4) The polypeptide grows until a stop codon is reached - ther are no corresponding tRNAs for UAA,UAC or UGS so the chain is completed