Topic 3.5: Transcription and translation

3.5.1 Compare the structure of RNA and DNA

3.5.2 Outline DNA transcription in terms of formation of an RNA strand complementary to the DNA strand by RNA polymerase

The DNA transcription process aims to create mRNA (messenger RNA - carries the codons).

• The DNA helix is unwound using RNA polymerase at the position of the gene
• RNA ribonucleotides triphosphate aligns with their complementary pairs. (Uracil replaces Thymine)
• Once RNA polmerase has full synthesized, the RNA polymerase will detach itself form the DNA molecule and the double helix will reform
• Transcription occurs in the nucleus.

3.5.3 Describe the genetic code in terms of codons composed of triplets of bases.

The genetic code is the set of rules by which information encoded in mRNA sequences is converted into proteins (amino acids sequences) by living cells

• Codons are a triplet of bases which encodes a particular amino acid
• As there are four bases, there are 64 different codon combinations 
• The order of the codon determines the amino acid sequence for the protein
• The coding region always starts with the codon (AUG) and end with a stop codon.

The genetic code is universal and degenerate. Every living animal and plants (except a few) uses the exact same coding system. This pronounces the genetic code as universal. It is degenerate because there are a lot of codons which map to the same amino acid. This has no effect seen whatsoever.

3.5.4 Explain the process of translation, leading to polypeptide formation

Translation is the process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids in a polypeptide chain.

• Ribosomes bind to mRNA  in the cell's cytoplasm and move along the mRNA molecule in a 5' - 3' direction until it reaches a start codon (AUG)
• Anti-codons on tRNA molecules align opposite appropriate codons according to complementary base pairing 
• Each tRNA carries a specific amino acid
• Ribosome catalyse the formation of peptide bonds between adjacent amino acids (via a condensation reaction)
• The ribosome moves along the mRNA molecule synthesizing a polypeptide chain until it reaches a stop codon, at this point translation stops and the polypeptide chain is relesed

3.5.5 Discuss the relationship between one gene and one polypeptide

One gene is transcribed and translated to produce one polypeptide

A gene sequence is converted into a polypeptide sequence via the processes of transcription (making an mRNA transcript) and translation (polypeptide synthesis)

The universality of the genetic code means all organisms show the same relationship between genes and polypeptides (indicating a common ancestry and allowing for transgenic techniques to be employed)

The exceptions to this rules are:

• Genes that are encoding for tRNA and rRNA, aren't creating polypeptides.
• A single gene may code for multiple polypeptides if alternative splicing occurs.