Protein synthesis (Edexcel GCSE Biology): Revision Notes

Protein synthesis

Protein Synthesis

Transcription:

Proteins are made in the cell cytoplasm by subcellular structures called ribosomes DNA is found in the cell nucleus and cant move out of it as its too big so cell needs to get information from DNA to ribosome in the cytoplasm

Molecule called messenger RNA (mRNA), like DNA it is a polymer of nucleotides but its shorter and only a single strand & uses uracil instead of thymine as a base

• RNA polymerase is the enzyme involved in joining together RNA nucleotides to make Mrna, here's how:
• DNA unwinds, and the two strands separate.
• mRNA (messenger RNA) nucleotides match up with their complementary DNA bases on one strand, forming a strand of mRNA that is a mirror image of the DNA template.
• The newly formed mRNA strand detaches and exits the nucleus, moving into the cytoplasm.
• Once made, the mRNA molecule moves out of the nucleus pores & joins with a ribosome

Translation:

Once Mrna is bound to a ribosome the protein can be assembled

1. The order in which the amino acids are brought to the ribosome matches the order of the base triplets in mRNA (codons)

• The ribosome reads the mRNA sequence in sets of three bases (codons), each specifying an amino acid.

2. Part of the tRNA's structure is called an anticodon which is complementary to the codon for the amino acid. The pairing of the codon and anticodon makes sure that the amino acids are brought to the ribosome in the correct order

• Amino acids are brought to the ribosome by another RNA molecule called transfer RNA (tRNA)
• The mRNA attaches to a ribosome in the cytoplasm.
• tRNA (transfer RNA) molecules carry the corresponding amino acids to the ribosome, where they are added to the growing polypeptide chain.

3. Amino acids are joined together (bonds) by the ribosome which makes a polypeptide (protein)

• Once the chain is complete, the polypeptide folds into a unique three-dimensional shape, forming a functional protein.

The triplet code

4 different bases in the structure of DNA Adenine, Thymine, Cytosine, Guanine In RNA thymine is replaced with uracil The sequence of bases determines the sequence of amino acids in the protein

• 3 bases in DNA code for 1 amino acid in final protein (codon)

Non-coding DNA affects the binding of RNA polymerase

Before any transcription can happen, RNA polymerase has to bind to a region of non-coding DNA in front of a gene If a mutation happens in this region of DNA then it could affect the ability of RNA polymerase to bind to it which may make it easier to bind to or more difficult

• How well RNA polymerase can bind to this region of DNA will affect how much mRNA is transcribed & therefore how much of the protein is produced
• Depending on the function of the protein, the phenotype of the organism may be affected by how much of it is made So genetic variants in non-coding regions can still affect the phenotype of an organism, even if they don't code for proteins themselves