D1.2 Protein Synthesis

Transcription

The process in which a DNA sequence is used to synthesise an mRNA molecule with the help of RNA polymerase. Takes place in the nucleus

mRNA

A single stranded RNA molecule that transfers the information in DNA from the nucleus into the cytoplasm

Steps of transcription

RNA polymerase binds to a site on the DNA at the start of a gene

As RNA polymerase moves along the gene, it separates the two strands DNA into two single strands

RNA polymerase synthesises a complementary strand of mRNA by matching RNA nucleotides to the antisense until it reaches a termination signal

The sugar-phosphate groups of these RNA nucleotides are bonded together to form the sugar-phosphate backbone of the mRNA molecule

RNA polymerase detaches from the DNA molecule and the double helix reforms

Antisense strand

The non-coding DNA strand of a gene, which serves as the template for mRNA synthesis during transcription

Sense strand

The DNA strand that has the same sequence as the resulting RNA molecule, except that uracil substitutes for thymine in RNA

Gene expression

The process by which information carried by a gene is turned into an observable effect on an organism

Translation

The process of using the sequence of a mRNA molecule to synthesise a sequence of amino acids. Takes place in the cytoplasm

Polypeptide

Short chains of amino acids linked by peptide bonds

Subunits of ribosomes

mRNA binds to the small subunit, 2 tRNA molecules can bind to the large subunit simultaneously

Steps of translation

The small ribosomal subunit binds to 5’ of the mRNA and moves along it until it reaches the start codon (AUG)

The corresponding tRNA molecule with the specific amino acid binds to the codon

A second tRNA molecule pairs with the next codon in the A site

The amino acids are covalently attached via a peptide bond

The ribosome moves along by 3 base pairs, and the tRNA that has given up its amino acid is released

The process repeats until it reaches a stop codon (UAA, UAG, UGA)

Codons

A sequence of 3 mRNA bases that codes for a specific amino acid

Anticodon

A sequence of 3 tRNA bases that are complementary to a codon

How many amino acids are there?

20

How many combinations of different codons are there?

64

Why is genetic code said to be degenerate?

Multiple codons can code for the same amino acids, this degenerate nature of the genetic code limits the effect of mutations

Gene mutation

A change in the sequence of bases in a DNA molecule, may result in a new allele

Features of mutations

They occur randomly as a result of copying errors in DNA replication. Most genes are harmful and neutral but some are beneficial

Point mutations

Mutations where one base in the DNA sequence is altered

Example of a point mutation

Sickle cell disease. In the hemoglobin gene a DNA triplet is changed from GAG to GTG on the coding strand. As a result valine replaces glutamic acid, causing a distortion in red blood cells. These cells have a limited oxygen-carrying ability

Promoter

A sequence of DNA that acts as a binding site for RNA polymerase

Transcription factors

Proteins that regulate gene expression by initiating transcription. They bind directly to the promoter

Introns

Non-coding sequences of DNA found within genes of eukaryotic organisms

Telomeres

Regions of repeated nucleotide sequences at the end of chromosomes that provide protection during cell division. Without them, chromosomes would be shorter after every cell division

Exons

Coding sequences of DNA that carry information for protein synthesis

Benefits of post-transcriptional modification

Helps prevent degradation, increases efficiency of protein synthesis

Pre-mRNA

The first form of RNA created through transcription. It contains both introns and exons

Mature mRNA

The processed form of mRNA in eukaryotic cells, containing a 5′ cap, 3′ poly-A tail, and spliced exons

3 steps of post-transcriptional modification

A methylated cap is added to the 5' end to protect against degradation by exonucleases

A poly-A tail (long chain of A nucleotides) is added to the 3' end for further protection and to help the transcript exit the nucleus

Splicing

Splicing

Introns are removed and the remaining exons are joined together

Alternative splicing

Exons from the same gene are joined in different combinations, leading to the formation of different polypeptides

Modification of polypeptides

Protein folding (the physical process by which a polypeptide folds into its functional 3D structure)

Example of modification of polypeptides

Insulin, when first synthesised it is in the form of pre-proinsulin. It is then modified by an enzyme, transforming it to proinsulin. The proinsulin folds and disulfide bonds form between different sections of the polypeptide

Proteasomes

Break down unneeded, damaged, or misfolded proteins by breaking the peptide bonds between amino acids, process is called proteolysis. The proteins are tagged with ubiquitin