Question

describe in words the process of protein synthesis

Answer 1

PROTEIN SYNTHESIS

It is an important biological process, occurring inside cells, which will balance the loss of cellular proteins by the production of new proteins. Proteins perform many functions as enzymes, structural proteins or hormones and therefore, the yy ar crucial biological components. Protein synthesis is a very similar process for both prokaryotes and eukaryotes but there are some distinct differences.

This protein synthesis is divided broadly into two phases they are ,transcription and translation.

During TRANSCRIPTION, a section of DNA encoding a protein, which is gene, is converted into a template molecule called MESSENGER RNA. This conversion is carried out by enzymes, known as RNA polymerases, in the nucleus of the cell . In eukaryotes, this messenger RNA , is initially produced in a premature form which undergoes post-transcriptional modifications to produce mature mRNA.The mature mRNA is exported from the nucleus via nuclear pores to the cytoplasm of the cell for translation to occur.

During TRANSLATION, the mRNA is read by ribosomes which use the nucleotidesequence of the mRNA to determine the sequence of amino acids. The ribosomes catalyse the formation of covalent peptide bonds between the encoded amino acids to form a polypeptide chain

Following the translation , polypeptide chain must fold to form a functional protein. In order to adopt a functional three-dimensional shape, the polypeptide chain must first form a series of smaller underlying structures called secondary structures. The polypeptide chain in these secondary structures then folds to produce the overall 3D tertiary structure.

When correctly folded, the protein can undergo further maturation through different post-translational modifications. Post-translational modifications can alter the protein's ability to function, where it is located within the cell and the protein's ability to interact with other proteins.

TRANSCRIPTION

This transcription is consisting of 3 stages: initiation, elongation & termination.

Initiation

Transcription is catalysed by the enzyme RNA polymerase. It attaches to and moves along the DNA molecule until it recognises a promoter sequence. There may be multiple promoter sequences in a DNA molecule. They indicate transcription start sites.It then unwinds a portion of the DNA double helix, exposing the bases on each of the singular DNA strands. One of the strands will act as a template to create the new mRNA strand.The incoming ribonucleotides are used by RNA polymerase to form the mRNA strand. It does this using complementary base pairing . The RNA polymerase then binds these ribonucleotides together by phosphodiester bonds.

Next is, Elongation

It is the process by which ribonucleotides are added to the template strand, enabling growth of the mRNA transcript. The mRNA transcript is made in a 5’ to 3’ direction.

Then,Termination

Elongation will continue until the RNA polymerase encounters a stop sequence. At this point, transcription will stop and RNA polymerase will release DNA template.

Next we can look at, RNA processing

The mRNA which has been transcribed up to this point is referred to as pre-mRNA. Processing must occur to convert this into mature mRNA.

Processing include, caping, tailing and splicing.

Capping

In caping ,addition of a methylated guanine cap confers protection to the mRNA. This is necessary as RNA is much more unstable than DNA. It involves:

• *Addition of a methylated guanine
• *Occurs at the 5’ end of the mRNA transcript
• *Protects the mRNA from degradation

Next is, Polyadenylation or tailing

Here , addition of a polyA tail confers protection to the mRNA. This is necessary as RNA is much more unstable than DNA. It involves:

• *Endonucleases* recognise a specific sequence along the mRNA transcript and cleaves it there
• *Several adenine nucleotides are added to the transcript by the enzyme polyA polymerase
• *Occurs at the 3’ end, protecting the mRNA from degradation

Next is,splicing

This Splicing allows one genetic sequence to code for different proteins. This conserves genetic material. It involves:

• *Introns (non-coding sequences) are removed via spliceosome excision
• *Exons (coding sequence) are then joined together by ligation
• It is sequence dependent and occurs within the middle of transcript.

This will allow many proteins.

TRANSLATION

Translation occurs in 3 phase,

They are

1. Initiation: The ribosome assembles around the target mRNA. The first tRNA will be attached at start codon.
2. Elongation: The tRNA transfers an amino acid to the tRNA corresponding to the next codon. The ribosome then moves (translocates) to the next mRNA codon to continue the process, creating an amino acid chain.
3. Termination: When a peptidyl tRNA encounters a stop codon, then the ribosome folds the polypeptide into its final structure.In the case of prokaryotes which are unicellular, translation occurs in the cytosol,where the medium and small subunits of the ribosome bind to the tRNA.

   In the case of eukaryotes, translation occurs in the cytosol or across the membrane of the endoplasmic reticulum in a process called co-translational translocation. In co-translational translocation, the entire ribosome/mRNA complex binds to the outer membrane of the rough endoplasmic reticulum and the new protein is synthesized and released into the ER; the newly created polypeptide can be stored inside the ER for future vesicletransport and secretion outside the cell, or will be secreted immediately.

Protein synthesis has a key role in disease as changes and errors in this process, through underlying DNA mutations or protein misfolding, are often the underlying causes of a disease. DNA mutations change the subsequent mRNA sequence, which then alters the mRNA encoded amino acid sequence. Mutations can cause the polypeptide chain to be shorter by generating a stop sequence which causes early termination of translation. Alternatively, a mutation in the mRNA sequence changes the specific amino acid encoded at that position in the polypeptide chain. This amino acid change can impact the proteins ability to function or to fold correctly . Misfolded proteins are often implicated in disease as improperly folded proteins have a tendency to stick together to form dense protein clumps. These clumps are linked to a range of diseases, often neurological, including Alzheimer's diseaseand Parkinson's disease.