Function of RNA

RNA is different DNA in the following ways:

RNA is usually single stranded

The sugar in RNA is ribose

It contains Uracil

This section will briefly mention three major types of RNA and the function of RNA:

Messenger RNA (mRNA)

mRNA carry copies of genetic instructions that are used to assemble amino acids into proteins/polypeptide chains.

Transcribes code from DNA strand

Transfer RNA (tRNA)

tRNA transfers each amino acid to the ribosome.
tRNA acts as the interpreter of the mRNA code

Ribosomal RNA (rRNA)

rRNA are combined with proteins to form ribosomes.

It is a component of the ribosome

Ribosome is the site of protein synthesis

Ribosome holds the mRNA and tRNA close together
It positions each new amino acid for addition to polypeptide
It combines each new amino acid together

Ribosome has two parts: large subunit and small subunit

There are three active sites

A-site - (aminacyl tRNA site)

binds new incoming tRNA

P-site - (peptidyl tRNA site)

position of the tRNA holding the growing chain
where tRNA adds its amino acid to the growing polypeptide chain

E-site - (exit site)

tRNA is released and it exits ribosome

The notes above detailed the function of RNA. The section below covers protein synthesis:

Protein Synthesis requires two important stages: Transcription and Translation

Transcription: RNA synthesis

RNA molecules are formed by using DNA as a template

Translation: Polypeptide synthesis

RNA molecules assemble amino acids into polypeptide

Transcription

During this stage, RNA polymerase binds and separates DNA into separate strands. The RNA polymerase then uses one DNA strand as a template in which nucleotides are assembled into a RNA strand

Occurs in the nucleus
mRNA is synthesized in transcription
Transcription occurs in three stages: initiation, elongation, termination

Initiation

Initiation requires a promoter

Promoter is a special sequence of DNA that signals the beginning of a gene to be transcribed

RNA polymerase binds to the promoter
RNA polymerase separates the DNA strands
Then the RNA polymerase adds each new complementary nucleotide to a new mRNA strand (elongation)

Elongation

The RNA polymerase moves down along the DNA template strand, unwinding and separating the DNA strand and adding new nucleotides to the new mRNA strand

RNA polymerase typically unwinds about 10 to 20 base pairs at a time.
RNA polymerase can only add nucleotides to the 3’ end
mRNA only synthesizes in the 5’ to 3’ direction

The RNA transcript is anti-parallel to the DNA template strand

Termination

Elongation will continue until a specific base sequence is reached

The terminator is the base sequence that signals the end of transcription

In prokaryotes, transcription will end at the terminator.
In eukaryotes, transcription can continue past the terminator sequence

RNA Processing

In eukaryotes, the pre-mRNA that was synthesized in the end of transcription needs some modifications.

A 5’ cap is attached to the 5’ end

5’ cap is a modified GTP
5’ cap provides stability and protects the 5’ end from degradation

It is also an attachment point for ribosome

Poly A tail is attached to the 3’ end of the mRNA

Poly A tail is a sequence of about 100 to 200 adenine nucleotides

It provides stability and protects the 3’ end from degradation

RNA splicing – portions of pre-mRNA are removed and then the remaining portions of the mRNA are spliced together

Introns are removed

Introns are the non-coding regions that gets removed

Exons are spliced together

Exons are coding regions that do not get removed

The processed mRNA leaves the nucleus and onto the cytoplasm

Translation

Essentially, information from the mRNA is used to produce polypeptides

Translation will begin when an mRNA molecule in the cytoplasm binds to a ribosome. The ribosome will move through the mRNA molecule. As each codon of the mRNA goes through the ribosome, a specific amino acid is brought by a tRNA molecule to the ribosome. The first codon to start translation is AUG.

Three stages: Initiation, Elongation and Termination

Initiation

Small ribosomal subunit will bind to the 5’ cap of the mRNA
The first codon that enters the A site is AUG

A tRNA will bring an anticodon (anticodon UAC) and is carrying an amino acid (methionine).

The first amino acidismethionine, which is attached to the mRNA by hydrogen bonds

This causes the large subunit to attach to the small subunit that is bound to the mRNA, which forms a complete ribosome.

Elongation

In this stage, the ribosome moves along the mRNA.

The start codon shifts to the P-site, and a new codon enters the A site

The complementary anticodon from the tRNA hydrogen bonds with the codon in the A-site

The large subunit catalyzes two reactions:

It breaks the bond between the tRNA in the P-site and its amino acid.

The amino acid chain separates from the tRNA in P-site, and then it is held by the tRNA in the A-site

It is a peptide bond that forms between the amino acid (separated from the tRNA in the P-site) and the amino acid that is on the tRNA in the A site.

The ribosome will again move along the mRNA in the 5’ to 3’ direction

A new mRNA codon enters the A-site
The mRNA codon and tRNA that was in the A-site will shift to the P-site
The tRNA in the P-site shifts to E-site

This tRNA separates from the mRNA and then exits the ribosome

Essentially, as each new codon (and anticodon) arrives to the A-site, the polypeptide chain will elongate by one new amino acid

Termination

Translation will end when a stop codon enters the A-site

Stop codons are: UAA, UGA, UAG

The stop codon binds a protein release factor

Release factor causes a water molecule to attach to the chain and hydrolyzes the H-bonds between the mRNA and tRNA molecules

Hydrolysis causes the separation of the mRNA and tRNA

The polypeptide separates from completely from the ribosome
The ribosome subunits separate from each other

End of notes on function of RNA

(Function of RNA and Protein Synthesis)

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