Structure and functions of RNA.
RNA is single stranded, contains uracil instead of thymine and ribose instead of deoxyribose sugar.
mRNA carries a copy of the DNA code from the nucleus to the ribosome.
Ribosomal RNA (rRNA) and proteins form the ribosome.
Each transfer RNA (tRNA) carries a specific amino acid.
RNA
Like DNA, RNA is a nucleic acid.
Differences between DNA and RNA
DNA
RNA
A DNA molecule is double stranded
An RNA molecule is single stranded
DNA contains the sugar deoxyribose
RNA contains the sugar ribose
DNA contains the base thymine
RNA contains uracil instead of thymine
Types of RNA and their functions
Type of RNA
Messenger RNA (mRNA)
Function
Carries a copy of the genetic code on DNA to the ribosomes
Transfer RNA (tRNA)
Binds to a specific amino acid and transports it to the ribosomes
Ribosomal RNA (rRNA)
Component of the ribosomes
Transcription
Transcription of DNA into primary and mature RNA transcripts to include the role of RNA
polymerase and complementary base pairing.
The introns of the primary transcript of mRNA are non-coding and are removed in RNA splicing.
The exons are coding regions and are joined together to form mature transcript.
This process is called RNA splicing.
Protein synthesis
2 stages:
1.
Transcription
Production of mRNA with a base sequence that depends on the base sequence of DNA.
Transcription occurs in the nucleus.
2. Translation
A protein is produced with an amino acid sequence that depends on the mRNA base
sequence.
Translation occurs on the ribosomes.
Transcription summary
Part of DNA molecule uncoils
this exposes the bases of the gene on one DNA strand
RNA nucleotides form hydrogen bonds with complementary
bases of the DNA strand
RNA nucleotides
U
A
C
G
G
C
U
A
C
A
T
G
C
C
G
A
T
G
Hydrogen
bonds
DNA strand
Bonds form joining nucleotides to make a mRNA molecule - this needs ATP and
an enzyme (RNA polymerase)
Transcription
Transcription is the synthesis of mRNA from DNA.
The base sequence of the mRNA is determined by the base sequence of the DNA and so is complementary
to the DNA base sequence.
RNA polymerase catalyses the synthesis of mRNA.
The RNA formed is called the primary transcript, it consists of regions called exons which actually code
for the protein and other regions called introns that are non-coding regions.
RNA splicing
The introns are cut from the primary transcript and the exons are spliced together to form the mature
transcript.
This mRNA then passes from the nucleus to the ribosomes where it is translated into a protein.
Primary transcript of mRNA
exons
introns
Introns cut out
Exons spliced to form mature
transcript of mRNA
Transcription
1.
The DNA double helix uncoils at the site of the gene
2.
Hydrogen bonds between the bases break and the 2 DNA strands
separate to expose the bases of the gene on one of the strands
3.
mRNA nucleotide bases form hydrogen bonds with the gene DNA
bases
adenine pairs with thymine on DNA
uracil pairs with adenine on DNA
guanine pairs with cytosine and vise versa
4.
While the mRNA nucleotides are held in position, sugar-phosphate
bonds are formed to join them into a mRNA molecule. This requires an
enzyme and ATP.
5.
Hydrogen bonds between the mRNA and DNA bases break to release
the mRNA molecule from the DNA
6.
The mRNA formed is the primary transcript – introns are cut out and the
exons spliced to make the mature transcript
7.
The mature transcript mRNA molecule leaves the nucleus through
pores in the nuclear membrane
tRNA folds due to base pairing to form a triplet anticodon site and an attachment site for a specific amino acid.
Transfer RNA (tRNA)
The function of tRNA is to bind to a particular amino acid and carry it to the ribosomes.
The diagram represents a tRNA molecule
RNA is a single strand but part of the tRNA molecule is folded
due to hydrogen bonds between it’s bases.
The molecule has three unpaired bases called an anticodon
that is able to form hydrogen bonds with a complementary
codon (consisting of three bases) on mRNA.
Another part of the tRNA molecule has an attachment site for
a specific amino acid.
Translation of mRNA into a polypeptide by tRNA at the ribosome.
Triplet codons on mRNA and anticodons translate the genetic code into a sequence of amino acids. Start and stop
codons exist.
Codon recognition of incoming tRNA, peptide bond formation and exit of tRNA from the ribosome as polypeptide is
formed.
RIBOSOMES contain enzymes essential for protein
synthesis, and have 3 tRNA binding sites; E, P and A.
- Site P holds the tRNA carrying the growing polypeptide
chain.
- Site A holds the tRNA carrying the next amino acid to be
joined to the growing chain by a peptide bond.
- Site E discharges a tRNA from the ribosome once its
amino acid has become part of the polypeptide chain.
E site
P site
A site
Translation starts when the START CODON on an mRNA molecule binds to the P site on a
ribosome.
Eventually, once peptide bonds have formed between amino acids (the order of which is decided
by the base sequence on mRNA) a STOP CODON is reached and the ribosome releases the
last tRNA and the now complete polypeptide chain.
The process of polypeptide formation from translation of mRNA at a ribosome is shown below:
Translation summary
tRNA molecules transport amino acids to the ribosome
3 bases on mRNA is a CODON
Hydrogen bonds form between anticodons and codons holding amino acids in the correct sequence
While amino acids are held
in the correct sequence,
peptide bonds form
between them to join them
into a polypeptide chain
This requires an
enzyme and ATP
Polyribosome
Several ribosomes can become attached to an mRNA molecule at the same time – a string of ribosomes on the same
mRNA is called a polyribosome.
Polyribosomes allow many copies of the polypeptide to be made from a single mRNA molecule.
Production of different mRNA molecules from the same primary transcript
The same primary transcript of mRNA can be used to produce different mature RNA molecules depending on which
sections are treated as exons and introns and therefore cut out or spliced.
So mRNA with different base sequences can be produced from the same gene (DNA base sequence). These different
RNA molecules produce different proteins after translation.
Therefore a single gene can give rise to several different proteins.
Post translation changes to protein structure
The polypeptide chain formed by translation may be modified by:
Cutting the chain using enzymes and combining sections, e.g. insulin is formed from a polypeptide that
has its central section cut out leaving the two remaining sections joined by sulphur bridges
Polypeptide
chain
Sulphur bridges
Polypeptide chain
folds and sulphur
bridges form
Enzymes cut out
middle
section
Insulin consisting of 2
polypeptide chains
joined by sulphur
bridges
The protein can be modified by adding other molecules, e.g. addition of a carbohydrate makes a glycoprotein,
e.g. mucus or phosphate may be added to activate a regulatory protein.