Gene expression

GENE EXPRESSION

by A.Arputha Selvaraj

Two steps are required

1. TranscriptionThe synthesis of mRNA uses the gene on the DNA molecule as a templateThis happens in the nucleus of eukaryotes

2. TranslationThe synthesis of a polypeptide chain using the genetic code on the mRNA molecule as its guide.

© 2010 Paul Billiet ODWS

RIBONUCLEIC ACID (RNA)

Found all over the cell

(nucleus, mitochondria, chloroplasts, ribosomes and the soluble part of the cytoplasm).


Types

Messenger RNA (mRNA) <5% Ribosomal RNA (rRNA) Up to 80% Transfer RNA (tRNA) About 15% In eukaryotes small nuclear

ribonucleoproteins (snRNP).


Structural characteristics of RNA molecules Single polynucleotide strand which may

be looped or coiled (not a double helix) Sugar Ribose (not deoxyribose) Bases used: Adenine, Guanine, Cytosine

and Uracil (not Thymine).


mRNA

A long molecule 1 million Daltons Ephemeral Difficult to isolate mRNA provides the plan for the

polypeptide chain


rRNA Coiled Two subunits:

a long molecule 1 million Daltonsa short molecule 42 000 Daltons

Fairly stable Found in ribosomes Made as subunits in the nucleolus rRNA provides the platform for protein

synthesis© 2010 Paul Billiet ODWS

tRNA Short molecule about 25 000 Daltons Soluble At least 61 different forms each has a

specific anticodon as part of its structure. tRNA “translates” the message on the

mRNA into a polypeptide chain


Transcription: The synthesis of a strand of mRNA (and other RNAs)

Uses an enzyme RNA polymerase Proceeds in the same direction as replication (5’

to 3’) Forms a complementary strand of mRNA It begins at a promotor site which signals the

beginning of gene is not much further down the molecule (about 20 to 30 nucleotides)

After the end of the gene is reached there is a terminator sequence that tells RNA polymerase to stop transcribing

NB Terminator sequence ≠ terminator codon.© 2010 Paul Billiet ODWS

Editing the mRNA

In prokaryotes the transcribed mRNA goes straight to the ribosomes in the cytoplasm

In eukaryotes the freshly transcribed mRNA in the nucleus is about 5000 nucleotides long

When the same mRNA is used for translation at the ribosome it is only 1000 nucleotides long

The mRNA has been edited The parts which are kept for gene expression are

called EXONS (exons = expressed) The parts which are edited out (by snRNP

molecules) are called INTRONS.


Transcription plan

Transcription

DNA

messenger RNA

Gene

Nucleus


Translation plan

TRANSLATION

Complete protein

Polypeptide chain

Ribosomes

Stop codon Start codon


Translation

Location: The ribosomes in the cytoplasm that provide the environment for translation

The genetic code is brought by the mRNA molecule.


What is the genetic code?

The genetic code consists of the sequence of bases found along the mRNA molecule

There are only four letters to this code (A, G, C and U)

The code needs to be complex enough to represent 20 different amino acids used to build proteins.


How many combinations?

If one base represented one amino acid this would only be able to produce

4 different combinations. (A, C, G and U) If pairs of bases represented each amino acid this would

only be able to produce 4 x 4 = 16 combinations. (AA, AC, AG, AU, CA, CC, CG, CU

etc) If triplets of bases represented each amino acid, this would

be able to produce 4 x 4 x 4 = 64 combinations

This is enough combinations to code for the 20 amino acids but is the code actually made of triplets?


Nature is logical!

Over 10 years biochemists synthesised bits of mRNA with different combinations

Then they used them to synthesise polypeptides

The results proved the logical answer was correct

The genetic code is made of triplets of bases called codons.


The Central Dogma Proposed by Francis Crick 1958 DNA holds the coded hereditary information in

the nucleus This code is expressed at the ribosome during

protein synthesis in the cytoplasm The protein produced by the genetic information

is what is influenced by natural selection If a protein is modified it cannot influence the

gene that codes for it Therefore there is one way flow of information:

DNARNAProtein© 2010 Paul Billiet ODWS

An important discovery Retro viruses (e.g.

HIV) carry RNA as their genetic information

When they invade their host cell they convert their RNA into a DNA copy using reverse transcriptase

Thus the central dogma is modified:

DNA↔RNAProtein This has helped to explain an important paradox

in the evolution of life.

Image Credit: Reverse transcriptase

The paradox of DNA DNA is a very stable molecule It is a good medium for storing genetic material

but… DNA can do nothing for itself It requires enzymes for replication It requires enzymes for gene expression The information in DNA is required to synthesise

enzymes (proteins) but enzymes are require to make DNA function

Which came first in the origin of life DNA or enzymes?


RIBOZYMES: Both genetic and catalytic Certain forms of RNA have catalytic properties RIBOZYMES Ribosomes and snRNPs are ribozymes RNA could have been the first genetic

information synthesizing proteins… …and at the same time a biocatalyst Reverse transcriptase provides the possibility of

producing DNA copies from RNA


The ribosome a ribozyme

Image Credit: Ribosome

Thank You

Gene expression

Health & Medicine

Transcript of Gene expression