Gene Expression, RNA,

Transcription and

Translation

What you should know about gene expression

• Phenotype is determined by the proteins produced as the result of gene expression.

• Only a fraction of the genes in a cell are expressed.

• Gene expression is influenced by intra- and extra-cellular environmental factors.

• Gene expression is controlled by the regulation of both transcription and translation.

Phenotype/Genotype

• A cell’s genotype is determined by the sequence of DNA bases in its genes

• Phenotype is determined by the proteins produced as the result of gene expression

Selective Gene Expression

• Although a specialised cell has a complete set of the organism's genes, only those needed for its specialised functions are switched on. All other genes are switched off.

• So only a fraction of genes in a cell are expressed

• The process of differentiation proceeds in a stepwise fashion and involves changes in gene expression. 

• Gene expression is the activation of a gene that results in the formation of a protein.

• Gene expression is influenced by intra- and extra-cellular environmental factors.

Gene activity in different specialised cells

Cell type Activity of genes coding for production of:  antibodies lysosome

enzymesenzymes for synthesis of

ureapepsin

phagocyte off on off off

liver off off on off

stomach lining

off off off on

lymphocyte on off off off

Gene expression is controlled by the regulation of the processes of TRANSCRIPTION and TRANSLATION (protein synthesis).

DNA Base -> Amino Acid -> Protein• 4 different organic bases, which make up

the genetic code (DNA)

• Bases are taken in groups of 3, gives rise to 64 different combinations (4³)

• Amino acids are produced according to the sequence of every 3 bases on DNA• Each individuals’ base triplets are arranged

in a specific order for coding for the particular proteins needed by that individual

Gene expression- Do you know ?

• Phenotype is determined by the proteins produced as the result of gene expression.

• Only a fraction of the genes in a cell are expressed.

• Gene expression is influenced by intra- and extra-cellular environmental factors.

• Gene expression is controlled by the regulation of both transcription and translation.

RNA, Transcription and Translation

What you should know about RNA

• RNA is single stranded • RNA contains uracil instead of thymine RNA

contains 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.

Structure of RNA• Differences between DNA and RNA

RNA:• Single Stranded• Bases: Adenine, Uracil, Guanine and

Cytosine• Ribose Sugar

Types of RNA

• Messenger RNA (mRNA) – carries the genetic information (in base triplets called codons) from the nucleus to the cytoplasm for protein synthesis

• Transfer RNA (tRNA) – Carries amino acids on the anti-codon to the appropriate codon to form a polypeptide

RIBONUCLEIC ACID

• DNA carries the code (“recipe”) for making proteins

• The “equipment” for protein synthesis is in the cytoplasm

• mRNA acts as a go-between molecule, carrying the “recipe” to the “equipment”.

Transcription

Transcription- what you should know

• Describe the transcription of DNA into primary and mature RNA transcripts and 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.

TRANSCRIPTION• The process of making mRNA from the

template provided on DNA• DNA to act as template• Free RNA nucleotides• Enzymes including RNA polymerase• ATP for energy

• When mRNA is formed, it has triplets of bases along it. These are called codons:

• A G C U U A C G G A U G

CODON

TRANSCRIPTION

• Transcription - copies the information in DNA into an RNA molecule - occurs in the nucleus.

1) RNA polymerase enzyme binds to the promoter region of the DNA, it moves along the DNA, unwinding the double helix & breaking hydrogen bonds holding the base pairs together

2) Free RNA nucleotides join onto the 3’ end of the growing mRNA & bond with their complementary base pairs on the DNA (Remember- A-U, G-C) 3) The RNA that has been produced at this stage is known as the primary transcript

• The primary transcript now undergoes splicing

TRANSCRIPTION

• Hydrogen bonds between DNA bases re-form

• DNA coils up into double helix

SPLICING

Introns and exons

• The primary transcript of RNA is composed of introns and exons.

• The introns are non-coding regions of genes and so do not appear in the mRNA in eukaryotic cells. The exons are coding regions of genes and so do appear in the mRNA.

• The introns of the primary transcript of mRNA are removed in RNA splicing.

RNA splicing•In RNA splicing, the primary transcript is cut at the boundaries between the introns and exons.

•The introns are removed and the exons are joined together, to leave a continuous sequence of nucleotides.

•The mRNA can then leave the nucleus via a nuclear pore and enter the cytoplasm to

move onto the next stage of protein synthesis- TRANSLATION.

DNA to Protein

Transcription- Do you know ?

• How to describe the transcription of DNA into primary and mature RNA transcripts and include the role of RNA polymerase and complementary base pairing.

• That 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.

Translation

Translation –what you should know

• Describe the translation of mRNA into a polypeptide by tRNA at the ribosome.

• tRNA folds due to base pairing to form a triplet anticodon site and an attachment site for a specific amino acid.

• 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 a polypeptide is formed.

TRANSLATION

• Changing the code on mRNA into a sequence of amino acids.

• Occurs in the cytoplasm

• Involves ribosomes

The ribosome• found in the cytoplasm either floating

freely or attached to the rough endoplasmic reticulum.

• Ribosomes floating freely are used to synthesise proteins for use within the cell; those attached to the endoplasmic reticulum synthesise proteins for export or inclusion in the membrane.

• Ribosomes are formed from proteins and a third type of RNA known as ribosomal RNA (rRNA).

• During translation they allow the mRNA and tRNA to come together.

David S. Goodsell RCSB PDB

tRNA

• Complementary base pairing occurs between nucleotides within the strand of tRNA, producing tRNA’s distinctive structure. This structure exposes a triplet anticodon site and attachment site for a specific amino acid.

• The triplet anticodon site is complementary to the triplet codon site on the mRNA.

Triplet anticodon

Amino acid attachment site

TRANSLATION

A G C UA AU CG CC U A AG GG

U C G

mRNA

tRNA

ANTI-CODON

RIBOSOME

AMINO ACID

CODON

1) During translation the mRNA passes through the ribosome.

2) The ribosome binds to the mRNA so that the start codon is in position

TRANSLATION

A G C UA AU CG CC U A AG GG

U C G

U A A

3) The codons are recognised by tRNA, each carrying a particular amino acid.

TRANSLATION

U A A

A G C UA AU CG CC U A AG GG

U C G

4) The appropriate tRNA brings its amino acid to the ribosome as it moves along the mRNA.

TRANSLATION

A G C UA AU CG CC U A AG GGU C G

U A A

U C G

TRANSLATION

A G C UA AU CG CC U A AG GG

U C G

U A A

U C G

G G A

5) Adjacent amino acids then join with a peptide bond. PEPTIDE

BOND

TRANSLATION

A G C UA AU CG CC U A AG GG

U C G

G G A

U A A

U C U

TRANSLATION

A G C UA AU CG CC U A AG GG

G A A

U C G

U C U

6) The tRNA then leaves the ribosome.

TRANSLATION

A G C UA AU CG CC U A AG GG

G G A

U C U

POLYPEPTIDE CHAIN

7) This process continues until a stop codon is reached and the polypeptide is released.

DNA – mRNA – tRNA - Protein

DNA - TTACGGCAATGCGGTACCGTTGGGGGCAG

mRNA -

Codons (set of 3 bases)

tRNA -

Anti-codons (set of 3 matching bases)

Protein -

Complete the sequences of mRNA and tRNA then using Torrance pg 39 find out the names of the amino acids.

DNA – mRNA – tRNA - Protein

DNA - TTACGGCAATGCGGTACCGTTGGGGGCAG

mRNA - AAUGCCGUUACGCCAUGGCAACCCCCGUC

Codons (set of 3 bases)

tRNA - UUACGGCAAUGCGGUACCGUUGGGGGCAG

Anti-codons (set of 3 matching bases)

Protein - aspn– ala – val – thr – pro – try – glu – pro - pro

Overview

Fate of Protein

• The protein is carried in a vesicle from the RER to the Golgi where it is processed and packaged into a vesicle which fuses with the cell membrane.

Translation –You should know

• The process of translation of mRNA into a polypeptide by tRNA at the ribosome.

• tRNA folds due to base pairing to form a triplet anticodon site and an attachment site for a specific amino acid.

• 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 a polypeptide is formed.

Protein synthesis storyboard

• You are going to produce a storyboard showing the process of protein synthesis in a number of steps.

• For each step you should draw a diagram and give a short description of what is happening.

• Use as many boxes as you require to describe the full process.

• The steps should be displayed in the correct order, with all the main molecules mentioned.

Protein synthesis storyboard