PROTEIN SYNTHESIS

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Amino acids

Sequence of amino acids is important

Peptide bond

Protein / Polypeptide A chain of amino acids A chain of amino acids Linked by peptide bondsLinked by peptide bonds

The sequence of amino acids

is important

The sequence of amino acids

is important

DNA

Carries information aboutCarries information about

the the sequence of aminoof amino

acids in proteins.acids in proteins.

The sequence of bases in DNA

is directly related to is directly related to

The sequence of amino acids in proteins

non-template non-template strandstrand

A T T A G G C T AA T T A G G C T A

T A A T C C G A TT A A T C C G A T

template strand template strand

The Genetic Code

Only one of the DNA strands carries the Only one of the DNA strands carries the code – code – template strand

Sequence of Sequence of 3 bases Codes for Codes for one amino acid Called a triplet

The Genetic Code

DNADNA

A T T A G G C T A A T T A G G C T A (non-template (non-template strand)strand)

T A A T C C G A T T A A T C C G A T (template strand)(template strand)

Amino acids Amino acids

The Genetic Code

Problem:DNA located :DNA located :

in the nucleusin the nucleus

But proteins made : But proteins made :

in cytoplasm at in cytoplasm at ribosomes

DNA is too large to leave DNA is too large to leave the nucleusthe nucleus

So…

A ‘copy’ of the relevant A ‘copy’ of the relevant section of DNA is madesection of DNA is made

Forming a molecule of Forming a molecule of messenger RNA (mRNA)

Then

mRNA leaves the mRNA leaves the nucleusnucleus

and travels to the and travels to the ribosomeribosome

Protein Synthesis

1.Transcription

DNA is used as a template to produce a DNA is used as a template to produce a

molecule of mRNAmolecule of mRNA

2.Translation the mRNA is used as a code to produce thethe mRNA is used as a code to produce the

chain of amino acids.chain of amino acids.

occurs in the

nucleus

occurs in the

cytoplasm

Tra

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n Transcription factors attach Transcription factors attach

to a promoter region of to a promoter region of DNA upstream of the gene DNA upstream of the gene to be transcribedto be transcribed

This enables This enables RNA polymerase to bind to the to bind to the DNA at the start of the DNA at the start of the genegene

RNA polymeraseRNA polymerase passes passes along the DNA causing along the DNA causing it to unwind and unzip – it to unwind and unzip – breaking hydrogen bonds breaking hydrogen bonds between bases between bases

Tra

nsc

rip

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n RNA nucleotides RNA nucleotides

attach to the attach to the exposed bases on exposed bases on the template strand the template strand by complementary by complementary base pairingbase pairing

RNA polymerase RNA polymerase enables these enables these nucleotides to nucleotides to attach to each other attach to each other to form mRNAto form mRNA

This initial molecule This initial molecule of mRNA is the of mRNA is the primary transcriptprimary transcript

single strandedsingle stranded contains contains uracil and not thymine and not thymine contains contains ribose not deoxyribose each sets of 3 bases called each sets of 3 bases called codons

mRNA

mRNA leaves the nucleus viaa nuclear pore

mRNA is edited by splicing

Translationoccurs in the cytoplasmoccurs in the cytoplasmmRNA associates with a ribosomemRNA associates with a ribosome

Also in the cytoplasm are Also in the cytoplasm are

tRNA’s (transfer RNA’s)

tRNA’s carry tRNA’s carry specific amino amino acids to the ribosomeacids to the ribosome

Translation

tRNAAt one end :At one end : a set of 3 unpaired bases a set of 3 unpaired bases

called the called the anticodon

The other end :The other end : an amino acid binding site an amino acid binding site binds to binds to specific amino acids amino acids depending on the anticodon depending on the anticodon

mRNA attaches to the ribosomemRNA attaches to the ribosome

Translation

A tRNA with a complementary anticodon

lines up against lines up against

the first codon on the mRNA

Translation

Translation

A second tRNA bonds with the next A second tRNA bonds with the next codoncodon

Translation

Translation

A A peptide bond forms between the 2 bond forms between the 2 amino acidsamino acids

Ribosome now moves 1 codon along Ribosome now moves 1 codon along the mRNA the mRNA

Translation

Translation

The first tRNA leaves The first tRNA leaves

returns to cytoplasm to attach to returns to cytoplasm to attach to another amino acidanother amino acid

A third tRNA brings the next amino acid A third tRNA brings the next amino acid

Translation

Translation

The ribosome continues to move along The ribosome continues to move along the mRNA until the polypeptide chain is the mRNA until the polypeptide chain is complete.complete.

Translation

Translation

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Exam Questions

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Sequence of bases is the code ;DNA strands separate /Hydrogen bonds break ;producing mRNA/transcription (linked to mRNA production) ;role of RNA polymerase ;complementary base pairing ;mRNA attaches to ribosome/RER ;tRNA bring amino acid ;anticodons of tRNA complementary to codons on mRNA ;amino acids join by peptide bonds/condensation reaction ;T

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Features of the Genetic Code

The Genetic Code is a :

Degenerate codeDegenerate code Each amino acid is coded for by more than one set of triplet Each amino acid is coded for by more than one set of triplet

basesbases

Because…Because…

Proteins are made from Proteins are made from 20 20 different amino acidsdifferent amino acids

6464 different combinations of triplet bases different combinations of triplet bases

The Genetic code is :

Non-overlappingNon-overlapping Each base is only used once to code for an Each base is only used once to code for an

amino acidamino acid

UniversalUniversal The same triplet of bases codes for the same The same triplet of bases codes for the same

amino acid in every organismamino acid in every organism

Control: Promoters and Transcription factors

A promoter is a region of DNA that aids the transcription of a gene. Promoters are typically located near the genes they regulate, on the same strand and upstream

a transcription factor is a protein that binds to a promoter region and thereby initiates the process of transcription. It does this by facilitating the binding of RNA polymerase.

Control: Promoters and Transcription factors

Control: Promoters and Transcription factors

Control: The example you have to learn - Effect of oestrogen on transcription

Oestrogens: a group of steroid hormones – roles include development of secondary sexual characteristics and are involved in the regulation of the oestrus cycle.

Control: The example you have to learn - Effect of oestrogen on transcription

Oestrogen enters the cell

Oestrogen binds to receptor molecules in

the cytoplasm

Control: The example you have to learn - Effect of oestrogen on transcription

And promote the transcription of these

genes

They enter the nucleus & bind to promoter regions

The oestrogen-receptor complexes act as

transcription factors

Control: Mechanism of RNA interference (RNAi).

•The appearance of double stranded RNA [usually as a result of viral infection] can start the process.

•The enzyme Dicer [blue] binds to the double stranded RNA and cleaves it into short pieces of approx. 20 base pairs in length known as small interfering RNA (siRNA) [orange-red].

• The siRNA binds to a protein called “RNA-induced silencing complex” (RISC) – this protein splits the siRNA one strand remains and RISC uses one strand of the siRNA to bind to mRNA molecules.

• The nuclease activity of RISC then breaks down the mRNA stopping it being translated.

Control: Mechanism of RNA interference (RNAi).