As you come in

• Make two lists under the 2 headings:- ‘Features I got from my mum and

dad’- ‘Features I did not get from my mum

and dad’

Learning objectives• Recall the differences between

environmental and inherited effects• What is a gene?• How does a gene code for a polypeptide

Success criteriaComplete bracelet models to describe

translation.Take away knowledge of key terms.

What is a ‘gene’?What is a ‘locus’?

• Write on a post it note….Stick it on to the chromosome

I think a gene is a type of mythical creature with 3 heads…….

Bracelet sequencing• Decide whether you are a chimp or a

human:

CTATTTGTGGT TCTGAGTTCTTAAAACCCAGTG CTTCGAAGG

Making DNA• Order the original dna sequence

using colour code.• Make a complimentary strand of DNA

(remember DNA is a double helix)

A = blue T = redC = green G = black

Learning objectives• Be able to describe the genetic code• Explain and compare the structure of

RNA(t and m) and DNA• Explain the preocess of transcription

and splicing.

The triplet codeGiven that there are four bases in DNA, and these code for 20 amino acids, what is the basis for the genetic code?

If three bases = one amino acid, possible aminoacids = 64 (4×4×4)

The existence of a three-base (triplet) code was confirmed by experiments by Francis Crick and his colleagues in 1961. The triplet code is degenerate, which means that each amino acid is coded for by more than one triplet.

What is mRNA?When a polypeptide is required, the triplet code of its gene is converted into a molecule of messenger RNA (mRNA).

This process is called transcription and is the first stage of protein synthesis.

Like DNA, mRNA is a nucleic acid, but it differs in that:

it is single stranded, not double stranded

it contains ribose instead of deoxyribose

it contains uracil instead of thymine.

mRNA strand during

transcription

U A G

tRNAmolecule

Differences between DNA and RNA

• Bases• Structure• Pentose sugar• Where is it

found?• Quantity in cells• Stability?

Transcription and codonsDuring transcription, the mRNA is built up by complementary base pairing, using the DNA as a template. The DNA’s base triplets are converted into mRNA codons.

What are the codons in the mRNA transcribed from this sequence of DNA base triplets?

DNA

mRNA

T A C G C A G A T T A C A U G C G U C U A A U G

The genetic code is non-overlapping: each base is only part of one triplet/codon, and each triplet/codon codes just one amino acid.

OVERLAPPING

AACGTAAGCACGTTCGCACCCCAAACACAC

EACH CODON CODES FOR ONE AMINO ACID. However these may be the same amino acid.

What is tRNA?

nucleotides

amino acidattachment site

anticodon

In the cytoplasm, amino acids become attached to transfer RNA (tRNA) molecules. Each tRNA is specific for one amino acid.

Each tRNA molecule has a sequence of three bases called an anticodon. These are complementary to codons on the mRNA molecule.

3’ end

5’ end

hydrogen bond

What is the anticodon for the codon A U G

U A C

What is translation?Once a molecule of mRNA has been transcribed, it moves out of the nucleus via a nuclear pore.

In the cytoplasm, the mRNA combines with a ribosome – the cellular structure on which the polypeptide chain will be built in a process called translation.

How are the correct amino acids transported to the ribosome, and how are they linked together in the correct order?

mRNA strand

ribosome

Next step in the polypeptide process

• Unwind your double helix• You are going to write the mRNA

sequence to your original DNA strand on your strip of card.

AGCUGUCUAGUAB

C

Next is the tRNA• Write on your tRNA molecules the

anticodons which are complimentary to the codons on your mRNA sequence.

• Next, write down the Amino Acids which are attatched to the specific amino acids.

AMINO ACID LINKED TO tRNA ANTICODONS

Valine - CAU/ CACAspargine – UUGSerine – AGAGlutamate - CUCPhenylalanine – AAA/AAGLeucine - AAUArganine - GAAProline - GGU

What happens during translation?tRNA molecules attach to the ribosome, and their anticodons pair up with the appropriate codons on the mRNA.

The amino acids transported by the tRNA link together, and the tRNA molecules then return to the cytoplasm.

The ribosome moves along the mRNA, and amino acids continue to join together until all the codons have been translated and the polypeptide is complete.

Draw a chromosome

SIMILARITIES

DIFFERENCES

mRNAANDtRNA

CODONAND

ANTICODON

DNAANDRNA

Extension ‘ how do genes code for polypeptides?’

Answer this question using the following key words:

Gene complimentary transcribe Ribosome tRNA Amino acids peptide bonds DNA translate mRNA

Learning objectives• What is a gene?• How does a gene code for a

polypeptide

Success criteriaComplete bracelet models to describe

translation.Take away knowledge of key terms.