A sequence of bases on a DNA molecule which codes for a sequence of amino acids in a polypeptide...

A sequence of bases on a DNA molecule which codes for a sequence of amino acids in a polypeptide chain

A sequence of 3 DNA bases in a gene which code for a single amino acid

4 Bases = A C G TExamples AGC

CTA

A code that starts at one end of the gene and reads each codon

independently of the next

AUGAGCACCGAGamino acid 2amino acid 1 amino acid 3 amino acid 4

More than one base triplet

can code for a particular amino acid

If the order of the bases is important there are 64

different ways of arranging four bases in a triplet code.HOWEVER only 20 different

amino acids have to be coded for.

• Some codes are used as initiation codons to indicate the beginning of a gene

• Some are stop codons and indicate where a gene terminates

• Others are used as alternative codons for some amino acids

TRANSCRIPTION

This involves 2 processes

Copying a gene in the nucleus to make mRNA

translationUsing the mRNA to make proteins in the

cytoplasm

Ribosome

POLYPEPTIDE

DNA

mRNA

tRNA

ENERGY (ATP)

transcriptiontranslation

IN NUCLEUS IN CYTOPLASM

DNA cannot move into the cytoplasm where

polypeptides are produced.This reduces the possibility

of damage to the DNA.

The DNA code is transcribed (copied) into a single stranded

messenger RNA (mRNA) molecule in the nucleus.

The process is controlled by the enzyme RNA polymerase.

Only one of the DNA strands of the double helix carries

the genetic code. This is called the coding or

sense strand. This sense strand acts as a

template for mRNA production.

DNA helicase separates the 2 strands by causing hydrogen bonds to break between the complementary base pairs in the gene and the DNA to unwind.

RNA polymerase attaches to the initiation code of the gene and moves along the template adding free RNA nucleotides to the exposed DNA nucleotides

Free complementary RNA nucleotides in the nucleoplasm

attach to the exposed DNA bases.

adenine on the RNA

thymine on the DNA

uracil on the RNA

adenine on the DNA

pairs with

pairs with

whilst

guanine always pairs with cytosine

RNA polymerase joins the sugar-phosphate backbone of the mRNA by

condensation reactions.The completed mRNA molecule passes

through the nuclear pores in the nuclear membrane into the cytoplasm.There are only 2 copies of each gene in

the nucleus, but transcription allows many copies of mRNA to be available for translation and protein synthesis.

During transcription the whole gene is copied into mRNA.

However genes contain long sequences of non-coding DNA

called introns.

These are not required to make the protein therefore before the mRNA is used,

these sequences are removed.

Part of DNA / gene to be copied

INTRON(noncoding)

EXON

DNA templatedouble stranded

transcription

mRNA after

Transcriptionsingle stranded

modification

Modified mRNAused by ribosometo make protein

• interactive transcription

• Carries amino acids to the ribosomes during translation

• tRNA is single stranded• Folded back in itself to form a clover

shape, held by hydrogen bonds• Specific amino acids are attached to

one end• A 3 base anticodon at the other end

is complementary to a specific mRNA codon

Put the sentences in the correct order and then either rewrite or cut and paste

• This is an enzyme which copies RNA into DNA

• Transcription involves unpairing of bases in one region of the DNA helix (the gene being read), followed by the synthesis of a strand of mRNA.

• This carries a triplet code sequence complementary to the template strand of DNA (i.e. DNA acts as a template)

• mRNA acts as a messenger to carry information to the ribosome in the cytoplasm.

• Translation involves decoding/reading the triplet message on mRNA.

• Each codon, 3 bases, has a natural complementary sequence of 3 bases, called the anticodon.

• This set of 3 bases is attached to a specific tRNA molecule that carries and transfers a specific amino acid.

• The specific amino acid is determined by the sequence of the triplet code and anticodon.

• The complementary triplets on mRNA and tRNA pair together in ribosomal sites (peptidyl & aminoacyl).

• Ribosomes are situated in the cytoplasm, hence the need for mRNA & tRNA.

• When 2 amino acids sit side by side, at the two sites on the ribosome, a condensation reaction occurs and a peptide bond will form between them.