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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.