DNA as the genetic code

a GENE is 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 a BASE TRIPLET is A sequence of 3 DNA bases in a gene which code for a single amino acid 4 Bases = A C G T Examples AGC CTA

and reads each codon independently of the next THE GENETIC CODE IS NON-OVERLAPPING A code that starts at one end of the gene and reads each codon independently of the next AUG AGC ACC GAG amino acid 1 amino acid 2 amino acid 3 amino acid 4

More than one base triplet can code for a particular amino acid THE GENETIC CODE IS DEGENERATE More than one base triplet can code for a particular amino acid

HOWEVER only 20 different amino acids have to be coded for. 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 translation PROTEIN SYNTHESIS This involves 2 processes Copying a gene in the nucleus to make mRNA translation Using the mRNA to make proteins in the cytoplasm

tRNA IN NUCLEUS IN CYTOPLASM Ribosome mRNA ENERGY (ATP) transcription translation DNA POLYPEPTIDE

DNA cannot move into the cytoplasm where polypeptides are produced. TRANSCRIPTION DNA cannot move into the cytoplasm where polypeptides are produced. This reduces the possibility of damage to the DNA.

The process is controlled by the enzyme RNA polymerase. 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.

This is called the coding or sense strand. 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

RNA nucleotides in the nucleoplasm attach to the exposed DNA bases. Free complementary RNA nucleotides in the nucleoplasm attach to the exposed DNA bases. adenine on the RNA thymine on the DNA pairs with uracil on the RNA adenine on the DNA whilst pairs with 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 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 template double stranded transcription mRNA after Transcription single stranded modification Modified mRNA used by ribosome to make protein

structure of mRNA p107 Froggy single stranded U replaces T short Ribose replace deoxyribose

tutorial 12.1 Transcription animations animation3 animation4 tutorial 12.1 Transcription transgenic plant animation

AS GURU VIDEO

interactive transcription

draw structure of tRNA p109 Froggy transfer RNA (tRNA) 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 draw structure of tRNA p109 Froggy

translation animation

read building a protein p110 draw diagram 1.4 p110 complete worksheet translation Put the sentences in the correct order and then either rewrite or cut and paste complete questions 1-10 p111-112

reverse transcriptase This is an enzyme which copies RNA into DNA

summary 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.

One gene one polypeptide theory