Genetic Code Protein Synthesis 3.2.2 DNA Structure Lesson 3 Genetic Code Protein Synthesis

Genes-What do they code for?

Genes-What do they code for? Genes code for proteins How?

Protein What is a protein? Your genes control the production of a protein. Your sequence of DNA codes for a specific sequence of amino acids. This agrees with Beadle and Tatum’s ONE GENE ONE ENZYME hypothesis

The Genetic Code A T C G

The Genetic Code All proteins are synthesised by just 20 amino acids. However the sequence and number of amino acids can produce a limitless range of proteins. The code of bases in a gene relates to a specific code for a protein-this is the genetic code.

thousands more bases in gene (not shown) The genetic code of an organism is the sequence of bases along its DNA. It contains thousands of sections called genes or cistrons. Each gene codes for a specific polypeptide. one gene/cistron thousands more bases in gene (not shown) All polypeptides are made from amino acids, so the sequence of bases in a gene must code for amino acids. Teacher notes The genetic code in mitochondria and chloroplasts (which both contain their own DNA) is slightly different to genomic DNA. The size of genes varies enormously. In humans, the average gene contains 10,000–15,000 bases (10–15 kb). The genetic code is almost universal – the same sequence of bases codes for the same amino acids in all organisms.

HOW DOES IT WORK? How many bases do you think code for a particular amino acid? One? Two? Three? Four?

Given that there are four bases in DNA, and these code for 20 amino acids, what is the basis for the genetic code? If one base = one amino acid, possible amino acids = 4 If two bases = one amino acid, possible amino acids = 16 (4×4) If three bases = one amino acid, possible amino acids = 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.

DEGENERATE, NON-OVERLAPPING CODE A triplet of bases codes for an amino acid How many possible amino acids can be produced? But there are only 20 amino acids so what do all the other …….. Codes represent

2ND T C A G 1ST T C A G 3RD T C A G PHENYLALANINE SERINE TYROSINE CYSTEINE LEUCINE STOP TRYPTOPHAN PROLINE HISTIDINE ARGININE GLUTAMINE ISOLEUCINE THREONINE ASPARAGINE LYSINE METHIONINE VALINE ALANINE ASPARTIC ACID GLYCINE GLUTAMIC ACID

DEGENERATE, NON-OVERLAPPING CODE

DNA Double Strand- Coding Strand and template Strand

Questions A section of DNA has the following sequence TACGCTCCGCTGTAC. What is the template strand? How many amino acids does this code for? What are the amino acids? What is the advantage of the degenerate code?

PROTEIN SYNTHESIS Where does it happen? How can the TRIPLET CODE get here? How do the Amino Acids know that they are going to be synthesised?

Transcription The copying of DNA on to messenger RNA (mRNA) The messenger RNA takes the sequence of bases of a gene from the nucleus to the ribosome Coding /Sense Template Antisense Strand Strand G A T C C T A G

Transcription You want a copy of the DNA Coding Sense strand So what strand do you copy and why?

Transcription Transcribe a particular sequence of DNA to produce a sequence of amino acids that corresponds to a protein. A sequence of DNA, called a Cistron, is unwound by a HELICASE enzyme DNA dependent RNA Polymerase copies the Template Antisense strand, producing an exact copy of the Coding sense strand.

mRNA The sequence of bases on the mRNA molecule codes for a sequence of bases. The code is split into triplet codes On the mRNA, this triplet code that corresponds to an amino acid is known as a CODON

mRNA What’s not on the messenger RNA molecule that is on the DNA molecule that was copied?

mRNA strand during transcription 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 mRNA strand during transcription it contains uracil instead of thymine.

T A C G C A G A T T A C A U G C G U C U A A U G During 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 T A C G C A G A T T A C mRNA 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.

Translation Transfer RNA

In the cytoplasm, amino acids become attached to transfer RNA (tRNA) molecules. Each tRNA is specific for one amino acid. amino acid attachment site 3’ end Each tRNA molecule has a sequence of three bases called an anticodon. These are complementary to codons on the mRNA molecule. 5’ end hydrogen bond What is the anticodon for the codon A U G? nucleotides U A C anticodon

Translation The mRNA arrives at the ribosome and is recognised. The first two codons enter the ribosomal structure. tRNA units, complementary to the codon travel from the cytoplasm to the ribosome. The tRNA molecule binds to the mRNA molecule in the ribosome

Translation A second tRNA unit complementary to the second codon then enters the ribosome and binds to the mRNA molecule. A peptide bond then forms between the two amino acids The mRNA then moves through the ribosome allowing more mRNA codons to attach to tRNA units.

Translation The tRNA molecules that have already connected to a mRNA molecule and have had their amino acids attached to a peptide chain are released back in to the cytoplasm. This process continues until a STOP codon is reached More than one ribosome may be attached to a mRNA molecule at any one time. This creates a POLYSOME unit. Up to 50 a second

TRANSLATION Peptide bonds form between the amino acids. Once the entire mRNA molecule has been translated to a sequence of amino acids, the created protein can be used.

Once a molecule of mRNA has been transcribed, it moves out of the nucleus via a nuclear pore. ribosome 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. Teacher notes Ribosomes are composed of about 65% ribosomal RNA (rRNA) and 35% ribosomal protein. They consist of a large and small sub-unit that only come together at the start of protein synthesis. mRNA strand How are the correct amino acids transported to the ribosome, and how are they linked together in the correct order?

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. Teacher notes Ribosomes translate mRNA at a rate of 15 amino acids per second. This can be increased by the use of polysomes – multiple ribosomes each translating a different part of the same mRNA strand.

Teacher notes The students should be reminded that, when identifying the amino acids, it is the bases in mRNA codons – and not the tRNA anticodons – that should be used.