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1 of 36 © Boardworks Ltd 2008 2 of 36 © Boardworks Ltd 2008 Introduction to nucleotides Nucleotides are nitrogen-containing organic substances that form the basis of the nucleic acids DNA and RNA. All nucleotides contain the following three groups: a phosphate group a nitrogencontaining base a pentose sugar In DNA the sugar is deoxyribose, whereas in RNA the sugar is ribose. 3 of 36 © Boardworks Ltd 2008 Bases There are five bases, split into two types: adenine (A) and guanine (G) are purine bases. A thymine (T), cytosine (C) and uracil (U) are pyrimidine bases. T G C U DNA contains A, G, T and C, whereas RNA contains A, G, U and C. 4 of 36 © Boardworks Ltd 2008 Identifying bases 5 of 36 © Boardworks Ltd 2008 Base pairing rules 6 of 36 © Boardworks Ltd 2008 Formation of nucleotides 7 of 36 © Boardworks Ltd 2008 Formation of polynucleotides 8 of 36 © Boardworks Ltd 2008 Determining the structure of DNA The double-stranded structure of DNA was determined in 1953 by the American biologist James Watson and the British physicist Francis Crick. X-ray diffraction studies by British biophysicist Rosalind Franklin strongly suggested that DNA was a helical structure. The Austrian chemist Erwin Chargraff had earlier showed that DNA contained a 1:1 ratio of pyrimidine:purine bases. 9 of 36 © Boardworks Ltd 2008 Structure of DNA 10 of 36 © Boardworks Ltd 2008 How is DNA packaged? In eukaryotic cells, DNA is packaged as chromosomes in the nucleus. There is around 2 m of DNA in a cell, so to fit it needs to be tightly coiled and folded. Eukaryotic DNA is associated with proteins called histones. Together, these form chromatin – the substance from which chromosomes are made. In prokaryotic cells, DNA is loose in the cytoplasm – there are no histones or chromosomes. 11 of 36 © Boardworks Ltd 2008 Structure of eukaryotic chromosomes 12 of 36 © Boardworks Ltd 2008 13 of 36 © Boardworks Ltd 2008 DNA replication The discovery of DNA’s structure by Watson and Crick provided evidence that complementary base pairing was key to DNA’s ability to replicate. Scientists proposed that DNA ‘unzipped’ as hydrogen bonds between base pairs were broken. New polynucleotide strands could then be synthesized using the originals as a template. Several hypotheses were proposed as to the specific mechanism by which new strands are created. 14 of 36 © Boardworks Ltd 2008 Mechanisms of DNA replication 15 of 36 © Boardworks Ltd 2008 Meselson–Stahl DNA experiment 16 of 36 © Boardworks Ltd 2008 DNA replication 17 of 36 © Boardworks Ltd 2008 DNA replication enzymes 18 of 36 © Boardworks Ltd 2008 DNA replication 19 of 36 © Boardworks Ltd 2008 Errors in replication: gene mutations 20 of 36 © Boardworks Ltd 2008 21 of 36 © Boardworks Ltd 2008 The genetic code: timeline 22 of 36 © Boardworks Ltd 2008 What is the genetic code? 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. The genetic code is almost universal – the same sequence of bases codes for the same amino acids in all organisms. 23 of 36 © Boardworks Ltd 2008 The triplet code 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. 24 of 36 © Boardworks Ltd 2008 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. 25 of 36 mRNA strand during transcription © Boardworks Ltd 2008 Transcription and codons 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 TAC GCA GAT TAC mRNA AUG CGU CUA AUG The genetic code is non-overlapping: each base is only part of one triplet/codon, and each triplet/codon codes just one amino acid. 26 of 36 © Boardworks Ltd 2008 The genetic code 27 of 36 © Boardworks Ltd 2008 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. ribosome mRNA strand How are the correct amino acids transported to the ribosome, and how are they linked together in the correct order? 28 of 36 © Boardworks Ltd 2008 What is tRNA? In the cytoplasm, amino acids become attached to transfer RNA (tRNA) molecules. Each tRNA is specific for one amino acid. amino acid 3’ end attachment site Each tRNA molecule 5’ end has a sequence of three bases called an hydrogen bond anticodon. These are complementary to codons on the mRNA molecule. What is the anticodon for the codon A U G? UAC 29 of 36 nucleotides anticodon © Boardworks Ltd 2008 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. 30 of 36 © Boardworks Ltd 2008 Stages of protein synthesis 31 of 36 © Boardworks Ltd 2008 From DNA to amino acids 32 of 36 © Boardworks Ltd 2008 33 of 36 © Boardworks Ltd 2008 Glossary 34 of 36 © Boardworks Ltd 2008 What’s the keyword? 35 of 36 © Boardworks Ltd 2008 Multiple-choice quiz 36 of 36 © Boardworks Ltd 2008