e the structure of nucleic acids DNA bases

(e) the structure of nucleic acids: • DNA bases: purines-adenine and guanine, pyrimidines-cytosine and thymine; • Complementary base pair rule; • Hydrogen bonding and the double helix; • Antiparallel strands

Nucleic acids, DNA and RNA Pg 66 DNA nucleotides have the pentose sugar deoxyribose and the bases adenine, thymine, cytosine or guanine. RNA nucleotides have the pentose sugar ribose and the bases adenine, uracil, cytosine and guanine.

Nitrogenous bases There are two groups of nitrogenous bases: Pyrimidines (CUT) Purines (AG) A pyrimidine always pairs up with a purine via hydrogen bonding. Pyrimidines Cytosine Single ringed structures Always pair up with purines Purines U Uracil Thymine Adenine Double-ringed structures Always pair up with pyrimidines Pentose Sugar: deoxyribose Organic base Guanine

Complimentary base pairing Adenine bonds with thymine or uracil using 2 hydrogen bonds. A=T A=U Cytosine bonds with guanine using 3 hydrogen bonds. Pyrimidines Single ringed structures Always pair up with purines C G In data questions complimentary bases percentages/masses are very similar If a sample of DNA has 10% adenine it must also have 10% thymine, as the bases are complimentary and must pair up. The remaining 80% of bases belong to cytosine and guanine (40% each). (This would be the same if masses were given instead of percentages) DNA contains C, T, A and G but RNA contains C, U, A and G. (Don’t abbreviate base names in the exam). Cytosine Purines U Uracil Thymine Adenine Double-ringed structures Always pair up with pyrimidines Guanine

DNA

Introduction to DNA is found in the nucleus of eukaryotic cells and has two functions – replication in dividing cells and protein synthesis.

Introduction to DNA has two functions – replication in dividing cells and protein synthesis. • Each DNA molecule contains thousands of genes. • A gene is a short section of the DNA. ØThe sequence of bases on the DNA forms the genetic code. This codes for amino acids. • Genes code for proteins and functional RNA products.

The DNA molecule CB Pg 67 The DNA molecule • The pentose sugar in DNA is always deoxyribose. • The 4 nitrogenous bases are – adenine (A) and thymine (T). guanine (G), cytosine (C)

The DNA molecule CB Pg 67 • DNA is a nucleic acid. It is made up of two chains of nucleotides bonded together by hydrogen bonding. • Each chain is a polymer of nucleotides or polynucleotide. • The alternating phosphate groups and pentose sugars form the sugar-phosphate backbone of the polynucleotide.

Making a polynucleotide chain The nucleotides in one chain join together by condensation reactions between their phosphate groups and deoxyribose sugars: • The sugar of one nucleotide joins to the phosphate group of another at position 3’ • and a molecule of water is released. This forms a sugar-phosphate backbone to which the bases are • attached. In this manner a polynucleotide chain is made – it consists of millions of nucleotide units joined together.

• The polynucleotide chains are antiparallel to each other (go in opposite directions) with a different carbon atom ‘leading’ in each 3’ ribose sugar. • The directions are: 5’ 3’ and 3’ 5’). 5’ 5’ 3’ 3’ 5’

Complimentary base pairing joins the two chains together Complimentary base pairing links the two polynucleotide chains: • Purine bases (A/G) bond with pyrimidine(T/C) bases by hydrogen bonding. • Adenine bonds with thymine with 2 hydrogen bonds • and guanine bonds with cytosine with three hydrogen bonds • (A=T and G C); this is called complimentary base pairing. • The DNA molecule is then twisted to from a double helix. The shape of the twisted double helix is maintained by hydrogen bonding.

Read and highlight your booklet pg 67

RNA

RNA CB pg 68 Ribonucleic acid • RNA is a single stranded polymer of nucleotides. It has ONE polynucleotide chain. • RNA contains the pentose sugar ribose • RNA contains the organic bases adenine, guanine, cytosine and uracil. It does not contain Thymine. • The bases in RNA do not occur in equal amounts as it only has one polynucleotide chain so no complimentary base pairing. • RNA is much shorter than DNA. • There are 3 types of RNA and all are involved in Protein Synthesis.

3 types of RNA (table CB pg 68) Messenger RNA (m. RNA) • • • - m. RNA is a long single-stranded molecule. It is synthesised in the nucleus by a process called Transcription and carries the genetic code from the DNA to the ribosomes in the cytoplasm. Each strand of m. RNA contains the genetic code for one gene. Each gene codes for a particular polypeptide Every 3 bases is called a CODON Each codon is the code for one amino acid It ensures the correct t. RNA binds, bringing the correct amino acid into the ribosome This ensures the primary structure of the polypeptide is correct

3 types of RNA Ribosomal RNA (r. RNA) • • • r. RNA is found in the cytoplasm and is a component part of ribosomes. Ribosomes are made of r. RNA and protein and are synthesised in the nucleolus of the nucleus (they leave the nucleus via the nuclear pores). Ribosomes are the site of protein synthesis by a process called Translation.

3 types of RNA Transfer RNA (t. RNA) • • • t. RNA is a small single stranded molecule folded into the shape of a clover leaf. Each t. RNA molecule has an amino acid binding site CCA. At the opposite end of the t. RNA molecule there is a triplet of bases called an anticodon. t. RNA molecules transport amino acids to the ribosomes. The anticodon bases form a codonanticodon-complex with complimentary bases on the m. RNA molecule (codon). This allows translation to take place. Amino acid binding site

(f) the similarities and differences in the structure of RNA and DNA Action Complete the table of differences between DNA and RNA pg 69. Highlight and learn the 3 most important differences for the examination. H/W: PPQs Pg 103 -106