2.1.3 Nucleotides and Nucleic Acids

What is a nucleotide?

A monomer of a nucleic acid

What is the structure of a nucleotide?

What is the difference between DNA and RNA nucleotides?

In DNA, the sugar is deoxyribose

In RNA, the sugar is ribose

What is a purine base?

A double ring structure.

e.g. adenine (A) & guanine (G) are purine bases

What is a pyrimidine base?

A single ring strucutre.

e.g. thymine (T), cytosine (C) & uracile (U) are pyrimidine bases

What is the difference between DNA and RNA’s bases?

DNA contains: A, G, T & C

RNA contains: A, G, U & C

How are polynucleotides formed?

Pentose carbon 3 on one nucleotide bonds to a phosphate from another nucleotide, through condensation reactions→ nucleotides are linked by phosphodiester bonds

What is the structure of ATP?

It is a phosphorylated nucleotide:

• Adenine (base)

• Ribose (pentose sugar)

• 3 Phosphate groups

What is the structure of DNA?

Double helix composed of two polynucleotide strands, joined together by hydrogen bonds between complementary bases

• Backbone is a sugar-phosphate arrangement

• 2nd polynucleotide chain runs in the opposite direction to this 1st (anti-parallel)

What happens during semi-conservative DNA replication?

1. Hydrogen bonds break (DNA helicase)

2. Double helix structure unwinds

3. Free activated DNA nucleotides join the unpaired bases (DNA polymerase)

4. Hydrogen bonds form

5. Phosphodiester bonds form between nucleotides

What are the features of the genetic code?

• Triplet code

• Degenerate

• Non-overlapping

• Widespread

What happens during transcription of DNA?

1. Hydrogen bonds between bases broken by DNA helicase - DNA unzips

2. Free activated RNA nucleotides diffuse into position with exposed bases on reference stand, using complementary base pairing

3. Condensation reactions catalysed by RNA polymerase creates mRNA

4. RNA molecule breaks free from DNA & leaves nucleus through a nuclear pore

5. Arrives in cytoplasm & moves to a ribosom

What happens during translation?

1. In the ribosome, there is complementary base pairing between codons & anti-codons

2. Another tRNA molecule approaches & joins

3. A condensation reaction forms a peptide bond between the 2 amino acids

4. mRNA moves, & the first tRNA leaves

5. Another amino acid is bought in

6. A chain of amino acids continues to be built up until a stop codon is reached on the mRNA

What is the role of tRNA?

tRNA is used in translation to transport specific amino acids from the cytoplasm to the ribosome

What is the role of rRNA?

Forms part of the ribosome