Nucleic Acids

Describe the structure of DNA (6)

• polymer with anti-parallel strands made up of nucleotides

• nucleotide containing: phosphate group, pentose sugar, nitrogenous base

• 4 bases: A-T and C-G which are complimentary pairs

• hydrogen bonds in between bases 

• phosphodiester bond between sugar + phosphate to form backbone 

• glycosidic bond between sugar + base

What are nucleic acids

• deoxyribonucleic acid (DNA): acts as the information store

• ribonucleic acid (RNA): reads & translates information

What are the 4 enzymes involved in DNA replication

• DNA gyrase + DNA helicase (scissors)

• DNA polymerase (builds)

• DNA ligase (glues)

Explain why complimentary base-pairing is important in DNA replication

• so DNA is replicated without error, producing the same sequences of nucleotides

• reduces the occurrence of mutations

Describe the process of DNA replication

• original DNA molecule unwinds

• DNA helicase and DNA gyrase causes the 2 strands of DNA to unzip, breaking the hydrogen bonds

• both strands act as templates

• nucleotides are free in nucleoplasm, attracted to their complimentary base-pairs, A-T and C-G

• DNA polymerase rejoins the sugar phosphate backbone using phosphodiester bonds to form 2 identical strands of DNA

• each strand contains half of the original DNA and half of the new DNA (SEMI-CONSERVATIVE)

What is the template strand

• the nucleotides only pair with bases on one strand of the DNA molecule

• this is known as the template strand and is used to produce the mRNA molecule

• when A pairs, it’ll pair with uracil (U)

How does RNA polymerase move across the strand

• RNA polymerase moves along the template strand in the 3’ to 5’ direction

• this means the mRNA molecule grows in the 5’ to 3’ direction

What is the coding strand

• non-template strand

• the base sequence of the coding strand will be the SAME as the base sequence as the mRNA apart from T which will be replaced with U

What are okazaki fragments

• the other strand of DNA is replicated discontinuously in the opposite direction with the formation of a series of short DNA segments called okazaki fragments

• this is called the lagging strand

How did Meselson and Stahl use the replication of DNA in E.Coli to prove the semi-conservative theory

• they used isotopes of N15 and N14 (isotopes of N), growing bacteria on the heavy N

• after growth the DNA was spun in centrifuge to distribute the mass

• the bacteria was then grown on a light only containing medium

• the data showed that the DNA contained both N15 and N14 within the DNA and none of the DNA contained only one of the isotopes