2.1.2 AS OCR biology


  • Created by: Maryan
  • Created on: 31-03-12 15:22

The structure of DNA

DNA-Deoxyribonucleic acid

  • Has two complimentary nucleotides-double stranded
  • Nucleotide: made up of a deoxyribose sugar, phosphate group and a nitrogenous base
  • Each nucleotide has the same sugar and phosphate group but its the base that varies(http://t3.gstatic.com/images?q=tbn:ANd9GcRyQ_QHwQZrN8k-fSS-z0_vpGhNvR9akHPKfuu2MaDa3f7hap0S)
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For DNA there are four possible bases: A,T,C,G

  • Adenine
  • Thymine
  • Cytosine
  • Guanine
  • Adenine+Guanine= Purine
  • Thymine+ Cytosine= Pyrimidine (think Top Cat)
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Structure of RNA

RNA-Ribonucleic acid

Similar structure to DNA:

  • Has one polynucleotide- Single stranded
  • Has phosphate group, nitrogenous base but a ribose sugar- note that its not deoxyribose sugar
  • Makes proteins using the instructions from the DNA
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Differences between RNA and DNA

  • Sugar in RNA is ribose and not deoxy ribose like DNA
  • Has only polynucleotide so its single stranded, whereas DNA has two polynucleotides therefore double stranded
  • RNA's pyrimidine consists of Adenine and Uracil
  • And its purine Cytosine and Guanine
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Formation of a DNA molecule

  • DNA nucleotides join together and become polynucleotides
  • The nucleotide joins between its own phosphate group and the sugar of another nucleotide
  • 2 DNA polynucleotides are joined together by Hydrogen bonding between the bases(http://www.dynamicscience.com.au/tester/solutions/chemistry/bonding/DNA.gif)
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Formation of a DNA- Pairings of the strands

To form the double helix:

  • Adenine always pairs with Thymine (A-T)
  • Cytosine always pairs with Guanine (C-G)
  • 2 hydrogen bonds form between A-T
  • and 3 hydrogen bonds between C-G
  • 2 Antiparallel polynucleotides twist and form a double helix

To form the double helix complimentary pairing happens

A-T 2H bonds

C-G 3H bonds

The two antiparralel polynucleotides twist and form a double helix

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Anti parallel visual aid


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DNA replication

The DNA copies itself before cell division

  • The hydrogen bonds break between the two strands- The DNA unzips to form two single strands
  • Free floating nucleotides join to exposed bases on template strand by complimentary pairing A-T, C-G
  • Nucleotides of the new and old strand are joined together by DNA polymerase (an enzyme) this enzyme also checks for any errors.
  • The two DNA strands one original and one new are joined by Hydrogen bonding between the bases.
  • This process is known as semiconservative replication
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Gene: A length of DNA that codes for a polypeptide

  • Proteins are made up of amino acids
  • Different proteins = different numbers and order of amino acids
  • Each amino acid is coded by 3 bases in a gene, so different bases codes for different Amino acids
  • Guanine, Thymine, Cytosine (GTC) makes valine
  • Thymine, Cytosine, Adenine (TCA) makes serine
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Roles of DNA and RNA in living organisms

DNA- Deoxyribonucleic acid

  • Contains the cell's genetic information to grow and develop
  • Its long and tightly coiled together and fits into the nucleus
  • The molecules paired structure makes it easier for cell replication- this makes it easier for cell division and passing on the genetic information down generations
  • DNA can be specific and form specific amino acids needed for a particular protein
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Outline of protein synthesis

All reactions and processes in living organisms need proteins

  • DNA carries instructions to make proteins (in the form of genes) and its located in the nucleus
  • The DNA undergoes transcription where the instructions to make the protein is now in the form of mRNA
  • Once the mRNA leaves the nucleus through the nuclear pores it heads to the ribosome where a 3 step process with tRNA synthesises the protein
  • This protein is then packed off to the Golgi apparatus via vesicles, where it is further modified and processes
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