AS Biology - Unit 2

AS Biology AQA, on unit 2 topic DNA (3.2.2)

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  • Created by: Kaimlee
  • Created on: 21-02-11 13:39

Structure of DNA

DNA - polymer of repeating units - nucleotides (momer) join in a condensation reaction to form a sugar-phosphate back bone (1 chain upwards, other chain down) - antiparallal

3 components of a nucleotide:

  • Phosphate
  • Deoxyribose (pentose sugar)
  • 1 of 4 organic base   Adenine & Thymine   Guanine &  Cytosine

Pair due to complementary shape - form hydrogen bonds between bases A & T double bond, G & C triple bond.

Purines (double ring) A & G  Pyrimidines (single ring) T & C so distance between stands are constant.

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Structure of DNA - Questions

Q1.) Draw a nucleotide? and label all components

Q2.) What is meant by the term nucleotide?

Q3.) If in a DNA stand there 35% Adenine calculate the amount of Cytosine in the stand?

Q4.) What element does an organic base contain?

Q5.) Explain why A & T pair and why G & C pair?

Q6.) Explain why a purine bonds with a pyrimidine?

Q7.) Name the reaction when necleotides join forming a polynecleotide strand?

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Double Helix

2 polynucleotide strands are twisted, the phosphate and deoxyribose wind around each other to form a double helix, for each complete turn there are 10 base pairs.

Function of DNA

  • Acts as hereditary material to pass on from each generation.
  • Carries a code that instructs ribosomes which amino acids to fit together to form a functioning protein, helping to develop features and allow cell to function.

How DNA is adapted to its functions?

  • Its v.stable - passed from generations without being changed
  • Stands joined by H bonds - easy to seperate during DNA replication
  • Extremely large molecule - carrying a lot of genetic information
  • Genetic code protected from damage by sugar phosphate backbone
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Double Helix - Questions

Q1.) Draw a sugar - phosphate backbone?

Q2.) Explain the function of DNA?

Q3.) Describe what is meant by the term double helix?

Q4.) Explain how DNA is well adapted to its function?

Q5.) What is the importance of DNA?

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Genetic Code and Protein Synthesis

DNA within the nucleus carries a code which dictates what proteins the cell will make. Different areas along the DNA - genes code for specific proteins.

  • Triplet Code (3 bases instructing protein primary structure)
  • Transcription - coded into mRNA (smaller to fit through membrane of nucleus)
  • Translation - mRNA in cytoplasm travels to ribosomes. Ribosomes use instructions from triplet code to fit amino acids into correct primary structure (order) = a polypeptide

Gene Mutation - a random change in one or more nucleotide bases in the DNA of an organism, which may be inherited. Increased risk by - exposure to mutagens (i.e. UV, X-ray, chemicals - tar) Difference in triplet code - non functioning proteins (i.e. enzyme with a wrongly shaped active site)

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Genetic Code and Protein Synthesis - Questions

Q1.) Describe how DNA helps with the synthesis of polypeptides?

Q2.) Explain the function of a ribosome?

Q3.) Define what is meant by a gene mutation?

Q4.) Explain how a mutation in the DNA of an organism may effect future generations?

Q5.) Describe and Explain the 3 main areas of protein synthesis?

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DNA Definitions

Triplet Code - A sequence of 3 bases from 1 strand of the DNA codes for one particular amino acid.

Universal - All organisms use the same triplet base code for an amino acid.

Non overlaping - Each set of 3 bases are read in one triple, a base is not read gain in the next frame.       AAT          GTA         CTG   ...

                                                1st triplet .. 2nd triplet .. 3rd Triplet

Degenerate - some amino acids have more than one triplet base code. (GAA & GAG both code for amino acid Glu)

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More on mutations

Phenotype - physical characteristics that you inherit according to your genotype (height, hair colour ...)

Mutations normally occur during cell division (mitosis, metosis)

2 types:

  • Chromosome mutations - there is a change in the number of chromosomes (e.g. down's syndrome extra on chrom.21)
  • Gene mutations - change in the sequence of bases within DNA

How mutations effect phenotypes:

  • ...steps of protein synthesis = protein = enzymes
  • enzymes control chemical reactions, effects features developed 
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Mutations - Questions

Q1.) When do mutations normally occur?

Q2.) Define the 2 types of mutations?

Q3.) Explain how DNA effects phenotype?

Q4.) Define what is meant by phenotype?

Q5.) Explain why there may not be a mutation even if there is a change in the DNA base sequence?

Q6.) If there is a removal of part of a base sequence explain what this would mean for the rest of the sequence?

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Deletions and Additions

This effect of shifting the whole sequence is known as a frame shift


if the  T was removed (deletion) there is a frame shift & from there onwards isn't working, because can't be read. AG backwards still working.

- often a non - functioning protein is produced, also may be vital if an enzyme.

Mutations only have an effect on natural section and evolution - if occurs during meiosis, because these cells are passed on.

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Exon, Introns and multiple reapeating sequences

Multiple repeating sequence - non coding DNA between genes

Introns - non coding areas of DNA

Exon - coding areas of DNA

Gene - section of DNA on a chromosome coding for one polypeptide, the code is formed in specific sequences - triplet code.

Allele - one of a number of alternative forms of gene, for example eye colours.

Homologous pairs - pair of chromosomes which code for the same characteristic but aren't idential, in humans 23 from each parent making a diploid (total) of 46 chromosomes.

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Chromosome Structure

Only visible when cell is dividing (mitosis, meiosis) if stained, using a light microscope.

When visible appears as 2 threads (chromatids) joint at a single point (centromere)


  • DNA molecule held in position by proteins
  • DNA - protein complex is coiled -- looped -- further coiled
  • Single molecule of DNA packed into chromosome
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Differences in Prokaryotic and Eukaryotic DNA


  • fewer genes - so shorter
  • circular - forms a loop
  • DNA not held in position by proteins  - NO chromosomes
  • Has no non-coding areas of DNA


  • lots of genes - so longer
  • linear - forms chromosomes
  • DNA held in position by proteins (histones)
  • Has non-coding areas of DNA (Introns present within genes, between genes = multiple repeating sequence
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Exam Questions

Q1.)The scientists found that, in a second species of bacterium, 29% of the bases were guanine.
Explain the difference in the percentage of guanine bases in the two species of

(ANS -Different DNA base sequence, codes for different proteins/different genes)

Q2.) Name 1 difference in the DNA of a Prokaryotic cell and Eukaryotic cell?

Q3.) If the DNA of the cell is damaged, a protein called p53 stops the cell cycle.
Mutation in the gene for p53 could cause cancer to develop. Explain how?

(ANS - Due to mutation p53 protein not formed, cancer cells often have damaged DNA, so the cell can complete cell cycle and divide rapidly.

Q4.) The structure of DNA in a virus is not the same as other orgaisums suggest why? (ANS - virus has a single strand of DNA)

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