Genetic Control of Protein Structure and Function

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Protein Synthesis- Translation

The second stage of protein synthesis and takes place in the ribosomes in the cytoplasm. During this stage, amino acids are joined together by a ribosome to make a polypeptide chain (protein).

  • The mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to the ribosome
  • A tRNA molecule, with an anticodon that's complementary to the first condon on the mRNA, attches itself to the mRNA by specific base pairing
  • A second tRNA molecule attches itself to the next codon on the mRNA in the same way
  • The twon amino acids attached to the tRNA molecules are joined by a peptide bond
  • The first tRNA molecule moves away, leaving its amino acid behind
  • A third tRNA molecule binds to the next codon on the mRNA, its amino acid binds to the first two and the second tRNA molecule moves away
  • This process continues, producing a chain of linked amino acids (polypeptide chain), until there's a stop condon on the mRNA molecule
  • A polypeptide chain then moves away from the ribosome and translation is complete
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Protein Synthesis- Splicing

  • Involves the removal of introns which is present in DNA and do not code for amino acids
  • During transcription the introns and exons (code for amino acids) are both copied into the mRNA this is called pre-RNA
  • The introns are removed from the pre-RNA and the exons are joined together in a process called splicing
  • This takes place in the nucleus and forms the m-RNA that can then leave the nucleus for the next stage of photosynthesis
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Protein Synthesis- Transcription

3. RNA polymerase moves down the DNA strand

  • The RNA polymerase moves along the DNA, seperating the strands and assembling the mRNA strand
  • The hydrogen bonds between the uncoiled strands of DNA re-forms once the RNA polymerase has passed by and the strands coil back into a double helix

4. mRNA leaves the nucleus

  • When RNA polymerase reaches the stop codon, it stops making mRNA and detaches from the DNA
  • The mRNA moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm where the next stage of protein synthesis takes place
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Types of RNA

mRNA (messenger RNA)

  • Made in the nucleus during transcription
  • Carries the genetic code from the DNA in the nucleus to the cytoplasm where it's used to made a protein during translation
  • Groups of three adjacent bases are calles codons

tRNA (transfer RNA)

  • A single polynucleotide strand folded into a clover shape, hydrogen bonds between specific base pairs hold the molecule in this shape
  • Every tRNA molecule has a specific sequence of three bases at one end called the anticodon
  • They have an amino acid binding site at the other end
  • Found in the cytoplasm where it's involved in translation
  • Carries the amino acids that are used to make proteins to the ribosomes 
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Protein Synthesis- Transcription

1. RNA polymerase attches to the DNA

  • The enzyme RNA polymerase attaches to the DNA double helix at the beginning of the gene
  • The hydrogen bonds between the two DNA strands in the gene break, seperating the strands
  • The DNA molecule uncoils at that point
  • One of the strands is used as a template to make an mRNA copy

2. Complementary mRNA is formed

  • RNA polymerase line up free RNA nucleotides alongside the template strand
  • Specific base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand (the base T is replaced with U)
  • Once the RNA nucleotides have paired up with their specific bases on the DNA strand they're joined together, forming an mRNA molecule
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DNA (Deoxy-ribose Nucleic Acid)

  • Double helix formed from two seperate strands of polynucleotide chains
  • Each nucleotide in the chain is made from a phosphate group, a pentose sugar (5 carbon atoms)- deoxyribose and a nitrogenous base--> Adenine, Thymine,Cytosine and Guanine
  • Many nucleotides join up between the phosphate group of one nucleotide and the sugar of another, creating a sugar-phosphate backbone
  • Cytosine and Guanine join together with 3 hydrogen bonds and Thymine and Adenine join together with 2 hydrogen bonds

RNA (RiboNucleic Acid)

  • The sugar in RNA nucleotides is a ribose sugar
  • The nucleotides form a single polynucleotide strand
  • The bases in the nucleotides are: Uracil, Adenine (pair), Cytosine and Guanine (pair)
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The Genetic Code and Nucleic Acids

The Genetic Code

  • The sequence of base triplets/ codons in mRNA which code for specific amino acids
  • non-overlapping: each base triplet is read in sequence, seperate from the triplet before it and after it
  • Degenerate: some amino acids are coded for by more than one base triplet/ codon. Not all triplet codes code for an amino acid, some are start and stop codons
  • Universal: the same specific base triplets code for the same amino acids in all living things

Interpreting data on nuclei acids

  • Giving the complementary DNA sequence to the mRNA codon--> made up of the bases that would pair with the mRNA sequence e.g. UCU (mRNA)= AGA (DNA)
  • Giving the tRNA anticodons from the mRNA codons--> tRNA codons are complementary copies of mRNA codons e.g. CUA (mRNA)= GAU (tRNA)
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Gene Mutation

  • Mutation: a change in the base sequence of DNA, can be caused by errors during DNA replication or by mutagenic agents


  • One base is substituted for another e.g. ATG becoming ATT (G being substituted for a T)
  • Nonsense mutation- occurs if the base change results in the formation of one of the three stop codons that mark the end of a polypeptide chain. As a result the chain would bestopped prematurely resulting in a significant difference to the protein formed
  • Mis-sense mutation- occurs when the base change results in a different amino acid being coded for. The significance of this mutation will depend upon the precise role of the original amino acid- if it is important in the forming of the bonds in the tertiary structure of the protein then substituting this amino acid will have a significant effect functionality of the protein fomed
  • Silent mutation- occurs when the substituted base still codes for the same amino acid. This is due to the genetic code being degenerate meaning that most amino acids have more than one codon/amino acid
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Gene mutation

Deletion of bases

  • One base is deleted e.g. ATGCCT becoming ATCCT (G is deleted)
  • Known as a frame shift as the triplet code has shifted to the left by one letter
  • Deletion at the start of the base sequence has a far more devastating effect than if it was towards the end as the whole genetic message will be altered

Addition/ insertion

  • One or more bases are added in e.g. ATGCCT becoming ATGACCT (A is inserted)

The order of DNA bases in a gene determines the order of amino acids in a particular protein, so a mutation in a gene can cause the sequence of amino acids that it codes for to be altered. If the sequence of amino acids in a protein is altered, this may affect the protein's tertiary structure which can result in the protein not being able to function at all.

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Mutagenic Agents

Substances that increase the rate of mutations- UV radiation, ionising radiation, some chamicals, some viruses. They increase the rate of radiotion by:

  • Acting as a base- Chemicals called base analogs can substitute for a base during DNA replication, changning the base sequence in the new DNA
  • Altering bases- Some chemicals can delete of alter bases
  • Changing the structure of DNA- Some types of radiation can change the structure of DNA, which causes problems during DNA replication
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Hereditary and Acquired Mutations

  • Hereditary- You inherit the mutation from your parents, they are present in the gametes meaning the fetus will have the mutation
  • Acquired- You develop the mutation during your lifetime, they occur in individual cells after fertilisation
  • Some hereditary mutation can cause genetic disorders- inherited disorders caused by abnormal genes or chromosomes e.g. cystic fibrosis
  • Other hereditary mutations can increase the likelihood or developing certian cancers
  • Acquired mutations can also cause cancer
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Genetic Control of Cell Division

Tumour Supressor Genes

  • Slow cell division by producing proteins that stop cells dividing or cause them to self-destruct (apoptosis)
  • If a mutation occurs in a tumour supressor gene, the gene will be inactivated, the protein it codes for isn't produced and the cells divide uncontrollably (rate of division increases) resulting in a tumour


  • Stimulate cell division by producing proteoins that make cells divide
  • If a mutation occurs in a proto-oncogene, the gene can become overactive, tis stimulates the cells to divide uncontrollably (the rate of division increases) resulting in a tumour
  • A mutated proto-oncogene is called an oncogene

Mutations in tumour supressor genes and proto-oncogenes are often acquired but some are inherited.

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1. Diagnosing and Treating Cancer and Genetic Diso

Cancer caused by acquiring mutations

  • Prevention- reduce your exposure to mutagenic agents e.g. wear suncream when exposed to the sun
  • Diagnosis- by knowing the cause is an acquired mutation, high risk individuals can be screened for cancers that the general population isn't normally screened for or they can be screened earlier and more frequently e.g. people with Crohn's disease are at a higher risk of getting colon cancer. Some cancers are often caused by a particualr mutation, knowing this means that more sensitive tests can be developed which lead to a more accurate and early diagnosis
  • Treatment- Individuals diagnosed with cancer can have their DNA screened in order to indentify the mutation than has caused it, this allows for a more personalised treatment to be given. The type of mutation can also effect the aggressiveness of the treatment, e.g. if that mutation is known to cause a fast growing cancer then higher doses of radio therapy may be given. Gene treatment can also be used 
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2. Diagnosing and Treating Cancer and Genetic Diso

Cancer cause by hereditary mutations

  • Prevention- Hereditary mutations often mean there is a family history of that certain type of cancer. The individual can therefore avoid specific mutagenic agents to prevent acquired mutations e.g. those with a family history of lung cancer shouldn't smoke. Individuals could also be screened for the known defective gene and then meausures put in place reduce risk e.g. a mastectomy to prevent the development of breat cancer
  • Diagnosis- Increased/earlier screening if there is a family history can lead to early detection and increased chances of recovery
  • Treatment- Often diagnosed earlier than an acquired mutation meaning a different treatment can be used. New treatments are being developed that are tailored to cancers by hereditary mutations in specific genes

Genetic disorders

  • Prevention- Carriers/sufferers can undergo preimplantation genetic diagnosis during IVF
  • Diagnosis- If a person had a family history then they can have their DNA screened
  • Treatment- Gene therapy can be given, treatment mostly depends on the genetic disorder
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