Chapter 14

Structure of RNA

Transcription and splicing

Translation

Gene mutation

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Structure of RNA- the genetc code

Sections of DNA are transcribed onto a single-stranded molecule called RNA

2 types of RNA:

  •  mRNA- copies genetic code and trnsfers it to the cytoplasm from the nucleas, where it acts as a messenger . Small enough to leave through nuclear pores
  • tRNA

Main features of genetic code are:

  • Each amino acid id coded for by 3 bases on mRNA stand
  • A few amino acids have only one codon
  • The code is degenerate, so some amino acids can be coded for by different codons
  • there are 3 stop codons that don't cod for an amino acid
  • Stop codons mark the end of a polypeptide chain
  • Code doesn't overlap
  • Universal code that works for all organisms
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Structure of RNA

Single strand which each nucleotide is made up of:

  • Ribose (pentose sugar)
  • An organic base (A,G,C,U)
  • A phosphate group
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Structure of RNA- mRNA

mRNA is a long strand that is arranged into asingle helix.

Is a mirror image of copied DNA strand

mRNA leaves through nucleas through nucleus pores and assoicates with ribosomes

Acts as a template onto which protiens are built

Can be easily broken down

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Structure of RNA- tRNA

Single stranded chain folded into a clover shape

Part of the molecule extends out and allows for amino acids to attach

At the opposite end is an anti-codon, which will pair with 3 bases in the mRNA molecule

There are different types of tRNA each with a different anti-codon

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Polypeptide Synthesis

Basic process:

  • DNA provides the blueprint in theform of a sequence of ucleotides
  • A complementory section of DNA is made from pre-mRNA (transcription)
  • Pre-mRNA is spliced to form mRNA
  • The mRNA is used as a template for the attachment of complementory tRNA molecules carrying amino acids which are then linked together (translation)
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Transcription

The process of making pre-mRNA from DNA a a template.

The process is as follows:

  • DNA helicase breaks the H bonds in a specific region of the DNA molecule, which exposes unbased pairs
  • Enzyme RNA polmerase moves along a template DNA strand and causes nucleoides in the DNA strand to bond with pre-exisiting free nucleotids in the nucleas
  • As RNA polymerase moves along the molecule causing complementory bases to join up with one another, the DNA molecule recombines behind it
  • Eventually RNA polymerase reaches a stop codon on DNA molecule and detaches and competes production of pre-mRNA 
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Splicing

Exons code for protiens, but introns don't

Introns would interfere wit mRNA synthesis so needs to be removed from pre-mRNA forming mRNA.

Splicing is the removal of interfering introns and combinig of exons

Exon sections that have introns removed from them can be recombined in a numbe of different wys.

This means one that one section of DNA (a gene) can code for a variety of different protiens

Mutations can affect the splicing of pre-mRNA 

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Translation- synthesising the polypeptide

Each amino acid has a corrosponding tRNA molecule with its own anticodon bases

The process of polypeptide formation is as follows:

  • A ribosome becomes attached to the starting codon at one end of the mRNA molecule.
  • The tRNA molecule with the complementory anticodon sequence moves to the ribosome and pairs up with the sequence on the mRNA, carrying an amino acid
  • Another tRNA molecule with its anticodon binds into the next codon on the mRNA strand whilst carrying another amino acid
  • The ribosome moves along the mRNA, bringing together 2 tRNA molecules each pairing up with correspnding 2 codons on mRNA
  • Enzymes along with ATP join together the amino acids on adjacent tRNA molecules
  • The ribosome moves long to the third codon and links the amino acids on the second and thrid tRNA together
  • As this happens, the first tRNA is released from its amino acid and is free to collect another amino acid from the amino acid pool
  • The process continues as the polypeptide chain is built up
  • The synthesis continues until a ribosome reaches a stop codon. At this point, the ribosome, mRNA and tRNA all seperate leaving behind the polypeptide chain
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Translation- assembling a protien

A protien ma consist of one or may different polypeptide chains

Process:

  • The polypeptide is coiled or folded, producing a secondary stucture
  • The secondary stucture may be further folded producing a tertiary structure
  • Different polypeptide chains, along with any non-protien goups are linked to form a quaternary structure
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Gene mutation- subsitution of bases

Mutuaions that occur in gametes can be inherited

When one nucleotide is replaced by another it is called a subsitution mutation. A change to a single base could result in a:

Nonsense mutuation- occurs when the base subsitution results in a stop codon being transcribed onto mRNA. This occurs when the polypeptide is stopped prematurely and will often not function

A mis-sense mutation- occurs when the base subsitution resluts in a different amino acid being coded for. Since there is a different amino acid in the polypeptide, it may not function correctly as the intermolecular bonds that give the unique shape of the tertiary stucture may be changed so the whole shape of the protien will be different

Silent mutation- occurs when the subsitution doesn't result in a different amino acid being coded for. The polypeptide will therefore contain the same sequence of amino acids and will still function correctly

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Gene mutation- deletion of bases

Occurs when a nucleotide is lost

The polypeptide chain is often completely different due to the fact that there is a frame shift

The reason for the frame shift is because the nucleotides are read in 3's, so when a base is removed, the bases are read in different units of 3

A deletion at the end of a polypeptide chain is more likely to have a smaller effect that if it was at the start

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Gene mutation- causes

Can arise spontaneously in DNA replication

The rate of gene mutation can be influenced by mutagenic agents

High energy radiation can disrupt the DNA molecule

Chemicals can interfere with transcription or DNA structure

Mutation can increase species diversity

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Gene mutation- genetic control of cell division

The rate of cell division is controlled by 2 genes:

Proto-oncogenes

  • Stimulate cell division.
  • Growth factors attach to a protien on the cell surface membrane.
  • Relay protiens in the cytoplasm then switch on the genes nessary for DNA replication.
  • Mutations can turn proto-oncogenes into oncogenes. This can cause the receptor protien in a cell surface mebrane to be permanetly activated and cell division occurs without growth factors
  • The oncoene may code for an excessive amount of growth factor

Tumour Suppressor genes

  • Slow cell division
  • Mutation can make tumour suppressor genes inactivated so cell division isn't inhibited
  • The mutated cells are normally structurally different from normal cells
  • The cells that don't die can clone themselves and form a tumour
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