Nucleic Acids and Protein Synthesis

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Nucleic Acids and Protein Synthesis

• Chromosomes are thread-like structures found
  inside the nucleus of a cell. Each body cell
  contains 46 chromosomes arranged as 23 pairs. Sex
  cells (eggs and sperm) contain only 23
  chromosomes in total

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Chromosomes are the stuctures that hold genes.
Genes are made from DNA
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What are chromosomes?

• Chromosomes hold genes. Genes are made of DNA. A
  gene holds information that decides our colour of
  hair, colour of eyes etc. It also holds
  information for making proteins that may become
  enzymes or hormone for use within the body.

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Two of these strands become joined together by
weaker hydrogen bonds forming between there
bases. However this union is temporary in that
hydrogen bonds can be easily broken when this
becomes necessary e.g during the process of
transcription
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Base pairing rules

• Each base can only pair with one other type of
  base adenine(A) always bonds with thymine(T),
  and Guanine(G) always bonds with Cytosine(C). A-T
  and G-C are called Base Pairs. Each member of a
  pair are complimentary to its partner.

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• This twisted strand a bit like a spiral ladder is
  called a double helix

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How to carry out protein Synthesis

• In order to make a protein which may be a
  globular, fibrous or conjugated protein using the
  information from genes, we must start with DNA
  use it to make an RNA strand called a messenger
  RNA, this in turn is used to make another RNA
  strand called a Transfer RNA. The transfer RNA is
  used to link amino acids in a specific order.
  This in turns forms a polypeptide. Several
  polypeptides link together to form our completed
  protein

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Structure of RNA

• Now that we understand the structure of DNA, we
  must now understand the structure of our second
  type of nucleic acid RNA. An RNA molecule is
  similar to DNA

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Differences between DNA and RNA
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Starting protein synthesis-

• A single stranded Messenger RNA (mRNA) must be
  Transcribed from a single strand of double
  stranded DNA. This process is called
  Transcription.

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Stages of Transcription of a mRNA

• The 2 DNA strands becomes unwound at stage 1
• The DNA strands separate as weak hydrogen bonds
  between the bases are breaking at stage 2
• Free RNA nucleotides join up with exposed bases
  on one of the DNA Strands at stage 3. Uracil
  joins with Adenine, Cytosine with Guanine
• Weak hydrogen bonds are forming between the new
  base pairs at stage 4
• The nucleotides on the RNA strand are being
  linked in a chain by strong chemical bonds
  between the sugar of one RNA nucleotide and the
  phosphate of the next one in the chain at stage
  5. This linking into a chain is controlled by an
  enzyme called RNA Polymerase.
• At stage 6 the weak hydrogen bonds between the
  DNA and RNA bases are breaking allowing the
  molecule of transcribed mRNA to become separated
  from the DNA template.
• At stage 7 the transcribed mRNA is ready to begin
  its journey out of the nucleus and into the
  cytoplasm.
• At stage 8 the two exposed DNA strands reunite
  with weak hydrogen bonds forming between them.
  The DNA becomes wound into a double helix once
  more. This whole process is continuous.

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mRNA

• The completed molecule of mRNA leaves the nucleus
  through the pore in the nuclear membrane and
  enters the cytoplasm. Each triplet of bases on
  mRNA is called a codon

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tRNA

• A second type of RNA is found in the cells
  cytoplasm. This is called transfer RNA (tRNA).
  Each molecule of tRNA has a triplet of bases
  exposed. This triplet is known as an anticodon.

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Picking up amino acids

• The anticodon corresponds to a particular amino
  acid. Each tRNA molecule picks up the appropriate
  amino acid from the cytoplasm at its site of
  attachment. There are 20 different amino acids.

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Ribosomes

• These are small almost spherical structures found
  in all cells. Some occur freely in the cytoplasm,
  others are found attached to endoplasmic reticulum

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Translation

• Ribosomes are the site of translation of mRNA
  into protein. The ribosome becomes attached to
  one end of the mRNA molecule about to be
  translated. Inside the ribosome there are sites
  for attachment of tRNA molecules, two at a time.

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Stages of translation

• The anticodon of the first tRNA molecule forms
  weak hydrogen bonds with the complimentary codon
  on the mRNA

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Stages of translation

• When the second tRNA molecule repeats this
  process, the first two amino acids molecules are
  brought into line with one another.The two amino
  acids become joined together by a strong peptide
  bond.

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Stages of translation

• The first tRNA becomes disconnected from its
  amino acid and from the mRNA and leaves the
  ribosome

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Stages of translation

• The amino acids continue to align form peptide
  bonds and disconnect. The growing chain of amino
  acids is known as a polypeptide chain.

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Stages of translation

• The completed polypeptide chain consisting of
  very many amino acids is then released into the
  cytoplasm. The tRNA and mRNA are reused.
• The polypeptide then may be folded and rearranged
  to become the final protein. Sometimes several
  polypeptide chains combine to form the protein.

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• Protein synthesised in free ribosomes is for use
  within the cell. Protein made in ribosomes
  attached to endoplasmic reticulum is for export.

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Protein made in Ribosomes attached To
endoplasmic Reticulum is for export
Protein made In free Ribosomes is For use Within
the cell
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Coiling and folding of the polypeptide

• When a protein is made on the ribosome that is
  attached to endoplasmic reticulum, the
  polypeptide is injected into the ER and then
  coiled and folded. The protein is then passed to
  the golgi apparatus for packaging (adding for
  example a carbohydrate part to make it into a
  glycoprotein which is a conjugated protein) and
  secretion from the cell

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Function of the nucleus and nucleolus

• Nucleus- contains DNA which holds the genetic
  instructions for the manufacture of proteins and
  the control of inherited characteristics
• Nucleolus- controls the synthesis of RNA and
  other components needed to build ribosomes

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Nucleus
Nucleolus