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Nucleic Acids and Protein Synthesis


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 ... – PowerPoint PPT presentation

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Title: Nucleic Acids and Protein Synthesis

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
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
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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.

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

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

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

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

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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
  • 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.

  • 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

  • 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.

  • 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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  • 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

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

Stages of translation
  • The first tRNA becomes disconnected from its
    amino acid and from the mRNA and leaves the

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.

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.

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

Protein made in Ribosomes attached To
endoplasmic Reticulum is for export
Protein made In free Ribosomes is For use Within
the cell
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

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