DNA

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Topic 3.5 Transcription and Translation
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Central Dogma of Biology

• How does the information flow in biological
  systems?

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Two Main Process Transcription and Translation

• DNA copy itself
• Replication
• DNA synthesize RNA
• Transcription
• RNA synthesize protein
• Translation

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3.5.1 Compare the structure of RNA and DNA
DNA (deoxyribonucleic acid) RNA (Ribonucleic acid)
Two strands forming a double helix One strand only
Deoxyribose Ribose
A, C, G and T A, C, G and U
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3.5.2 DNA Transcription

1. DNA helix unzips
2. One of the 2 DNA strands acts as a template
3. RNA nucleotides line up along one strand of the
   DNA following the base-pairing rules.
4. No T, Instead U

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3.5.2 DNA Transcription

1. RNA polymerase forms covalent bonds between
   nucleotides.
2. Transcription continues until one entire gene has
   converted to RNA.
3. The single-stranded messenger RNA separate and
   the DNA strands rejoin.

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3.5.3The Genetic Code

• How do nucleotides specify 20 amino acids?
• 4 different nucleotides (A, G, C, T)
• Possible codes
• 1 letter code ? 4 AAs lt20
• 2 letter code ? 4 x 4 16 AAs lt20
• 3 letter code ? 4 x 4 x 4 64 AAs gtgt20
• Three letter code with 64 possibilities for 20
  amino acids suggests that the genetic code is
  degenerate (i.e., more than one codon specifies
  the same amino acid).

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3.5.3The Genetic Code

1. Triplete Code
2. Degenerated
3. Universal

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Nucleic Acids RNA types

• There are 3 different types of RNA
• Messenger RNA Transcription
• Single strand
• Transport nucleus Cytoplasm
• Ribosomal RNA
• Structural Functional
• component of Ribosomes
• Transfer RNA Translation
• Bring aa to Ribosomes for protein
• assembly

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3.5.4 DNA Translation

• The base sequence of mRNA is used as a guide for
  assembling the sequence of amino acids that will
  be a polypeptide.
• 2. This process of protein production using a
  mRNA as a guide is called TRANSLATION.

DNA
A
C
C
G
A
G
G
C
U
U
U
G
mRNA
Metionine
Alanine
Tyrosine
Polypeptide
Arginine
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3.5.4 DNA Translation
Translation take place on cell structures called
Ribosomes, using mRNA tRNA. Ribosomes are in
the cytoplasm, outside the cell nucleus.
Transfer RNA 1. Amino acids are carried to
the ribosomes by smaller RNA molecules called
transfer RNA (tRNA). 2. Single strands folded
back on themselves Clover Leaf
structures 3. Role Carry aa to the
ribosomes -aminoacid attachment site
-anticodon codon attachment site
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3.5.4 DNA Translation Process

• mRNA binds the small subunit of the ribosome
• tRNA molecules are present, each one carrying the
  specific aa anticodon
• tRNA binds to the ribosome at the site where the
  anticodon matches the codon on the mRNA
• Two tRNAs binds at once and the first one in
  transfers the growing polypeptide chain to the
  second one in.
• The ribosome moves along the mRNA and the process
  continues until a stop codon is reached when the
  polypeptide is realeased.

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3.5.4 DNA Translation Process
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3.5.4 DNA Translation Overview
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3.5.5 One Gene One Polypeptide Theory
One gene is transcribed and translated to produce
one polypeptide.
Some protein are composed of a number of
polypeptides and in this theory each polypeptide
has its own gene. e.g. haemoglobin is composed
of 4 polypeptides (2 of each type) and there is
a gene for each type of polypeptide. This
theory, like so many in biology has exceptions.
1) Some genes code for types of RNA which do
not produce polypeptides. 2) Some genes control
the expression of other genes.