Gene Function

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Chapter 16

• Gene Function

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Genes

• DNA provides the instructions to make protein
• One gene one protein
• RNA is the link between the gene and the protein
• DNA codes for RNA and RNA codes for the protein

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Transcription/Translation

• The DNA and RNA molecules are composed of
  nucleotide monomers.
• When converting from DNA to mRNA you transcribe
  the code from the language of DNA nucleotides to
  RNA nucleotides
• Proteins are written in the language of amino
  acids.
• When converting from mRNA to protein we translate
  from the nucleotide language to amino acid
  language

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Transcription

• Process whereby a gene is copied into a
  complementary strand of mRNA
• WHY does this occur??
• DNA is too large to leave the nucleus
• mRNA is small enough to carry the message to the
  ribosome

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RNA

• In what ways are RNA molecules different from
  DNA?
• RNA is single stranded
• In RNA Uracil replaces Thymine
• Nucleotides have ribose instead of deoxyribose.
• In eukaryotes RNA leaves the nucleus

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What are the functions of the 4 different types
of RNA?
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4 types of RNA

• mRNA carries message from DNA to ribosome.
  Message carried in codons
• tRNA delivers a specific amino acid to the
  ribosome. tRNA has an anti-codon.
• rRNA forms the ribosome and helps form peptide
  bonds
• snRNA - removal of introns

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Transcription

• 4 steps to transcription
• RNA polymerase
• attaches to a specific site on DNA
• separates the 2 strands of DNA
• synthesizes a complete mRNA strand
• mRNA splits off and DNA reforms

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Transcription

• Transcription factors (proteins) determine which
  genes are transcribed.
• Contain motifs 3-D conformations that allow
  transcription factors to bind to DNA
• Promotors control sequence (TATA box) that
  attract transcription factors and RNA polymerase

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Transcription

• Usually, only one strand of the DNA double helix
  is transcribed
• Sense or coding strand
• Always built in 5 to 3 direction
• (strand not transcribed anti-sense or
  non-coding)

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The Genetic Code

• RNA consists of four letters A, U, G, and C
• Proteins consist of 20 letters the amino acids
• If 1 RNA base codes for 1 amino acid, then only 4
  amino acids can be coded for.

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• How many different amino acids can be coded for
  if 2 RNAs code for 1 amino acid?
• 42 16 Not enough!
• How many different amino acids can be coded for
  if 3 RNAs code for 1 amino acid?
• 43 64 More than enough for the 20 different
  amino acids.

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There is redundancy in the code but not ambiguity!
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The Code is Universal

• The code is shared by almost all organisms
• CCG codes for what amino acid?
• Proline. This holds true for all species of
  living organisms.
• Bacteria, therefore can be programmed to
  synthesize human proteins by inserting human DNA

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In prokaryotes, RNA is directly translated into
the polypeptide
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RNA in eukaryotes is processed before translation
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• The function of the cap is
• prevent mRNA degredation by hydrolytic enzymes
• helps attach to the ribosome
• Function of the 3 (poly-A) tail
• same functions as the 5cap
• also helps facilitate export of mRNA from nucleus

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RNA splicing

• Removes noncoding regions called introns
• snRNP (short nuclear ribonucleoproteins)
  recognize the splicing signals at the ends of
  introns
• The RNA in the snRNP is called snRNA (small
  nuclear RNA)
• spliceosome is the larger protein assembly that
  surrounds the snRNP
• The spliceosome cuts and releases the introns,
  and then joins exons together

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Role of introns

• Introns may play regulatory role
• Different intron removal may lead to different
  proteins
• Introns may enhance crossing over between
  homologous regions by increasing the distance
  between exons

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Peptide Bonds Join Amino Acids
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Messenger RNA consists of leader, reading frame,
and trailer sequences.
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Ribosomes, consist of two subunits, each of which
contains rRNA and ribosomal proteins.
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tRNAs bring amino acids to the ribosomes during
translation to be assembled into polypeptide
chains.
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tRNA Assembly
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Initiation of Translation
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Elongation
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Termination
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review
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Polyribosomes
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