An Introduction to Genes and Genomes

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Chapter 2

• An Introduction to Genes and Genomes

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Introduction to Molecular Biology
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Prokaryotic Cell Structure

• Prokaryotic Cell

• Eukaryotic Cell

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Eukaryotic Cell Structure
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Eukaryotic Cell Structure

• Animal Cell

• Plant Cell

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Lets in on a cell!
Zoom

• DNA Zoom Interactive

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DNA Discovery (visit DNAi.org)

• Miescher identified a nuclear substance he
  called nuclein
• Griffith performed the first transformation
• Avery, McCarty, and Macleod identified
  Griffiths transforming factor as DNA
• Chargaff proved that the percentage of the DNA
  bases adenine always equaled thymine and guanine
  always equaled cytosine
• Wilkins, Franklin, Watson Crick demonstrated
  the structure of DNA

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Structure of DNA

• Deoxyribose Sugar
• Phosphate
• Nitrogen Base

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Structure of DNA

• Purines double ring
• Pyrimidines single ring

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Structure of DNA
Nucleic Acid Overview
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Structure of DNA
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DNA Replication

• When DNA makes an exact copy of itself

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DNA Replication
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DNA Replication

• The first step in DNA

replication is for the enzyme, helicase, to unzip
the double stranded DNA molucule.
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DNA Replication

• Proteins hold the two strands apart.
• An RNA primer lays down on each strand of DNA.

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DNA Replication

• DNA polymerase extends the primer by adding
  complementary nucleotides.
• DNA polymerase can only extend in the 5 ? 3
  direction

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DNA Replication

• Leading strand follows helicase.
• Lagging strand must wait for replication fork to
  open and therefore forms discontinous Okazaki
  fragments.
• Ligase seals the nicks in the DNA backbone
  between the Okazaki fragments.

helicase
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Lets put it all together

• Click on the animation below.
• Select the button for the whole picture.
• DNA Replication Animation

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Transcription

• Making an RNA copy from a DNA template

RNA polymerase
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RNA Structure

• Uracil instead of thymine
• Ribose sugar instead of deoxyribose sugar
• Single stranded
• Can leave the nucleus

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RNA Structure

• mRNA RNA copy of DNA that carries genetic
  information from the nucleus to the ribosomes
• rRNA makes up the ribosomes
• tRNA carries amino acids to ribosomes for
  protein synthesis

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Transcription

• RNA polymerase binds to a promoter region on
  double stranded DNA and unzips the double helix.

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Transcription

• Free RNA nucleotides pair with the complementary
  DNA of the template strand

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Transcription

• RNA is processed
• Introns are spliced out
• 7 methyl guanosine cap
• Poly-A tail

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Transcription

• mRNA leaves the nucleus and travels to the
  ribosomes in the cytoplasm

ribosome
nucleus
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Lets put it all together

• Transcription Animation

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Practice
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Central Dogma of Molecular Biology
Animation

• Click to see Video

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Translation

• Making protein from mRNA

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Translation

• Important Definitions
• A codon is composed of 3 RNA nucleotides
• Each codon codes for one amino acid
• Protein does the work in a cell

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Translation
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Translation
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Third Base
First Base Second Base
U C A G
U U C A G phenylalanine serine tyrosine cysteine phenylalanine serine tyrosine cysteine Leucine serine (stop) (stop) Leucine serine (stop) tryptophan
C U C A G leucine proline histidine arginine leucine proline histidine arginine leucine proline glutamine arginine leucine proline glutamine arginine
A U C A G isoleucine threonine asparagine serine isoleucine threonine asparagine serine isoleucine threonine lysine arginine met (start) threonine lysine arginine
G U C A G valine alanine apartic acid glycine valine alanine apartic acid glycine valine alanine glutamic acid glycine valine alanine glutamic acid glycine
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Translation
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Translation
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Translation
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Translation

• Asparagine, Serine, Methionine

• Tryptophan, Glycine, Lysine

• Proline, Leucine, Serine

• Aspartic acid, Histidine, Threonine

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Translation

• Always begins at a start codon and ends at a stop
  codon.
• The region between the start and stop codons is
  called the open reading frame (ORF)

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Practice

• Click on the animation to transcribe and
  translate a gene.

Click to see animation
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Translation Initiation

• mRNA attaches to the small subunit of a ribosome
• tRNA anticodon pairs with mRNA start codon
• Large ribosomal subunit binds and translation is
  initiated

amino acid
tRNA anticodon
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Translation Elongation

• Anticodon of tRNA carrying next amino acid binds
  to codon on mRNA
• A peptide bond joins the amino acids and the
  first tRNA is released.

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Translation Termination

• Amino acid chain continues until a stop codon is
  read. The amino acid chain is released and all
  of the translation machinery is recycled to
  translate another protein.

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Lets put it all together

• Click on the animation below

Translation Animation
Translation Video
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Lets put it all together
5-GATCTGAATCGCTATGGC-3
3-CTAGACTTAGCGATACCG-5 mRNA
5-GAUCUGAAUCGCUAUGGC-3
CUAGACUUAGCGAUACCG Asp,
Leu, Asn, Arg, Tyr, Gly
Coding Template mRNA tRNA amino acid
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Control of Gene Expression
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Control of Gene Expression
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Control of Gene Expression

• Prokaryotes cluster genes into operons that are
  transcribed together to give a single mRNA
  molecule.

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Control of Gene Expression

• Lac Operon
• Promoter region allows RNA polymerase to attach
  and begin transcription.
• Operator region is in the middle of the promoter.
  

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Control of Gene Expression

• If a repressor protein is bound to the operator,
  RNA polymerase cannot pass to transcribe the
  genes.

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Control of Gene Expression

• When the inducer (lactose) binds to the repressor
  protein, it changes shape and falls off of the
  operator region.
• Now RNA polymerase can pass and transcribe the
  genes into mRNA.

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Lets put it all together

• Click on the animation below.

Animation of lac operon
Video of lac operon
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Mutations

• Mutations are changes in the DNA sequence.
• Mutations can be inherited or acquired.

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Mutations

• Mutagens are agents that interact with DNA to
  cause mutations.
• Examples are chemicals and radiation.

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Mutations

• Point mutation changes a single base
• Point mutations can be silent, meaning they code
  for the same amino acid.

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Mutations

• Point mutations can also code for a structurally
  similar amino acid.

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Mutations

• Point mutations are not always harmless.
• If the mutation occurs on a critical amino acid
  in the active site of the protein, it can be
  detrimental, as in the case of sickle cell anemia.

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Mutations

• Frameshift mutations cause a shift in the reading
  frame by adding or deleting nucleotides.

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Mutations

• An example of a deletion causing a premature stop
  codon.