The Living World Chapter 9

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How Genes Work
Associate Professor Pamela L. Pannozzo Palm Beach
Community College Concepts in Biology BSC 1005
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What is a Gene?

• The work of Sutton and Morgan established that
  genes reside on chromosomes
• But chromosomes contain proteins and DNA
• So which one is the hereditary material proteins
  or DNA?

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The Griffith Experiment

• In 1928, Frederick Griffith discovered
  transformation while working on Streptococcus
  pneumoniae
• The bacterium exists in two strains
• S
• Forms smooth colonies in a culture dish
• Cells produce a polysaccharide coat and can cause
  disease
• R
• Forms rough colonies in a culture dish
• Cells do not produce a polysaccharide coat and
  are therefore harmless

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Fig. 9.1 How Griffith discovered transformation
Thus, the dead S bacteria somehow transformed
the live R bacteria into live S bacteria
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The Avery Experiments 1944

• Avery and his colleagues prepared the same
  mixture of dead S and live R bacteria as Griffith
  did
• Removed proteins from the dead S strain
• Subjected dead cells to experiments
• All of the experiments revealed that the
  properties of the transforming principle
  resembled those of DNA
• 1. Same chemistry and physical properties as DNA
• 2. Not affected by lipid and protein extraction
• 3. Not destroyed by protein- or RNA-digesting
  enzymes
• 4. Destroyed by DNA-digesting enzymes

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The Hershey-Chase Experiment 1952

• Viruses that infect bacteria have a simple
  structure
• DNA core surrounded by a protein coat
• Hershey and Chase used two different radioactive
  isotopes to label the protein and DNA
• Radioactive phosphorous in DNA
• Radioactive sulfur in protein coats

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Thus, viral DNA directs the production of new
viruses
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Discovering the Structure of DNA

• DNA is made up of nucleotides
• Each nucleotide has a central sugar, a phosphate
  group and an organic base
• The bases are of two main types
• Purines Large bases
• Adenine (A) and Guanine (G)
• Pyrimidines Small bases
• Cytosine (C) and Thymine (T)

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Fig. 9.3 The four nucleotide subunits that make
up DNA
Nitrogenous base
5-C sugar
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• Erwin Chargaff made key DNA observations that
  became known as Chargaffs rule

• Purines Pyrimidines

• A T and C G

• Rosalind Franklins X-ray diffraction experiments
  revealed that DNA had the shape of a coiled
  spring or helix

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• In 1953, James Watson and Francis Crick deduced
  that DNA was a double helix

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Dimensions suggested by X-ray diffraction
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How the DNA Molecule Replicates

• The two DNA strands are held together by weak
  hydrogen bonds between complementary base pairs
• A and T
• C and G

• Each chain is a complementary mirror image of the
  other
• So either can be used as template to reconstruct
  the other!

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• There are 3 possible methods for DNA replication

Daughter DNAs contain one old and one new strand
Old and new DNA are dispersed in daughter
molecules
Original DNA molecule is preserved
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• Meselson-Stahl Experiment 1958

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Thus, DNA replication is semi-conservative
Fig. 9.6
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DNA Replication is Semi-conservative
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How DNA Copies Itself

• The process of DNA replication uses 3 main
  enzymes

• Helicase
• unwinds the double helix
• Primase
• lays down a short piece of RNA termed the primer
• DNA polymerase
• reads along each naked single strand adding the
  complementary nucleotide
• Ligase
• Glues ministrands together on lagging side

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Fig. 9.7 How nucleotides are added in DNA
replication
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How Do Cells Make Proteins from
Genes?Transcription and Translation
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Transcription

• Produces mRNA
• The transcriber is RNA polymerase
• It binds to one DNA strand at a site called the
  promoter
• It then moves along the DNA pairing complementary
  nucleotides

• RNA bases
• Adenine
• Guanine
• Cytocine
• Uracil replaces Thymine
• It disengages at a stop signal
• mRNA falls off, exits nucleus through nuclear
  pores, goes to cytoplasm

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DNA to mRNA

• Original DNA code TACCGCTTCAGAATT
• What will be the mRNA code?

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DNA TACCGCTTCAGAATT mRNA AUGGCGAAGUCUUAA
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Translation

• Translation converts the order of the nucleotides
  of mRNA into the order of amino acids in a
  protein
• The rules that govern translation are called the
  genetic code
• mRNAs are the blueprint copies of nuclear genes
• mRNAs are read by a ribosome in
  three-nucleotide units, termed codons
• Each three-nucleotide sequence codes for an amino
  acid or stop signal
• AUG-GCG-UUG-UCU-UAA

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Translation

• DNA TACCGCTTCAGAATT
• mRNA AUGGCGAAGUCUUAA
• Amino acids methionine-alanine-leucine-serine-st
  op

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The Genetic Code is Universal

• Genetic code codes for the same amino acids
  across species!!!

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Ribosomes

• The protein-making factories of cells

• They use mRNA to direct the assembly of a protein

• A ribosome is made up of two subunits

• Each of which is composed of proteins and rRNA

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Transfer RNA
Hydrogen bonding causes hairpin loops

• tRNAs bring amino acids to the ribosome
• They have two business ends
• Anticodon which is complementary to the codon on
  mRNA
• 3OH end to which the amino acid attaches

3-D shape
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Making the Protein

• mRNA binds to the small ribosomal subunit
• The large subunit joins the complex, forming the
  complete ribosome
• mRNA threads through the ribosome producing the
  polypeptide

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Fig. 9.15 How translation works

• The process continues until a stop codon enters
  the A site
• The ribosome complex falls apart and the protein
  is released

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What does a gene actually look like?

• In eukaryotes, genes are fragmented
• They are composed of
• Exons Sequences that code for amino acids
• Introns Sequences that dont
• Eukaryotic cells transcribe the entire gene,
  producing a primary RNA transcript
• This transcript is then heavily processed to
  produce the mature mRNA transcript
• This leaves the nucleus for the cytoplasm

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• Different combinations of exons can generate
  different polypeptides via alternative
  splicingWOW!!!

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Mutation

• The genetic material can be altered in two ways
• Recombination
• Change in the positioning of the genetic material
• Mutation
• Change in the content of the genetic material

Bithorax mutant
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Mutation

• Mutation and recombination provide the raw
  material for evolution
• Evolution can be viewed as the selection of
  particular combinations of alleles from a pool of
  alternatives
• The rate of evolution is ultimately limited by
  the rate at which these alternatives are
  generated
• Mutations in germ-line tissues can be inherited
• Mutations in somatic tissues are not inherited

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Kinds of Mutations

• There are two main types of mutations
• Sequence Changes
• Mistakes during DNA Replication
• Mutagens damaging DNA
• Changes in Gene Position
• Transposons, chromosomal rearrangements

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Mutation Sequence Changes

• The sequence of DNA can be altered in one of two
  main ways
• Point mutations
• Alteration of one or a few bases
• Base substitutions, insertion or deletion
• Frame-shift mutations
• Insertions or deletions that throw off the
  reading frame

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Point Mutation
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Base Pair Substitution
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Frameshift Mutation
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Mutation Changes in Gene Position

• The position of genes can be altered in one of
  two main ways
• Transposition
• Movement of genes from one part of the genome to
  another
• Occurs in both eukaryotes and prokaryotes
• Chromosomal rearrangements
• Changes in position and/or number of large
  segments of chromosomes in eukaryotes

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