DNA and Genes

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DNA and Genes

• Unit 4
• Chapter 11

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DNA structure

• DNA controls cellular activity because it
  regulates the production of proteins.
• DNA is the blueprint for proteins that are
  necessary for cellular metabolism.

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Why are proteins so important?

• Some proteins become important structures, such
  as the filaments in muscle tissue.
• Other proteins, such as enzymes, control chemical
  reactions that perform key life
  functionsbreaking down glucose molecules in
  cellular respiration, digesting food, or making
  spindle fibers during mitosis.

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Who discovered that DNA is the blueprint for life?

• In 1952 Alfred Hershey and Martha Chase performed
  an experiment using radioactively labeled viruses
  that infect bacteria.
• Because viruses are protein and DNA only, they
  figured out that viral DNA (not viral protein)
  could force the bacteria to make new viruses.
• This was evidence that DNA can determine cell
  activity.

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DNA is a polymer.

• Polymer chemical structure made of repeating
  units
• DNA is made of repeating nucleotide units.
• DNA nucleotides always have a phosphate group,
  deoxyribose sugar, and a nitrogen base.

DNA nucleotide
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Four DNA nitrogenous bases

• A nitrogenous base is a carbon ring with nitrogen
  atoms and determines the name of the nucleotide.

• In DNA, there are four possible nitrogenous
  bases adenine (A), guanine (G), cytosine (C),
  and thymine (T).

Cytosine (C)
Guanine (G)
Thymine (T)
Adenine (A)
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Section 11.1 Summary pages 281 - 287
The structure of nucleotides

• Nucleotides join together to form long chains,
  with the phosphate group of one nucleotide
  bonding to the deoxyribose sugar of an adjacent
  nucleotide.

• The phosphate groups and deoxyribose molecules
  form the backbone of the chain, and the
  nitrogenous bases stick out like the teeth of a
  zipper.

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DNA is a double helix and looks like a twisted
ladder.

• The outer parts are the sugar-phosphate backbone.
• Two nitrogen bases of the nucleotides face inward
  and form the rungs of the helix ladder.
• Adenine always binds to thymine.
• Cytosine always binds to guanine..

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Who discovered the double helix structure?

• In 1953, Watson and Crick proposed that DNA is
  made of two chains of nucleotides held together
  by nitrogenous bases and twisted together.
• They used Rosalind Franklins X-ray
  crystallography work to figure this out.

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The importance of comparing DNA nucleotide
sequences

• Each species has its own unique DNA sequence.
• The more closely related two individuals are, the
  more likely they will share the same DNA
  nucleotide sequence.
• Comparing DNA base pairs of two species will show
  their evolutionary history.

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DNA replication making copies of the DNA code

• Necessary before a cell undergoes mitosis, occurs
  in interphase

Click on image to play video.
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Copying DNA
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How does DNA code for proteins?

• The sequence of nucleotides in each gene contains
  information for assembling the string of amino
  acids that make up a single protein.
• The ribosomes required to make proteins cannot
  read DNA.
• Therefore, for DNA to code for proteins, an RNA
  molecule must be made.
• Ribosomes can read RNA.

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RNA is another nucleic acid, nucleotide polymer.

• RNA differs from DNA structure in three ways.
• Single stranded instead of double stranded
• Ribose sugar instead of deoxyribose
• Uracil instead of thymine nitrogen base

Ribose sugar
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Three types of RNA

• Messenger RNA carries the DNA code (message) to
  the ribosomes
• Ribosomal RNA makes up the ribosomes that reads
  the mRNA to build the correct amino acid sequence
• Transfer RNA brings the amino acids to the
  ribosome

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Transfer RNA
Click on the image to play the video.
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Transcription

• The process of building an RNA strand from the
  DNA template

In eukaryotes, this occurs inside the nucleus. In
prokaryotes, this occurs in the cytoplasm.
Click on image to play the video.
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Transcription
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mRNA processing in eukaryotes

• Since much of the DNA code is useless or codes
  for multiple proteins, the unnecessary portions
  of DNA that were coded into mRNA must be removed.
• The useless portions of RNA (introns) are
  removed. The coding portions (exons) are linked
  together to make the final mRNA.

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mRNA codes for amino acids.

• Three mRNA nucleotides code for one amino acid,
  but more than one combination codes for the same
  amino acid.

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Translation

• The process of ribosomes reading the mRNA code to
  properly make an amino acid chain that is folded
  into a usable protein

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Translation

• The ribosome binds to AUG, the starting code
  (codon). The ribosome directs the methionine tRNA
  to bring transfer the methionine (met) amino
  acid.

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Translation

• The ribosome read the next codon and directs the
  appropriate tRNA to transfer the amino acid.

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Translation process

• The ribosome joins the amino acids together and
  continues this process until the codon indicates
  stop.

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What happens if there is a mistake (mutation) in
the DNA code?

• Possibly proteins wont be made or are made
  improperly.
• If the mutations occur in the gametes, the
  offsprings DNA will be affected positively,
  negatively, or neutrally.
• What can cause a mutation?
• Replication error
• Transcription error
• Cell division error
• Chemical agents (mutagens)
• Spontaneous changes

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Point mutation

• A point mutation is a change in a single base
  pair in DNA.
• A change in a single nitrogenous base can change
  the entire structure of a protein because a
  change in a single amino acid can affect the
  shape of the protein.

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Point mutations

• May change the amino acid code if the mutations
  occurs in the right place in the code.

mRNA
Normal
Protein
Stop
Replace G with A
Point mutation
mRNA
Protein
Stop
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Frameshift mutations

• Losing a single nucleotide base
• This mutation would cause nearly every amino acid
  in the protein after the deletion to be changed

Deletion of U
mRNA
Protein
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Changes to the chromosome

• When a part of a chromosome is left out, a
  deletion occurs
• When part of a chromatid breaks off and attaches
  to its sister chromatid, an insertion occurs.

A B C D E F G H
A B C E F G H
Deletion
A B C D E F G H
A B C B C D E F G H
Insertion
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Changes to the chromosome

• When part of a chromosome breaks off and
  reattaches backwards, an inversion occurs.
• When part of one chromosome breaks off and is
  added to a different chromosome, a translocation
  occurs.

A B C D E F G H
A D C B E F G H
Inversion
G
E
H
F
A
B
F
C
G
D
E
D
C
B
X
A
W
H
W
X
Y
Z
Y
Z
Translocation
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Repairing DNA

• Enzymes proofread the DNA and replace incorrect
  nucleotides with correct nucleotides.
• The greater the exposure to a mutagen such as UV
  light, the more likely is the chance that a
  mistake will not be corrected.