Chapter 4 Section 1: What does DNA Look Like?

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Chapter 4Section 1 What does DNA Look Like?
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DNA

• DNA stands for Deoxyribonucleic acid.
• DNA stores and passes on genetic information from
  one generation to the next.

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Discovering DNA

• In the 1950s Rosalind Franklin Maurice Wilkins
  took x-ray pictures of DNA, which suggested that
  it had a spiral shape.

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Discovering DNA

• In 1953, James Watson and Francis Crick used the
  x-ray pictures to determine that the shape of DNA
  is a double helix.

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Examples Double Helix
Calling DNA a double helix is the same as saying
its a twisted ladder.
A staircase in the shape of a double helix, in
the Vatican Museum
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The sides of the ladder are made up of millions
of alternating sugars phosphates.
Structure of DNA

• The sugar is named deoxyribose.

• The sides of the ladder are held together by
  rungs (steps) attached to the sugars.

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The rungs (steps)of the DNA ladder are made up of
two bases that contain nitrogen, called
nitrogenous bases.
Structure of DNA

• The 2 bases of each rung are held together by a
  hydrogen bond.
• There are 4 bases to choose from
• Adenine Thymine
• Guanine Cytosine

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Erwin Chargraff discovered that
Erwin Chargraff

• The amount of adenine (A) is always equal the
  amount of thymine (T)
• The amount of cytosine (C) is always equal the
  amount of guanine (G).

• He concluded that a rung can have
• A and T or it can have C G
• This is called complementary base pairing.

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States that only these bases pair together
Complementary Base Pairing

• A always pairs with T
• T always pairs with A
• G always pairs with C
• C always pairs with G.

If you know one base pair of the DNA, you will be
able to figure out the other!
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Try it Name the nitrogenous base thats missing
from each spot.
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Nucleotide

• Nucleotide is a subunit (small piece) of DNA
  which contains
• 1 base
• 1 sugar
• 1 phosphate

Lots of nucleotides connect to form a large DNA
molecule.
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Length of DNA

• Human genome has 3 billion base pairs
• Largest genome South American marbled lungfish
  has 133 billion base pairs.
• Smallest genome Symbiotic insect bacteria,
  Carsonella ruddii has159,662 base pairs.

South American marbled lungfish
Symbiotic insect bacteria, Carsonella ruddii
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DNA Replication

• DNA Replication - process in which DNA makes an
  exact copy of itself.
• When does DNA replication occur?
• During interphase before mitosis begins.

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DNA Replication is semi-conservative.
Semi-conservative - each new DNA is made of ½ new
and ½ old nucleotides. The old nucleotides were
from the original strand (saved).
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DNA Replication
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Steps of DNA Replication

1. DNA unzips at the hydrogen bonds.
2. New complementary nucleotides move in to match
   both halves of the DNA ladder.
3. They form hydrogen bonds with the old
   nucleotides.
4. Two identical DNA molecules are formed!

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DNA Replication

• Why does DNA replication occur?
• So when a cell splits, each cell has identical
  DNA in each cell.

• How is it possible that new DNA and original DNA
  are identical?
• The bases are complementary so they always pair
  with the same exact base.

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DNA Replication
DNA replication animation
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Recap Questions

• 1.) What does DNA stand for?

Deoxyribonucleic acid.
2.) In the 1950s these two scientists took x-ray
pictures of DNA, which suggested that it had a
spiral shape.
Rosalind Franklin Maurice Wilkins
3.) In 1953, these two scientists used the x-ray
pictures to determine that the shape of DNA is a
double helix.
James Watson Francis Crick
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Recap Questions

• 4.) What are the sides of a DNA molecule made up
  of?

Alternating sugars phosphates
5.) What is the name for the sugar on a DNA
molecule?
Deoxyribose
6.) What are the steps attached to on a DNA
molecule?
Sugar
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Recap Questions

• 7.) The rungs (steps)of the DNA ladder are called
  what?

Nitrogenous bases
8.) How many nitrogenous bases attach together on
each step (rung) of a DNA molecule?
Two
9.) What are the two bases of DNA held together
by?
Hydrogen Bond
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Recap Questions

• 10.) What bases always pair together?

Adenine/Thymine Guanine/Cytosine
11.) What scientists discovered that the amount
(A) the amount of (T) and the amount of (C)
the amount of (G).
Erwin Chargraff
12.) Determine the complementary base pairs for
the sequence ATG CCT AGC
TAC GGA TCG
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Recap Questions

• 13.) A subunit of DNA that contains 1 base, 1
  sugar and 1 phosphate is called?

A nucleotide
14.) What do lots of nucleotides together form?
DNA molecule
15.) What is the process in which DNA makes an
exact copy of itself?
DNA replication
16.) When does DNA replication take place?
Interphase (before mitosis)
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Recap Questions

• 17.) What is it called when each new DNA is made
  of ½ new and ½ old nucleotides(which were from
  the original strand)?

Semi-Conservative
18.) Why does DNA replication occur?
So when a cell splits, each cell has identical
DNA in each cell.
19.) How is it possible that new DNA and original
DNA are identical?
The bases are complementary so they always pair
with the same exact base.
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Recap Questions

• 20.) Put the steps of DNA replication in order
• A.) New complementary nucleotides move in to
  match both halves of the DNA ladder.
• B.) Two identical DNA molecules are formed!
• C.) They form hydrogen bonds with the old
  nucleotides.
• D.) DNA unzips at the hydrogen bonds.

D, A, C, B
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Chapter 4Section 2 How DNA works
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DNA

• DNA is like the blueprint for a house.

• DNA contains instructions that determines the
  traits an organism inherits and needs to live.

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• DNA is made up of smaller segments called genes.

No, not those jeans!
No, not that GENE.
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DNA molecule
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How genes work

• A gene consists of a string of nucleotides that
  give the cell instructions about how to make a
  specific trait.
• Genes do not do the work themselves, they carry
  the code for making specific proteins.

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What are Proteins?

• Found throughout cells
• Control cellular functions
• Determine physical traits such as
• Skin Color
• Eye color
• How tall you grow
• What colors you can see
• Whether your hair is curly or straight

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DNA, Genes, Proteins

• DNA has instructions how to make all traits for
  an organism.
• Gene carries instructions for how to make one
  specific trait.
• DNA and genes NEVER leave the nucleus so a copy
  of the gene is needed to make the protein.
• Proteins are made from a copy of one specific
  gene. Proteins do all the work! Yet, DNA gets
  all the credit!

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How genes work

• When a particular protein is needed, that part of
  the DNA (the gene) is copied.
• The copy of the gene is called mRNA or messenger
  ribonucleic acid.

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How genes work

• When a particular protein is needed it goes
  through a process called protein synthesis.

• Two parts of protein synthesis
• Transcription gene is copied in nucleus
  (because DNA never leaves the nucleus)
• Translation protein is made at the ribosome

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Transcription is like taking the blueprints and
copying a single section about one specific part
of the house.
Transcription
Ex. Copying the blueprint only for the kitchen.
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5 Steps of Transcription
Help, a protein is needed!
1.) The cell gets a message that a protein is
needed.
2.) DNA unzips at hydrogen bonds exposing only
one gene.
ONLY one gene being exposed
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Steps of Transcription

• 3.) Complementary nucleotides move into one side
  of the gene forming mRNA.
• mRNA (messenger ribonucleic acid) the copy of
  the gene.
• 4.) mRNA leaves the nucleus to go into the
  cytoplasm.
• 5.) DNA rezips at the hydrogen bonds.

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1.) mRNA goes through the cytoplasm to meet the
ribosome.
Steps of Translation
mRNA
2.) mRNA goes through the ribosome in groups of 3
bases at a time called codons

• Codons 3 bases in a row on an mRNA molecule.

codon
codon
codon
codon
codon
codon
codon
codon
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Steps of Translation

• 3.) The arrangement of bases in a codon creates a
  specific amino acid.
• Amino acid - The combination of the 3 letters on
  a codon. ( ex. AGG, CAG, AGC)
• Example AGG brings a different amino acid than
  CAG, and AGC, etc.)

codon
codon
codon
codon
codon
codon
codon
codon
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Steps of Translation
Amino acid
Amino acid
4.) Amino Acids get connected by a peptide bond.
Peptide bond
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• Peptide Bond bond that connects 2 amino acids

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Steps of Translation
5.) Many amino acids connected together makes a
protein.
protein
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Protein
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Differences between mRNA DNA
DNA mRNA
Longer shorter
Thousands/millions of genes 1 gene
Double stranded (helix) Single stranded (helix)
Bases - ATGC Bases AGCU (U Uracil)
Stays in the nucleus Goes from nucleus to cytoplasm and ribosome
Produced in DNA Replication Produced in Transcription
Deoxyribose Sugar Ribose Sugar
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Mutations

• Mutation- changes or mistakes that take place in
  the DNA or mRNA.
• Happens during DNA replication or transcription.
• Can occur randomly or caused by mutagens such as
• High energy radiation (from x-rays or sunlight)
• Asbestos
• Chemicals in cigarette smoke
• If mutations occur in sex cells, they can be
  passed on to offspring.
• Cells make proteins that can detect errors, so
  they are usually fixed.

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Mutations

• Mutations can be good, neutral, or bad.
• There are 3 types of mutation
• Deletion - one pair of bases is removed.
• Insertion- one pair of bases is added.
• Substitution - one pair of bases is replaced with
  another pair

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Deletion

• When one pair of bases is removed.

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Insertion

• When one pair of bases is added.

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Substitution

• When one pair of bases is replaced with another
  pair of bases.

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Examples using the following sequence

• THE DOG BIT THE CAT
• Deletion THE DOB ITT HEC AT
• Substitution THE DOT BIT THE CAT
• Insertions THE DAO GBI TTH ECA T