Download Chapter 11 – What is DNA and how does it work?

Chapter 4
Section 1: What does DNA
Look Like?
DNA
• DNA stands for: Deoxyribonucleic
acid.
• DNA stores and passes on genetic
information from one generation to
the next.
Discovering DNA
• In the 1950’s Rosalind Franklin &
Maurice Wilkins took x-ray pictures of
DNA, which suggested that it had a
spiral shape.
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.
Examples: Double Helix
Calling DNA a
double helix is
the same as
saying it’s a
twisted ladder.
A staircase in the
shape of a double
helix, in the
Vatican Museum
Structure of DNA
The sides of the “ladder” are made up of
millions of alternating sugars &
phosphates.
• The sugar is named deoxyribose.
• The sides of the
ladder are held
together by rungs
(steps) attached to
the sugars.
Structure of DNA
• The rungs (steps)of the DNA ladder are made up
of two bases that contain nitrogen, called
nitrogenous bases.
•
The 2 bases of each rung are held
together by a hydrogen bond.
• There are 4 bases to choose from:
Adenine
Thymine
Guanine
Cytosine
Erwin Chargraff
Erwin Chargraff discovered that:
•
•
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.
Complementary Base Pairing
• States that only these bases pair
together:
•
•
•
•
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!
1
Try it:
Name the
nitrogenous
base that’s
missing
from each
spot.
2
3
4
5
6
7
8
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.
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
DNA Replication
DNA Replication - process in which DNA makes
an exact copy of itself.
When does DNA replication occur?
• During interphase before mitosis begins.
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).
DNA Replication
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!
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.
Identical
base sequences
DNA Replication
DNA replication animation
Recap Questions
1.) What does DNA stand for?
Deoxyribonucleic acid.
2.) In the 1950’s 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 xray pictures to determine that the shape of
DNA is a double helix.
James Watson & Francis Crick
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
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
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
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)
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.
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
Chapter 4
Section 2: How DNA works
DNA
• DNA is like the blueprint for a house.
• DNA contains instructions that determines
the traits an organism inherits and needs
to live.
DNA is
made up of
smaller
segments
called
genes.
No,
not
those
jeans
No, not that GENE.
DNA
molecule
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.
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
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!
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.
How genes work
• When a particular protein is needed it goes
through a process called protein synthesis.
• Two parts of protein synthesis:
1. Transcription – gene is
copied in nucleus
(because DNA never
leaves the nucleus)
2. Translation – protein is
made at the ribosome
Transcription
• Transcription is like taking the blueprints
and copying a single section about one
specific part of the house.
Ex. Copying the
blueprint only for
the kitchen.
5 Steps of Transcription
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
“Help, a
protein is
needed!”
Steps of Transcription
3.) Complementary nucleotides move into one side of
the gene forming mRNA.
•
mRNA (messenger ribonucleic acid) – the copy of the gene.
A U G
G G C U U A A A G C A G
4.) mRNA leaves the nucleus to go into the
cytoplasm.
5.) DNA rezips at the hydrogen bonds.
Steps of Translation
1.) mRNA goes through the cytoplasm to meet the ribosome.
mRNA
ribosome
A U G
G G C U U A A A
G C A G U G C A C G U U A
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.
ribosome
A U G
G G C U U A A A
codon
codon
codon
G C A G U G C A C G U U A
codon
codon
codon
codon
codon
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.)
Amino acid that the
codon codes for
A U G
G G C U U A A A
codon
codon
codon
G C A G U G C A C G U U A
codon
codon
codon
codon
codon
Steps of Translation
Amino acid
Amino acid
A U G
G G C U U A A A
G C A G U G C A C G U U A
4.) Amino Acids get connected by a peptide bond.
Peptide bond
A U G
G G C U U A A A
G C A G U G C A C G U U A
• Peptide Bond = bond that connects 2
amino acids
Steps of Translation
5.) Many amino acids connected together
makes a protein.
protein
Protein
Differences between mRNA & DNA
DNA
mRNA
•Longer
•Thousands/millions of
genes
•Double stranded (helix)
•Bases - ATGC
•Stays in the nucleus
•Produced in DNA
Replication
•Deoxyribose Sugar
•shorter
•1 gene
•Single stranded (helix)
•Bases – AGCU (U = Uracil)
•Goes from nucleus to
cytoplasm and ribosome
•Produced in Transcription
•Ribose Sugar
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.
Mutations
• Mutations can be good, neutral, or bad.
There are 3 types of mutation:
1. Deletion - one pair of bases is removed.
2. Insertion- one pair of bases is added.
3. Substitution - one pair of bases is
replaced with another pair
Deletion
• When one pair of bases is removed.
Insertion
• When one pair of bases is added.
Substitution
• When one pair of bases is replaced with
another pair of bases.
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”