Download Chapter 12 - WordPress.com

Chapter 12
DNA & RNA
12 – 1 DNA
Deoxyribonucleic acid
Nucleotides
• Units that make up DNA
molecule
• Made of three parts
1. 5 carbon sugar (deoxyribose)
2. Phosphate group
3. Nitrogen bases
4 kinds of nitrogen bases
1.
2.
3.
4.
Adenine
Guanine
Cytosine
Thymine
(A)
(G)
(C)
(T)
Chargaff’s Rule
• A=T and G=C
X-Ray Evidence
• Rosalind Franklin
• British Scientist
• Used a technique
called X-Ray
diffraction
• Provided important
clues about the
structure of DNA
X-Ray Evidence
• There were 2
strands
• Strands were
twisted around
each other
(helix)
• The nitrogen
bases are in the
middle
Watson & Crick
The Double Helix
• Francis Crick & James Watson
• Trying to understand the structure of DNA by
building models
• Unsuccessful until early 1953, Watson was
shown a copy of Franklin’s X-ray pattern
• “The instant I saw the picture my mouth fell open
and my pulse began to race.”
– James Watson
• Within weeks Watson and Crick had figured out
the structure of DNA
• Published their results in a historic one page
paper in April of 1953
• Watson and Crick later discovered what
held the two strands together
• Hydrogen bonds could form between
certain nitrogen bases and provide
enough force to hold the two strands
together
• Hydrogen bonds could only form between
certain base pairs adenine and thymine
and guanine and cytosine
• This principal is called Base pairing
• This explains Chargaff’s Rule
Chromosomes
and DNA Replication
• To extract DNA for analysis, you
need to know where to find it and
how its organized
• DNA is located in the nucleus
• DNA is organized into
chromosomes
Prokaryotic Cells
• Prokaryotic cells have a single
circular DNA molecule that contains
nearly all of its genetic information
• Located in the cytoplasm
Eukaryotic Cells
• Much more complex
• 1000 times the amount of DNA as
prokaryotes
• DNA is located in the nucleus in
the form of chromosomes
Chromosome Structure
• Q: If eukaryotic DNA can contain
a meter or more of DNA, how
does it get packed in so tight into
chromosomes?
• A: Eukaryotic chromosomes
contain both DNA and protein that
form a substance called
chromatin
Histones
• Proteins that coil up DNA
• DNA + histone molecules form a
bead-like structure called a
nucleosome
• Nucleosomes pack together to form
thick fibers that loop and coil together
to form chromosomes
DNA coils around histones to form nucleosomes,
which coil to form chromatin fibers. The chromatin
fibers super coil to form chromosomes that are
visible in the metaphase stage of mitosis.
DNA Replication
• When Watson and Crick discovered
the double helix structure of DNA they
recognized immediately how DNA
could copy itself
• The strands are complementary
• If you could separate the two strands,
the rules of base pairing would allow
you to reconstruct the base sequence
of the other strand
Replication
• When the DNA splits into 2
strands, then produces 2 new
strands following the rules of base
pairing
Semiconservative Replication
• Parental strands of DNA separate, serve
as templates, and produce DNA molecules
that have one strand of parental DNA and
one strand of new DNA.
How Replication Occurs
1. Unwinding
2. Synthesizing
3. Joining
Getting Started
Origins of replication- short stretched of
DNA having a specific sequence of
nucleotides.
– The place where DNA replication begins.
Replication fork- Y-shaped region where the
parental strands of DNA are being unwound.
1. Unwinding
• Replication is carried out by enzymes
• Before DNA replicates, the double helix must
unwind and unzip.
• DNA Helicase-enzyme that is responsible for
unwinding and unzipping the double helix.
1. Unwinding
• When the double helix unwinds/unzips the
H+ bonds between the bases are broken,
leaving single strands of DNA.
• Then, proteins called single-stranded
binding proteins associate with the DNA to
keep the strands separate during
replication.
• As the helix unwinds, another enzyme,
RNA Primase, adds a short segment of
RNA, called RNA primer on each DNA
strand.
1. Unwinding
• The untwisting of the double helix causes
tighter twisting and strain ahead of the
replication fork.
Topoisomerase- helps relieve this strain by
breaking, swiveling, and rejoining DNA
strands.
1. Unwinding
Helicase
2. Synthesi
zing
• Within a bubble, the unwound sections of
parental DNA strands are available to
serve as templates for the synthesis of
new complementary DNA strands.
• As the helix unwinds, another enzyme,
RNA Primase, adds a short segment of
RNA, called RNA primer on each DNA
strand.
2. Base Pairing
DNA Polymerase:
• Joins individual nucleotides to produce a DNA
molecule, which is a polymer
• The nucleotides are added to the 3’ end of the new
strand.
• Also proof reads each new DNA strand
2. Base Pairing
• DNA polymerase continues adding new DNA
nucleotides to the chain by adding to the 3’
end of the DNA strand.
• A binds with T / C binds with G
• This allows for identical copies to be made
Leading Strand: elongated as the DNA unwinds
– Continuously added to 3’ end
Lagging Strand: elongates away from fork
– Done by adding small fragments. (Okazaki fragments)
3. Joining
• When the DNA polymerase comes to an
RNA primer on the DNA, it removes the
primer and fills in the place with DNA
nucleotides.
• When the RNA primer has been replaced,
DNA ligase links the two sections.
Do Now
Place the following steps of
Replication in order.
DNA unwinds
Sugar and phosphate groups
form the side of each new
strand
DNA Unzips
The bases attach from a
supply in the cytoplasm
Do Now
Place the following steps of
Replication in order.
1. DNA unwinds
2. DNA Unzips
3. The bases attach from a
supply in the cytoplasm
4. Sugar and phosphate
groups form the side of
each new strand
DNA replication results in 2 DNA
molecules
a. Each with two new strands
b. One with two new strands and
the other with two original
strands
c. Each with one new strand and
one original strand
d. Each with two original strands
12 – 3 RNA and Protein
Synthesis
Genes
• Coded DNA instructions that
control the production of proteins
• DNA never leaves the nucleus,
therefore the code must be copied into
• RNA, or ribonucleic acid
• There are 3 main differences between
RNA and DNA
1. It has the sugar ribose, instead of
deoxyribose
2. RNA is single stranded
3. RNA contains uracil in place of thymine
• RNA is like a disposable copy of a
segment of DNA
• RNA is like a working copy of a
single gene
Types of RNA
1. Messanger RNA (mRNA)
• Serve as messangers from DNA to
the rest of the cell
2. Ribosomal RNA (rRNA)
• Type of RNA that makes up parts of
ribosomes
3. Transfer RNA (tRNA)
• Transfers each amino acid to the
ribosome as it is specified by the
mRNA
Types of RNA
Transcription
• RNA molecules are produced by copying
part of the DNA sequence into RNA
• Transcription requires an enzyme known
as RNA polymerase
• During transcription, RNA polymerase
binds to DNA and separates the DNA
strands. RNA polymerase then uses one
strand of DNA as a template from which
nucleotides are assembled into a strand of
RNA.
• Q: How does RNA polymerase
“know” where to start and stop
making a RNA copy of DNA?
• A: promoters
• Signals in DNA that indicate to
the enzyme where to bind to
make RNA
• Similar signals in DNA cause
transcription to stop
RNA Editing
• Remember, a lot of DNA doesn’t
code for proteins
• Introns – not involved in coding
for proteins
• Exons – code for proteins
• The introns get cut out of the RNA
molecules before the final mRNA is made
The Genetic Code
• Proteins are made by joining amino acids
into long chains called polypeptides
• Each polypeptide contains a combination
of any or all of the 20 different amino acids
• The properties of proteins are determined
by the order in which different amino acids
are joined together
• The language of mRNA instructions is
called the genetic code
• The code is read three letters at a
time
• Each 3 letter “word” is called a codon
• Each codon corresponds to an amino
acid that can be added to the
polypeptide
UCGCACGGU
This sequence would be
read three bases at a time
as:
UCG-CAC-GGU
The codons represent the
different amino acids:
UCG-CAC-GGU
Serine-Histidine-Glycine
Translation
• The sequence of nucleotide
bases in an mRNA molecule
serves as instructions for the
order in which amino acids are
joined to make a protein
• Proteins are put together on
ribosomes
Translation
• Decoding mRNA into a protein
Steps of Translation
1. mRNA is transcribed from DNA in the
nucleus and released into the cytoplasm
2. mRNA attaches to a ribosome
3. as each codon of the mRNA molecule
moves through the ribosome, the proper
amino acid is transferred to the growing
amino acid chain by tRNA
• tRNA carries only one kind of amino acid
and three unpaired bases called the
anticodon
4. The amino acid chain continues
to grow until the ribosome
reaches a stop codon on the
mRNA molecule
The Roles of RNA and DNA
• You can compare the different roles played by DNA and
RNA molecules in directing protein synthesis to the two
types of plans used by builders. A master plan has all
the information needed to construct a building. But
builders never bring the valuable master plan to the
building site, where it might be damaged or lost. Instead,
they prepare inexpensive, disposable copies of the
master plan called blueprints. The master plan is safely
stored in an office, and the blueprints are taken to the job
site. Similarly, the cell uses the vital DNA “master plan”
to prepare RNA “blueprints.” The DNA molecule remains
in the safety of the nucleus, while RNA molecules go to
the protein-building sites in the cytoplasm—the
ribosomes
Genes and Proteins
• Q: If most genes contain nothing
more than instructions for
assembling proteins, what do
proteins have to do with traits?
• A: Everything, proteins are
microscopic tools designed to
build or operate a component of a
living cell
12 – 4 Mutations
Mutations
• Changes in the genetic material
Point mutations
• Changes in one or a few
nucleotides
Ex.) substitutions, insertions,
deletions
Frameshift mutations
• Mutation that shifts the “reading”
frame of the genetic message by
inserting or deleting a nucleotide
Chromosomal Mutations
Significance of Mutations
• Most mutations don’t do anything
• Mutations that cause drastic
changes in proteins produce
defective proteins that disrupt
normal biological activities
• Mutations are also a source of
genetic variability which can be
beneficial
Polyploidy
• When plants produce triploid (3N)
or tetraploid (4N) organisms
• These plants are often larger and
stronger
Do Now
• Look at the bottom strand of DNA on the
window blinds
• Suppose the second T was changed to a
C
• How would this specifically alter the
resulting amino acid chain?
• What kind of mutation is this?
Do Now #2
• What if we got rid of the first G in the
bottom strand of DNA
• How would this specifically alter the amino
acid chain produced?
• What kind of mutation is this?
Do Now #3
• How are substitution/point mutations and
frameshift mutations similar?
• How are they different?