Download Chapter 11 ~ DNA and the Language of Life

DNA and the Language
of Life
Chapter 10 and 11
Virus Experiments Provide More
Evidence
Hershey and Chase – Used radioactive isotopes on
virus components to further confirm that it was
DNA that transformed cells and contain hereditary
information
DNA by the numbers
• Each cell has about 2 m of DNA.
• The average human has over a trillion cells.
• The average human has enough DNA to go
from the earth to the sun more than 400
times.
• DNA has a diameter of only 0.000000002 m.
DNA
• DNA is often
called the
blueprint of life.
• In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
Why do we study DNA?
We study DNA for
many reasons
• its central
importance to all
life on Earth
• medical benefits
such as cures for
diseases
• better food crops
Nucleotides
•
•
Building blocks of nucleic acid polymers
Structure
1. Ring-shaped sugar
2. Phosphate group
3. Nitrogenous base
Nucleotides
•
Pyrimidines
– Single ring
1. Thymine
2. Cytosine
•
Purines
– Double ring
1. Adenine
2. Guanine
• Sugar-phosphate backbone
Discoverers of DNA Structure
• 1950s
• Rosalind Franklin
and Maurice Wilkins
• James Watson and
Francis Crick
• Nobel Prize in 1953
The Shape of the Molecule
• DNA is a very
long polymer.
• The basic shape
is like a twisted
ladder or zipper.
• This is called a
double helix.
Complimentary Base Pairs
• Individual structures of the nitrogenous
bases determine very specific pairings
between the two strands
• Adenine pairs with Thymine
(A is complimentary with T)
• Guanine pairs with Cytosine
(G is complimentary with C)
• The sequence of nucleotides along the
length of one side determine the sequence
of nucleotides along the length of the other
side
N
N
C
N
N
C
C
C
O
• The bases attract each
other because of
hydrogen bonds.
• Hydrogen bonds are
weak but there are
millions and millions of
them in a single
molecule of DNA.
• (The bonds between
cytosine and guanine
are shown here.)
C
N
Hydrogen Bonds
N
C
N
C
C
C
N
O
DNA Replicates
• Replication – makes a copy of itself
• Occurs just before cell division
• Necessary so that each new cell gets a
copy of the parent cell’s DNA
• Occurs in nucleus during interphase
Replication
Steps in DNA replication:
1. Double helix unwinds, then unzips.
2. Nucleotides on each of two single strands
are now available to form base pairs with
their complement from a new free-floating
nucleotide.
3. Both original strands can be copied, making
two double helices from one original.
4. The new helices are composed of half old
(original) and half new nucleotides.
Replication is complex, but catalyzed by
enzymes
a) The addition and matching of new bases is
catalyzed by DNA polymerase.
b) DNA ligase permanently attaches short
sections together to make one long
chromosome.
DNA is now copied so mitosis or
meiosis can take place.
• George Beadle and Edward Tatum
• Orange bread mold
• The function of an individual gene is to
dictate the production of a specific enzyme
Let’s Review!
• Where is the DNA stored?
• What is the membrane around that
organelle called?
• What are the openings in that membrane
that lead to other parts of the cell called?
• What organelle makes proteins?
Steps from DNA to Proteins
• While DNA has the information for all the cell’s
activities, it is not directly involved in the day to
day operations of the cell.
– Proteins are responsible for implementing the
instructions contained in DNA
• Each gene along a DNA molecule directs the
synthesis of a specific type of messenger RNA
molecule (mRNA).
• The mRNA interacts with the proteinsynthesizing machinery to direct the ordering of
amino acids in a polypeptide.
• RNA= ribonucleic acid
• Ribonucleic acid
• Ribose=
• Single stranded
• Needed for the production of proteins
• Nitrogenous base uracil instead of thymine
Steps from DNA to Proteins
1. DNA is copied or transcribed
into a single strand of mRNA
(messenger RNA)
2. The mRNA exits the nucleus
through a nuclear pore
3. The mRNA then is read by
Ribosomal RNA.
4. In the ribosome, the mRNA
is read and amino acids
attached to tRNA (transfer
RNA) are assembled to
make proteins.
DNA -> RNA -> Protein
Genetic code
• DNA contains a triplet code
• Every three bases on a DNA strand code
for one amino acid
• Each three-letter unit on mRNA is called a
codon
• Some amino acids can have more than
one codon
• The code is nearly universal to all living
organisms
– All animals, plants, fungi, bacteria, archaea, and viruses use this
same genetic code.
– All these organisms evolved from the same chemical basis, so
the same chemical process continued through evolution.
Transcription
• Transcription= process in which DNA’s
nucleotide sequence is converted to the form of
a single-stranded RNA molecule
• mRNA= messenger RNA
– RNA molecule transcribed from a DNA template
• DNA molecule unzips
• RNA bases pair with the complementary DNA
bases
• RNA has uracil instead of thymine
• RNA polymerases links the RNA nucleotides
together
RNA polymerase
 Enzyme found in the nucleus
 Separates the two DNA strands by
breaking the hydrogen bonds
between the bases
 Then moves along one of the DNA
strands and links RNA nucleotides
together
• In prokaryotes, the mRNA transcribed from a
gene is the direct messenger molecule needed
to make a protein
• In eukaryotes, the mRNA has to be modified
before it leaves the nucleus
• Noncoding nucleotides interrupt the nucleotide
sequences
• Internal noncoding sequences are called
introns
• Researchers are still trying to determine their
function
• The coding regions, or the parts of the gene that
will be expressed are called exons
• Before mRNA leaves the nucleus, the introns
are removed and the exons are connected
together in a process called RNA splicing
Translation
• tRNA= transfer RNA, translates the three-letter
codons of mRNA to the amino acids that make
up proteins (an “interpreter”)
• Picks up the appropriate amino acid floating in
the cytoplasm
• Transports amino acids to the mRNA
• Have anticodons that are complementary to
mRNA codons
• Recognizes the appropriate codons on the
mRNA and bonds to them with Hydrogen bonds
• (there is a different version of tRNA molecule
that matches each codon)
amino acid
attachment site
U A C
anticodon
Ribosome
• Coordinates the functioning of mRNA and tRNA
• Consists of two subunits made up of proteins
and rRNA (ribosomal RNA)
– Small subunit with a binding site for mRNA
– Large subunit with a binding site for tRNA
• rRNA= made in nucleolus, and functions to
decode the mRNA into amino acids
• The subunits of the ribosome act as a vise
holding the mRNA and tRNA close together
• The ribosome connects the newly arrived amino
acid to the growing polypeptide chain
Mutations
A. Mutation = permanent alteration in cell’s
DNA base sequence
B. Almost all cancers begin as a
mutation that is passed along at
replication.
1.
In somatic cells (body cells)
2.
Mutation rate is low, but after decades of
accumulated mutations, cells can become
malignant.
C. Heritable mutations occur in germ-line
cells (cells that divide to make sperm and
eggs).
D. Heritable mutations also create genetic
diversity.
Accidents Happen With Some “Accidents” (Base Mismatches)
Leading to Mutation
A mutation is a heritable change in DNA sequence.
X rays, ultraviolet light, and radioactive substances can change the
chemical nature of DNA, and are classified as mutagens.