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Molecular Biology
DNA Structure and Function
CHAPTER 9
PART A
Chromosome Review
oChromatin
◦ DNA and associated proteins
oChromosome
◦ Unduplicated and duplicated
◦ Sister chromatids
◦ Centromere
centromere
The Structure of DNA
oFrancis Crick and James Watson, 1953
◦ Used key pieces of information to determine the structure of DNA
◦ Four kinds of monomers
◦ Adenine, Guanine, Thymine, and Cytosine
◦ Chargaff Rules
◦ The amount of adenine and thymine is the same
◦ The amount of guanine and cytosine is the same
◦ Rosalind Franklin’s X-ray crystallography
◦ Basic shape, size, and spacing of DNA
The Structure of DNA
oFrancis Crick and James Watson, 1953
◦ DNA consists of two antiparallel chains of nucleotides coiled into a
right-handed double helix
The Structure of DNA
oNucleotides
◦ There are three parts to each nucleotide
◦ Phosphate group
◦ A five carbon sugar called deoxyribose
◦ One of four different nitrogenous bases
◦
◦
◦
◦
Adenine
Guanine
Thymine
Cytosine
The Structure of DNA
oNucleotide chains
◦ Nucleotides bond to each other creating a long chain
◦ The phosphate group of one nucleotide makes a covalent bond
with the sugar of the next nucleotide
◦ This creates an alternating phosphate-sugar backbone for the
chain (rails of the ladder)
◦ The bases extend out from the backbone (inside the helix)
◦ The order of the bases is tremendously variable and codes
for all inherited traits in all living organisms
The Structure of DNA
oNucleotide chains
◦ Two chains of nucleotides are held together by
hydrogen bonds
◦ The hydrogen bonds occur between the internally
positioned bases (creating the “rungs” of the ladder)
◦ Complementary base pairing
◦ A-T
◦ G-C
T-A
C-G
The Structure of DNA
oNucleotide chains
◦ The two strands are anti-parallel
◦ oriented in opposite directions
◦ One strand has the 3’ carbon in the upward position
◦ The other strand has the 5’ carbon in the upward position
The Structure of DNA
◦ DNA consists of two antiparallel chains of nucleotides coiled
into a right-handed double helix
Questions
1. Who were the scientists that described the structure of DNA?
2. DNA molecules are long chains of _____.
3. What are the three parts of a nucleotide?
4. What are the four nitrogenous bases used in DNA?
5. Which complementary base pairings form?
6. What type of chemical bond holds the two chains of DNA together?
The Structure of RNA
oRNA = ribonucleic acid
oVery similar to DNA with three distinct differences
◦ The 5 carbon sugar molecule is ribose instead of
deoxyribose
◦ RNA is single stranded
◦ The base uracil is used instead of thymine
◦ Complementary base pairing rules
◦
◦
◦
◦
A-U
T-A
G-C
C-G
DNA
RNA
The Structure of RNA
 Ribonucleic acid (RNA)
 There are three types of RNA transcribed
Messenger RNA (mRNA)
◦ Transcribed from genes that encode proteins
◦ Used to carry the instructions for synthesizing proteins from the nucleus
to the cytoplasm
Ribosomal RNA (rRNA)
◦ Major portion of the ribosomes
◦ the structures used to assemble polypeptide chains
Transfer RNA (tRNA)
◦ Delivers amino acids one by one to ribosomes, in the order specified by
the mRNA
◦ Amino acids = polypeptide monomers
How DNA is arranged in a cell
oDNA is protected and packaged in very
specific ways
◦ Must be able to be
◦ replicated when a cell is ready to divide
◦ “read” to produce proteins to carry out cellular functions
◦ packed into a small space
DNA Replication
oChromosomes typically have only one molecule of DNA (double helix)
◦ When a cell reproduces, each chromosome needs two DNA molecules
◦ One for each of the future offspring
oDNA replication during the S stage of the cell cycle copies the DNA of
each chromosome before cell division
◦ One unduplicated chromosome becomes one duplicated chromosome
DNA Replication
oDNA helicase unwinds DNA starting at the origin of
replication
◦ Eukaryotic: multiple origins of replication
◦ Prokaryotic: one origin or replication
DNA Replication
oDNA helicase unwinds DNA starting at the origin of
replication
◦ Two replication forks are formed
◦ Extend in both directions as replication proceeds
DNA Replication
oNew bases are added to the parental strands
◦ Each original strand is a template
◦ Complementarity makes it possible to recreate the other strand
◦ DNA polymerase matches base pairs
◦ A-T and G-C
DNA Replication
oDNA polymerase requires a “primer” as a starting point
◦ Short fragment of RNA which is removed later
DNA Replication
◦ DNA polymerase can only construct nucleotide strands in the 5’  3’
direction
◦ The leading strand is synthesized continuously
◦ The lagging strand is synthesized in short fragments (Okazaki fragments)
DNA Replication
◦ Finishing steps
◦ Primers are removed and replaced with DNA nucleotides
◦ Backbone is sealed by DNA ligase
DNA Replication
◦ Two identical sister chromatids are created
◦ Each has half of the original DNA molecule and half new nucleotides
◦ Replication is semiconservative
DNA Replication
◦ Telomere replication
◦ Ends of chromosomes
◦ Chromosome ends get progressively shorter with
each replication
◦ Lagging strand has no place for a primer to be made
◦ Associated with aging
◦ Telomerase is used in some cells to maintain
chromosome ends
Questions
1. Why do cells need two molecules of DNA when they reproduce?
2. What does DNA helicase do?
3. What does DNA polymerase do?
4. What does DNA ligase do?
5. Make the single-stranded DNA double stranded by using
complimentary base pairings.
ATGCCGA
6. Is replication conservative, semi-conservative, or nonconservative
DNA Repair
oErrors can be introduced into DNA
◦ DNA polymerase can make mistakes (“typos”)
◦ Environmental conditions: radiation and mutagenic chemicals
DNA Repair
oRepairs
◦ DNA polymerase proofreads and corrects mistakes
◦ Mismatch repair
◦ Uncorrected replication mistakes
DNA Repair
oRepairs
◦ Nucleotide excision repair
◦ Thymine dimers
◦ Caused by ultraviolet light
◦ If not removed they lead to DNA mutations
DNA Repair
oMutations
◦ Uncorrected mistakes may result in a mutation
◦ Mutation = a permanent change in the DNA sequence
◦ Serious consequences like cancer
◦ Positive consequences like a new adaptive trait
Summary
oThe structure of DNA
◦ History
◦ Chemical structure
oHow DNA is arranged in cells
oDNA replication
oDNA repair