The document discusses the structure and function of DNA. It describes DNA as a double helix composed of nucleotides that can replicate itself and transmit genetic information to direct cell function and allow for evolution through mutation. The four main roles of DNA are to replicate, encode information, control cells, and change over time. Gene expression is regulated and involves transcription of DNA into RNA and translation of RNA into proteins.

1. Molecular Biology
What Is DNA and How Does It
Work?

2. DNA Structure Must Be Compatible
with Its Four Roles
• DNA makes copies of itself.
– Occurs during S phase of the cell cycle before
mitosis or meiosis.
• DNA encloses information.
– Information that gives rise to discernible traits
in organisms.

3. DNA Structure Must Be Compatible
with Its Four Roles
• DNA controls cells and tells them what to
do.
– Determines function of the cell.
• DNA changes by mutation.
– Structure must be able to change.

4. Building Blocks of DNA
• Nucleotides
– Three components:
• Five-carbon sugar
• Phosphate group
• Nitrogen-containing
base

5. Building Blocks of DNA
• Four nitrogenous bases in DNA
– Adenine
– Thymine
– Guanine
– Cytosine

6. Structure of DNA
• Maurice Wilkins and
Rosalind Franklin
– Attempted to
determine structure of
DNA.
– Discovered DNA was
a helix.

7. Chargaff’s Ratios
• 1950
– Erwin Chargaff
• Observed that the four nitrogenous bases
conformed to a rule:
– Amount of Adenine = Amount of Thymine
– Amount of Cytosine = Amount of Guanine
• Served as a clue to help Watson and Crick
determine DNA structure!

8. Watson and Crick
• Early 1950s
– They were young scientists
at Cavendish Laboratory in
Cambridge, England.
• Using Chargaff’s ratios
and Franklin’s data,
Watson and Crick
determine DNA structure
is a double helix

9. DNA Double Helix
• Consists of two strands of
nucleotides.
• Nucleotides bonded
together with covalent
bonds.
– Adenine hydrogen bonds
with Thymine.
– Cytosine hydrogen bonds
with Guanine.
• Structure was compatible
with four roles of DNA

10. How Does DNA Copy Itself?
• DNA replication
– Precedes cell division.
– Process:
• DNA strands separate
• New complementary
base pairs are added
forming a new strand
– Result: two double
helices.
• Each containing one
old strand of DNA and
one new strand of DNA

12. Meselson and Stahl
• Proved the mechanism of DNA replication.
– Called semiconservative mechanism.
• Grew bacteria in medium containing
various radioactive nitrogen isotopes.
– Separated DNA by density using a dense,
viscous sugar solution.

14. How is the information in DNA
expressed?
• Genome
– Information to make proteins stored in all of
the DNA of a single set of chromosomes.
• Gene: blueprint for the synthesis of a protein.
• Proteins
– Polymers made of amino acids connected
end-to-end
• Similar to beads on a string.

15. How is the information in DNA
expressed?
• Chromosomes containing DNA contained
in nucleus.
• DNA codes for the construction of proteins
using an intermediary molecule:
– Ribonucleic acid or RNA.
• Decoding information in DNA requires two
processes:
– Transcription.
– Translation.

16. DNA vs. RNA
• RNA:
– Contains the sugar
ribose.
– Contains adenine, uracil,
cytosine and guanine.
– Single helix
• DNA:
– Contains deoxyribose.
– Contains adenine,
thymine, cytosine and
guanine.
– double helix.

17. DNA vs. RNA
• RNA:
– Smaller, mobile.
– Degrades easily.
– Travels form nucleus to cytoplasm.
• DNA:
– Larger, immobile.
– Lasts the life of cell.
– Resides in nucleus.

18. Types of RNA
• Messenger RNA
– Carries genetic
information from DNA
in nucleus to
cytoplasm.
• Information is used to
synthesize a protein.
– Codon: three
nucleotide sequence
that codes for one
amino acid.

19. Types of RNA
• Transfer RNA
– Functions as the
“interpreter”
– Transfer amino acids to the
sites where the information
in the mRNA is being used
to make a protein
– Anticodon: three
nucleotide sequence that is
complementary to a
particular codon in mRNA

20. Types of RNA
• Ribosomal RNA
– Combine with proteins
to form ribosomes
• Ribosomes
– Site of translation
– Large subunit
– Small subunit

21. Protein Synthesis
• Two processes:
– Transcription
• Occurs in the nucleus
• Produces RNA
– Translation
• Occurs in the
cytoplasm
• Produces proteins

22. Transcription

23. Translation
• To line up the
appropriate amino
acids in the proper
order requires:
– mRNA
– tRNA
– Ribosomes

24. Translation

25. Translation
• Codon (mRNA) must
be complementary to
the anticodon (tRNA).
• Translation continues
until ribosome
encounters a stop
codon.

26. Genetic Code
• Three nucleotides in mRNA (codon) code
for one amino acid.
• Some sequences serve as starting points.
• AUG codes for the amino acid methionine which
also indicates to start translation.
• Some sequences do not have
complementary tRNA.
– Indicate to the ribosome to stop translation.

27. Genetic Code

28. What Makes Cells Different From
Each Other?
• Due to the information in the DNA, a cell
could manufacture 50,000 different
proteins, but it doesn’t.
• The proteins a cell produces influences its
function.
– Example: red blood cells and hemoglobin

29. Gene Expression
• Some genes are always transcribed and
translated.
– Others can be turned on or off by
environmental signals
• Gene expression is highly regulated.

30. Gene Expression in Prokaryotes
• Jacob and Monod
– Studied digestion of
lactose in bacteria.
– Discovered the lac
operon.
• Prokaryotes regulate
gene expression at
the level of
transcription

31. Gene Expression in Eukaryotes
• Regulated at the level of transcription.
– Transcription requires transcription factors.
• They recognize and bind to DNA sequences called
regulatory sequences
• Transcription factors can increase or decrease the
rate of transcription
• Longevity of RNA molecule also
influences gene expression.

32. How Does DNA Change Over
Time?
• Mutations: a permanent change in the
genetic material of a cell or organism.
– Can be inherited.
– Can involve whole chromosomes or changes
in DNA sequences.

33. Whole Chromosome Mutations
• Polyploid: organism or cell containing
three or more sets of chromosomes.
– Occurs due to a cell division error.
– Frequently seen in plants, rare in animals.
– Can have advantageous results.

34. Whole Chromosome Mutations
• Nondisjunction:
instances when
paired chromosomes
fail to separate during
mitosis or meiosis
– Can result in an
aneuploid: individual
whose chromosome
number is greater or
less than normal

35. Whole Chromosome Mutations
• Down’s Syndrome
– Due to nondisjunction
with chromosome 21.
– Characterized by
mental retardation,
distinctive facial
features.

36. Whole Chromosome Mutations
• Transposons:
– Variety of DNA sequences that can randomly
insert themselves by transposition in various
non-homologous regions on chromosomes
and other DNA.
– Can generate new gene combinations
– Can also induce genetic errors

37. Mutations Involving Single DNA
Nucleotides
• Point Mutations:
– Change in a single nucleotide base pair.
– Example: sickle cell anemia.

38. Mutations Involving Single DNA
Nucleotides
• Frame-shift mutation:
– A change in the reading frame resulting from
an insertion or deletion of nucleotides in the
DNA sequence for a protein.
– Extremely harmful.
Normal:
JOE ATE THE HOT DOG
After deletion:
JEA THE OTD OG