biol 211 - chapter 6

8/13/2019 BIOL 211 - Chapter 6

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Chapter 6

Molecular Biology

What Is DNA and How Does It

Work?

8/13/2019 BIOL 211 - Chapter 6


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.

8/13/2019 BIOL 211 - Chapter 6


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.

8/13/2019 BIOL 211 - Chapter 6


Building Blocks of DNA

• Nucleotides – Three components:

• Five-carbon sugar

• Phosphate group

• Nitrogen-containing

base

8/13/2019 BIOL 211 - Chapter 6


Building Blocks of DNA

• Four nitrogenous bases in DNA – Adenine

– Thymine

– Guanine – Cytosine

8/13/2019 BIOL 211 - Chapter 6


Structure of DNA

• Maurice Wilkins and

Rosalind Franklin

– Attempted to

determine structure ofDNA.

– Discovered DNA was

a helix.

8/13/2019 BIOL 211 - Chapter 6


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!

8/13/2019 BIOL 211 - Chapter 6


Watson and Crick

• Early 1950s

– They were young scientists

at Cavendish Laboratory in

Cambridge, England.

• Using Chargaff’s ratiosand Franklin’s data,

Watson and Crick

determine DNA structure

is a double helix

8/13/2019 BIOL 211 - Chapter 6


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

8/13/2019 BIOL 211 - Chapter 6


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

8/13/2019 BIOL 211 - Chapter 6


8/13/2019 BIOL 211 - Chapter 6


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.

8/13/2019 BIOL 211 - Chapter 6


8/13/2019 BIOL 211 - Chapter 6


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.

8/13/2019 BIOL 211 - Chapter 6


How is the information in DNA

expressed?

• Chromosomes containing DNA containedin 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.

8/13/2019 BIOL 211 - Chapter 6


DNA vs. RNA

• RNA:

– Contains the sugarribose.

– Contains adenine, uracil,

cytosine and guanine. – Single helix

• DNA:

– Contains deoxyribose.

– Contains adenine,

thymine, cytosine andguanine.

– double helix.

8/13/2019 BIOL 211 - Chapter 6


DNA vs. RNA

• RNA:

– Smaller, mobile.

– Degrades easily.

– Travels form nucleus to cytoplasm.

• DNA:

– Larger, immobile.

– Lasts the life of cell.

– Resides in nucleus.

8/13/2019 BIOL 211 - Chapter 6


Types of RNA

• Messenger RNA

– Carries genetic

information from DNA

in nucleus to

cytoplasm.

• Information is used to

synthesize a protein.

– Codon: three

nucleotide sequencethat codes for one

amino acid.

8/13/2019 BIOL 211 - Chapter 6


Types of RNA

• Transfer RNA

– Functions as the

“interpreter”

– Transfer amino acids to the

sites where the informationin the mRNA is being used

to make a protein

– Anticodon: three

nucleotide sequence that is

complementary to aparticular codon in mRNA

8/13/2019 BIOL 211 - Chapter 6


Types of RNA

• Ribosomal RNA – Combine with proteins

to form ribosomes

• Ribosomes – Site of translation

– Large subunit

– Small subunit

8/13/2019 BIOL 211 - Chapter 6


Protein Synthesis

• Two processes:

– Transcription

• Occurs in the nucleus• Produces RNA

– Translation

• Occurs in the cytoplasm• Produces proteins

8/13/2019 BIOL 211 - Chapter 6


Transcription

8/13/2019 BIOL 211 - Chapter 6


Translation

• To line up the

appropriate amino

acids in the proper

order requires: – mRNA

– tRNA

– Ribosomes

8/13/2019 BIOL 211 - Chapter 6


Translation

8/13/2019 BIOL 211 - Chapter 6


Translation

• Codon (mRNA) must

be complementary to

the anticodon (tRNA).

• Translation continuesuntil ribosome

encounters a stop

codon.

8/13/2019 BIOL 211 - Chapter 6


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.

8/13/2019 BIOL 211 - Chapter 6


Genetic Code

8/13/2019 BIOL 211 - Chapter 6


What Makes Cells Different From

Each Other?

• Due to the information in the DNA, a cell

could manufacture 50,000 differentproteins, but it doesn’t.

• The proteins a cell produces influences its

function. – Example: red blood cells and hemoglobin

8/13/2019 BIOL 211 - Chapter 6


Gene Expression

• Some genes are always transcribed and

translated.

– Others can be turned on or off by

environmental signals

• Gene expression is highly regulated.

8/13/2019 BIOL 211 - Chapter 6


Gene Expression in Prokaryotes

• Jacob and Monod

– Studied digestion of

lactose in bacteria.

– Discovered the lacoperon.

• Prokaryotes regulate

gene expression at

the level oftranscription

8/13/2019 BIOL 211 - Chapter 6


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.

8/13/2019 BIOL 211 - Chapter 6


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.

8/13/2019 BIOL 211 - Chapter 6


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.

8/13/2019 BIOL 211 - Chapter 6



• Nondisjunction:

instances when

paired chromosomes

fail to separate duringmitosis or meiosis

– Can result in an

aneuploid: individual

whose chromosomenumber is greater or

less than normal

8/13/2019 BIOL 211 - Chapter 6



• Down’s Syndrome

– Due to nondisjunction

with chromosome 21.

– Characterized bymental retardation,

distinctive facial

features.

8/13/2019 BIOL 211 - Chapter 6



• Transposons:

– Variety of DNA sequences that can randomly

insert themselves by transposition in various

non-homologous regions on chromosomesand other DNA.

– Can generate new gene combinations

– Can also induce genetic errors

8/13/2019 BIOL 211 - Chapter 6


8/13/2019 BIOL 211 - Chapter 6


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

biol 211 - chapter 6

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