Molecular BiologyWhat Is DNA and How Does It

Work?

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.

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.

Building Blocks of DNA

• Nucleotides– Three components:

• Five-carbon sugar• Phosphate group• Nitrogen-containing

base

Building Blocks of DNA• Four nitrogenous bases in DNA

– Adenine– Thymine– Guanine – Cytosine

Structure of DNA

• Maurice Wilkins and Rosalind Franklin– Attempted to

determine structure of DNA.

– Discovered DNA was a helix.

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!

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

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

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

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.

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.

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.

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.

DNA vs. RNA

• RNA:– Smaller, mobile.– Degrades easily.– Travels form nucleus to cytoplasm.

• DNA:– Larger, immobile.– Lasts the life of cell.– Resides in nucleus.

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.

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

Types of RNA

• Ribosomal RNA– Combine with proteins

to form ribosomes

• Ribosomes – Site of translation– Large subunit– Small subunit

Protein Synthesis

• Two processes:

– Transcription• Occurs in the nucleus• Produces RNA

– Translation • Occurs in the

cytoplasm• Produces proteins

Transcription

Translation

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

Translation

Translation

• Codon (mRNA) must be complementary to the anticodon (tRNA).

• Translation continues until ribosome encounters a stop codon.

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.

Genetic Code

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

Gene Expression

• Some genes are always transcribed and translated.– Others can be turned on or off by

environmental signals

• Gene expression is highly regulated.

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

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.

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.

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.

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

Whole Chromosome Mutations

• Down’s Syndrome– Due to nondisjunction

with chromosome 21.– Characterized by

mental retardation, distinctive facial features.

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

Mutations Involving Single DNA Nucleotides

• Point Mutations:– Change in a single nucleotide base pair.– Example: sickle cell anemia.

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