Major Events in Genetics

1869 - Friedrich Meischer extracted a phosphorous rich material from nuclei of human white blood cells. He named this

material nuclein.

The Discovery of DNA

Deoxyribose- a sugar- acid

Four subunits belonging to class of compounds called nucleotides

DNA - deoxyribonucleic acid

Purines Pyrimidines

- Adenine - Thymine

- Guanine - Cytosine

Chromosomes are composed of DNA

The Chemical

Composition of DNA

A gene is a genetic sequence that codes for an RNA. In protein coding genes, the RNA codes for a protein.

A gene is a coding unit

DNA is the genetic material of bacteria, viruses, plant and animal cells

DNA, genetic material of bacteria

• The bacterium Pneumoccocus kills mice by causing pneumonia

• Pneumoccocus virulence is determined by its capsular polysacharide – component of cell surface allows the bacteria to escape destruction

by the host.

• Several types of pneumoccocus have different types of polysacharides. – Two groups S (smooth) and R (rough)

When heat-killed S and R pneumoccocus are jointly injected in the animal, the mouse dies. Virulent S bacteria is recovered from the mouse

1928 - Frederick Griffith Experiment

• Some properties from dead S-type bacteria can transform the live R-type bacteria and render it virulent (S-type)

Transforming Principle is DNA

Phages, viruses

1952..

• Phage coat and phage DNA were separated via centrifugation

• The DNA of the parent phage enters bacteria and becomes part of the progeny phages, as expected from genetic material

See pg. 290

Polynucleotide chains have nitrogenous bases linked to a sugar-phosphate backbone

• Three components– Nitrogenous base– Sugar– Phosphate

• The sugar phosphate backbone consists a of 5’-3’ phosphodiester linkages

• The nitrogenous bases “stick out” from the backbone– Pyrimidines ( C, T)– Purines ( A, G)

Nucleotide

DNA is a double helix

• By 1953..

• X-ray diffraction data showed that DNA has the form of a regular helix, making a complete turn every 34 A with a diameter of ~20 A

• The density of DNA suggested that the helix must contain two chains. – The constant diameter of helix

• Bases face inward, a purine is always opposite a pyrimidine

DNA Structure

• Sugar-phosphate backbone in the outside

• Phosphate negative charges are neutralized by positively charged proteins

DNA Structure

• Flat bases lie perpendicular to the sugar-phosphate backbone

• Each base is rotated 36o around the axis of the helix

• A complete turn of the helix include 10 bases

DNA Structure

• Major groove ( ~22 A) • Minor groove (~ 12A)• The double helix turns

clockwise, looking along the helical axis

• The model is also known as B-form of DNA

Replication of DNA is semiconservative

• Hydrogen bonds separate without requiring breakage of covalent bonds

• Each of the parental strains serve as template for synthesis of the daughter strand

• Each of the daughter duplexes is identical to the parent and contains one parental strand

DNA Structure & Replication

• The structure of DNA carries the information needed to perpetuate its sequence

• The unit conserved from one generation to the next is one of the two parental strands– Semiconservative replication

DNA strands separate at the replication fork

DNA Replication

• Requires the two parental strands to separate

• Only a small stretch of DNA is separated into single strands at any moment

• The double helix is temporarily interrupted at the replication fork

Replication Fork

• The fork moves along the parental DNA to continually unwind the parental strains and rewind the daughter strains

DNA Replication

• The synthesis of the complementary strand is catalyzed by polymerases (DNA or RNA polymerases)– Recognize the template– Catalyze the addition of subunits to the

polynucleotide chain

• Degradation of nucleic acids bonds is catalyzed by nucleases

Endonucleases

– Break individual bonds within DNA or RNA– Cleave either one or both strands

Exonucleases

– Remove residues one at a time generating monomers

– Function on a single strand, in a specific direction, either at 5’ or 3’ end

Nucleic acids hybridize by base pairing

• The double helix has the ability to separate the two strands without disrupting covalent bonds – The DNA strands can separate and reform

very rapidly

• The concept of base pairing is crucial to all processes involving nucleic acids

• Renaturation – Describes the reaction between two

complementary structures separated by denaturation

• Also known as” annealing” or “ hybridization”

• Nucleic acid hybridization constitute a test for their complementarity

Mutations change DNA sequence

Mutations

• Mutations are rare and spontaneous events– They may damage a gene or affect protein

functions

• The occurrence of mutations can be increased by treatment with certain compounds, mutagens

• A mutagen can modify a DNA base or can be incorporated into the nucleic acid

• Mutation Rate per Generation

– A base pair is mutated at 10-9-10-10

– A gene ~1 kb is mutated 10-6-10-7

– A bacterial genome is mutated at ~3 x 10-3-10-4

• There is a wide variation in rate of mutations given the large variation in genome sizes between organisms

• The overall rate of mutations has been subject to selective forces that balance the damaging effect and advantageous effects of mutations

Mutations may affect single base pairs or longer sequences

• Point mutations change only a single base pair

• Chemical modification of DNA changes one base to another

• A malfunction during DNA replication causes insertion of the wrong base

• The most common class of mutations– Transition; pyrimidine-pyrimidine or purine-

purine substitutions• A G-C pair is replaced with A-T and vice versa

• The less common class– Transversing; pyrimidine-purine

substitutions• An A-T pair is replaced by T-A or a G-C pair is

replaced by C-G

Mutations

• Point mutations can affect gene function

• Other factors– Deletions, insertions

The effect of mutations can be reversed

• A mutation caused by point mutations can be reverted by regaining a compensatory mutation elsewhere in the gene

• A mutation caused by insertion can revert by deletion of the same DNA sequence

• A mutation caused by deletion can not revert

Reversion of mutations

All organisms have repair systems that correct mismatched pairs

– The ability of these systems to recognize and repair mismatches determines whether mismatched pairs will result in mutations

A gene codes for a single polypeptide

The genetic code is a triplet

Triplets

• What is the genetic code?

• The genetic code is the relationship between a sequence of DNA and the corresponding polypeptde sequence

• One gene -> one polypeptide– Also, other regulatory sequences are

required for proper gene functioning

• The genetic code is read in nonoverlapping triplets from a fixed starting point

– Nonoverlapping• each codon consists of three nt; the successive codon is

represented by the successive trinucleotide

– Fixed starting point• The assembly of a protein starts at one point.

Mutations

• Mutations will have different effects on the peptide sequence– Point mutations might affect only one

amino acid– Single base mutations and deletions will

change the triplet sets for the entire polypeptide sequence, known as frameshift

– Frameshifts are likely to cause loss of function

• A gene is not directly translated into protein

• It is first expressed into mRNA

• Then translated into protein from the RNA intermediate

– Transcription into mRNA – Translation of mRNA into protein

mRNA– Synthesized by a DNA template by the

same concept of complementarity– Single stranded nucleic acid– Identical to DNA apart the replacement of T

with U– Includes additional sequences on either

end:• 5’ nontranslated region the leader• 3’ nontranslationl region the trailer

• The exons are represented in mRNA

• The introns are intervening sequences that are removed from the precursor RNA transcript

• Not all eukaryotic genes are interrupted– Yeast; the majority of genes have no

introns– Higher eukaryotes; only a minority of

genes has no introns