Molecular Genetics

Ch. 16, 17, 18, 19, 20

DNA Replication

Happens during interphase of mitosis. Semiconservative Replication 3 basic steps

Unwind and Unzip Build Rewind

Unwind

Helicase, an enzyme, unwinds the DNA helix forming a Y-shaped replication fork.

Single-stranded DNA binding proteins attach to each strand of the uncoiled DNA to keep them separate.

Topoisomerases help to keep the DNA from forming knots.

Build RNA primase initiates DNA replication at a

special nucleotide sequences called origins of replication with short segments of RNA called RNA primers.

DNA polymerase attaches to the RNA primers and begins elongation, the adding of DNA nucleotides to the complementary strand.

The leading complementary strand is assembled continuously.

The lagging complementary strand is assembled in short Okazaki fragments which are joined by DNA ligase.

RNA primers are placed by DNA nucelotides.

Leading vs Lagging

DNA is antiparallelThe uncoiled DNA consists of a 3’ 5’

template strand and a 5’ 3’ template strand.

DNA polymerase moves in the 3’ 5’ direction. 5’ or 3’?

The location of the carbon sugar next to the phosphate.

Leading vs Lagging Continued

Leading strand replication occurs regularly.

Lagging strand the replication occurs in fragments called Okazaki fragments.

Protein Synthesis

DNA mRNA tRNA Protein 3 types of RNA

mRNA A single strand of RNA that provides the template

used for sequencing amino acids into a polypeptide. tRNA

Is a short RNA molecule that is used for transporting amino acids to their proper place on the mRNA template.

rRNA Molecules are the building blocks of ribosomes.

Protein Synthesis (Transcription)

Transcribing DNA to RNAOccurs in the nucleus3 steps

Initiation Elongation Termination

Initiation

The RNA polymerase attaches to a promoter region on the DNA. TATA Box

DNA begins to unzip.

Elongation

RNA polymerase unzips the DNA and assembles RNA nucleotides using one strand of the DNA as a template.

Only one DNA strand is transcribed.

Termination

Occurs when the RNA polymerase reaches a special sequence of nucleotides that serve as a termination point.

Protein Synthesis (Translation)

After transcription mRNA, tRNA, and ribosomal subunits are transported across the nuclear envelope.

3 Steps Initiation Elongation Termination

Initiation

Small ribosomal subunit attaches to a special region near the 5’ end of the mRNA molecule.

tRNA with anticodon UAC carrying the amino acid methionine attaches to the mRNA at the start codon AUG with hydrogen bonds

The large ribosomal subunit attaches to the mRNA forming a complete ribosome with tRNA-Met, at the P site

Elongation

Begins when the next tRNA bearing the appropriate amino acid binds to the A site of the ribosome.

The old tRNA moves from the A to the E and leaves. The methionine is attached to the new amino acid in the A site.

A new tRNA moves into the P site.The cycle continues

Termination

Occurs when the ribosome encounters one of three “stop” codons.

At termination the completed polypeptide and the 2 ribosomal subunits are released.

Processing the Protein

Properties of the amino acids give it the secondary and tertiary structures.

Processes by the ER or Golgi make final modifications and quaternary structure.

Mutations

When DNA is copied or replicated mistakes may occur.

3 Types Substitution Insertion Deletion

The Molecular Genetics of Viruses

Parasite of cells.Typical virus penetrates a cell,

commandeers its metabolic machinery and makes copies of its self.

Viruses consist of a nucleic acid surrounded by a protein coat called a capsid.

2 basic replication cycles Lytic Cycle Lysogenic Cycle

The Molecular Genetics of Bacteria

ProkaryotesNo Nucleus“Chromosome”PlasmidsGenetic Variation

Conjugation Transduction Transformation

Conjugation

Process of DNA exchange between bacteria. Donor bacterium produces a pilus that connect

to a recipient. Donor sends chromosomal DNA or a plasmid to

the recipient. Plasmid Types

• F plasmid- contains genes that enable bacterium to produce pili.

• R plasmid- provide bacteria with resistance against antibiotics.

Transduction

Occurs when new DNA is introduced into bacteria by a virus. Occurs during the lytic cycle when some of the

bacterial DNA is used in place of viral DNA. When the virus infects another cell, the

bacterial DNA delivers can recombine with the resident DNA.

Transformation

Occurs when bacteria absorb DNA from their surroundings.

Specialized proteins on the cell membrane facilitate the DNA uptake.

Regulation of Gene Expression

RNA Processing (Eukaryotic Cells)DNA OrganizationOperon: sequences of DNA that direct

particular biosynthetic pathways.

RNA Processing

Prior to leaving the nucleus the mRNA strand is modified.

2 alterations A special nucleotide sequences is added to

both ends of the mRNA. 5’ gets a “cap” of a GTP molecule to provide stability

to the mRNA. 3” gets a poly-A tail to provide stability and to control

the movement of the mRNA across the nuclear envelope.

2nd Alteration

Some mRNA segments are removed.2 types of sequences in the mRNA strand:

Introns: intervening sequences that are noncoding.

Exons: express a code for a polypeptide. snRNPs delete out the introns and splice the exons

together. The modified mRNA leaves the nucleus to be

translated.

DNA Organization

DNA is arranged around proteins called histones. (Nucleosome)

Appears like beads on a string.Euchromatin

Regions where the DNA is loosely bound to the histone.

Actively transcribedHeterochromatin

Nucleosomes are tight and compacted. DNA is inactive

Operon

4 major components of an operon: Regulatory Gene Promoter Operator Structural Gene

Regulatory Gene

Produces a repressor protein A substance that can prevent gene expression

by blocking the action of RNA polymerase.

Promoter

A sequence of DNA to which the RNA polymerase attaches to begin transcription (TATA box)

Operator

Can block the action of the RNA polymerase if the region is occupied by a repressor protein.

Structural Gene

Contains DNA sequences that code for several related enzymes that direct the production of a particular end product.