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 antiparallel The 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 RNA Occurs in the nucleus 3 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 Prokaryotes No Nucleus “Chromosome” Plasmids Genetic 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 Organization Operon: 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.

2 nd 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 transcribed Heterochromatin  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.