Molecular Genetics

1
Molecular Genetics

• Ch. 16, 17, 18, 19, 20

2
DNA Replication

• Happens during interphase of mitosis.
• Semiconservative Replication
• 3 basic steps
• Unwind and Unzip
• Build
• Rewind

3
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.

4
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.

5
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.

6
Leading vs Lagging Continued

• Leading strand replication occurs regularly.
• Lagging strand the replication occurs in
  fragments called Okazaki fragments.

7
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.

8
Protein Synthesis (Transcription)

• Transcribing DNA to RNA
• Occurs in the nucleus
• 3 steps
• Initiation
• Elongation
• Termination

9
Initiation

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

10
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.

11
Termination

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

12
Protein Synthesis (Translation)

• After transcription mRNA, tRNA, and ribosomal
  subunits are transported across the nuclear
  envelope.
• 3 Steps
• Initiation
• Elongation
• Termination

13
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

14
(No Transcript)
15
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

16
Termination

• Occurs when the ribosome encounters one of three
  stop codons.
• At termination the completed polypeptide and the
  2 ribosomal subunits are released.

17
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.

18
Mutations

• When DNA is copied or replicated mistakes may
  occur.
• 3 Types
• Substitution
• Insertion
• Deletion

19
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

20
The Molecular Genetics of Bacteria

• Prokaryotes
• No Nucleus
• Chromosome
• Plasmids
• Genetic Variation
• Conjugation
• Transduction
• Transformation

21
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.

22
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.

23
Transformation

• Occurs when bacteria absorb DNA from their
  surroundings.
• Specialized proteins on the cell membrane
  facilitate the DNA uptake.

24
Regulation of Gene Expression

• RNA Processing (Eukaryotic Cells)
• DNA Organization
• Operon sequences of DNA that direct particular
  biosynthetic pathways.

25
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.

26
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.

27
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

28
Operon

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

29
Regulatory Gene

• Produces a repressor protein
• A substance that can prevent gene expression by
  blocking the action of RNA polymerase.

30
Promoter

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

31
Operator

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

32
Structural Gene

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