Exam 3 Genetics

Why are bacteria useful

• Short generation times

• Haploid Genomes (easy to observe mutations)

• Simple Genome

• Large numbers of progeny

• Ease of Propagation (cultures are easy, inexpensive, and do not require a lot of space).

• Numerous Heritable Differences

Binary Fission

The chromosome replicates and a copy is distributed to each of the progeny cells.

Bacterial Chromosome

Circular molecule of double stranded DNA

Bacterial Genome

Composed of a single small chromosome

Plasmids

Small Double-stranded circular DNA molecules that contain NONESSENTIAL genes

Are plasmids smaller than the bacterial chromosome

yes

High-copy-number plasmids

Replicate independently of the bacterial chromosome so number per cell can increase rapidly

Low-copy-number plasmids

Cannot replicate independently of bacterial chromosome and are present in one or two copies per bacterial cell.

Vertical Gene Transfer

The transfer of genetic material from a parent cell to an offspring cell through reproduction.

Horizontal Gene Transfer

Nonreproductive process: one way transfer of genetic material that occurs between a donor and recipient cell

What can be transferred during horizontal gene transfer

DNA from a plasmid, a portion of bacterial chromosome DNA, or both.

Conjugation

The transfer of replicated DNA from a donor to a recepient

Transformation

The uptake of DNA from the environment

Transduction

The transfer of DNA from one bacterium to another by a viral vector

Hayes: Donor cells

Possess an F factor: F+ cells

Hayes: Recipient Cells

Lack an F factor: F- cells

What is conjugation controlled by?

genes carried on the F plasmid

The exconjugant cell

Produced by conjugation: It is the recipient cell with its genetic information modified by receiving DNA from the donor cell.

Insertion Sequence (IS) elements

Shared by an F plasmid and bacterial chromosome. They allow for recombination between the two.

What is used during conjugation to contact recipient cell?

Conjugation pilus

High Frequency Recombination, Hfr strains

Transferred bacterial genes rather than F factor genes at a high rate.

Hfr chromosome

The F factor in the Hfr strains integrates into the bacterial chromosome. This occurs rarely and integration occurs at one of multiple IS elements that are shared by F plasmids and bacterial chromosome.

Hfr Gene Transfer

Homologous recombination occurs between the transferred linear DNA and the circular chromosome of the recipient. The new exconjugant cell may acquire one or more donor genes in this way. The F factor is not fully transferred during mating, therefore the recipient cell is not converted into a donor cell.

Transformation

occurs when a recipient cell takes up a fragment of donor DNA from surrounding growth medium.

Used in the lab to introduce DNA into microbial, plant, and animal cells.

Transduction

Transfer of genetic material from a donor to a recipient cell by way of a bacteriophage

Transductant

Integration of donor DNA into the recipient cell’s chromosome by homologous recombination

How does the lytic cycle begin?

a bacteriophage attacks bacteria with a variety of mechanisms involving it attaching to the host cell.

What does the lytic cycle lead to

Lysis of the host cell and release of progeny phage

Steps of Lytic Cycle

• Attachment of the phage to the host cell

• injection of phage chromosome into the host followed by circulization of the phage chromosome

• Replication of phage DNA

• Transcription and translation of phage genes

• Packaging of phage chromosomes into phage heads

• Lysis of the host cell and release of progeny phage particles.

Errors in Transduction

Excision of the prophage is inaccurate so that only part of it is removed and only some of the bacterial DNA.

Bacterial DNA is accidentally packaged with viral DNA during lytic cycle causing a specialized transducing phage. Since the tranductants dont receive a full phage genome they cannot produce new phages.

Bacteriophages

viruses that infect bacteria.

What is a DNA nucleotide composed of

sugar, one of four nitrogenous bases, and up to three phosphate groups.

DNA molecule structure

A nucleotide base is attached to the 1′ carbon, a hydroxyl

(OH) group is attached to the 3′ carbon, and one to three

phosphates are attached to the 5′ carbon

Pyrimidines

Single ring (thymine and cytosine)

Purines

double ring (adenine and guanine)

Polynucleotide chains

Catalyzes the formation of a phosphodiester bond between the 3 prime hydroxyl group of one nucleotide and the 5 prime phosphate of an adjacent one.

Base stacking

The offsetting of adjacent base pairs so that their planes are parallel. This leads to a twist in the double helix.

Major groove and minor groove

the major groove is approx 12 A wide and alternates with the minor groove which is approx 6 A wide. They are regions where DNA binding proteins can make direct contact with nucleotides.

Semiconservative Model

Each daughter duplex contains one parental and one daughter strand.

Conservative Model

One daughter duplex contains both parental strands and the other contains both daughter strands.

Dispersive Model

each daughter duplex contains interspersed parental and daughter segments

Replication initiating enzymes

Locate and bind to oriC consensus sequences; DnaA, DnaB, DnaC

DnaA

Bends DNA, breaks hydrogen bonds

DnaB (helicase)

breaks hydrogen bonds to separate strands and unwind helix

Single-Stranded Binding Proteins

Keep unwound DNA from reannealing

DNA Polymerase

elongates DNA by adding nucleotides to the 3 prime end of a preexisting strand

RNA Primers

Synthesized by primase (RNA polymerase).

Replisome

Large protein complex found at each replication fork that contains two copies of DNA poly 3

Leading Strand

Poly 3 copy synthesizes continuously in the same direction as fork progression.

Lagging strand

Other copy poly 3 elongates discontinuously in the opposing direction to fork progression via Okazaki fragments.

DNA poly 1

Its 5’-3’ exonuclease activity removes RNA primers while its polymerase activity adds DNA nucleotides to the 3 prime end of the DNA segment preceding the primer.

DNA ligase

seals gap among resulting DNA segments

Is replication of leading and lagging strand simultaneous

yes

Sliding clamp

Auxiliary protein complex that gives DNA polymerases the momentum on their own for daughter strand synthesis.

DNA proofreading

DNA polymerase undertake this to correct occasional errors. This is due to a 3’-5’ exonuclease activity.

Supercoiling

unwinding of chromosomes during DNA replication will create torsional stress

DNA topoisomerases

catalyze controlled cleavage and rejoining of DNA to relieve supercoiling.

Telomeres

Solve the problem of the lagging strand not being able to be completely replicated by being repetitive sequences at the end of linear chromosomes.

Telomerase

template RNA of telomerase allows new DNA replication to lengthen the telomere sequences. Once sufficiently elongated, DNA replication fills out chromosome ends.