10 eukaryotic, bacterial and viral geneticsmembers.optusnet.com.au/~dominicrb/notes/lecture...

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2004 Biology Olympiad Preparation Program 2

EUKARYOTIC GENETICS


Recombination frequencies again Body: grey b+, black b Wings: normal vg+,

vestigial vg

Mother: b+b vg+vg

Recombinant gametes

Parental types Recombinants

Recombination frequency =

# Recombinants ÷

Total offspring =

(206+185) ÷(2300) = 17%


Recombination frequencies again Recombination frequency (RF) is an indication of how close

2 genes are on a chromosome.

Smaller RF = closer genes = more likely to be inherited together = crossover event in between less likely.

High RF, genes further apart, crossover event in

between more likely. Low RF, genes closer

together, crossover event in between less likely.

RFs can therefore be used to create a linkage map – chromosome map based on RFs.


DNA packing

Chromosome

Chromatin is DNA associated with proteins (eg histones).

Euchromatin – looser than heterochromatin.

Chromosomes are supercoiled chromatin.


Repetitive DNA In eukaryotes, much of the genome (97% in humans) does not

code for proteins or RNA.

Some of this DNA are regulatory sequences. Most have unknown function, including introns and

repetitive DNA.

Satellite DNA – short nucleotide sequences repeated over and over again one after another.

Include microsatellites, minisatellites, satellites.

Interspersed repetitive DNA – nucleotide sequence found in many copies in a genome, not one after another.


Retrotransposons Transposons are present in all organisms. They can move from

one place to another within the genome – ‘jumping genes’.

Retrotransposons move by means of an RNA intermediate.

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Control of gene expression


DNA packing Nucleosomes & histones change shape and position to allow

RNA polymerase access to naked DNA.


Transcriptional regulation Transcription factors are crucial for eukaryotic gene transcription.

TFs bind to specific DNA sequences to help RNA

polymerase bind.

TF binding sequences distant from the gene are

enhancers.

Activators are special TFs that bind enhancers.

Activator binding bends DNA and brings many TFs

together with RNA pol. 2004 Biology Olympiad Preparation Program 11

Alternative splicing Splicing occurs in eukaryotes. Introns are removed from pre-

mRNA and exons are glued together by spliceosomes.

Alternative splicing occurs when some exons are

included while others are not.

Hence different proteins can result from the same gene.


BACTERIAL & VIRAL GENETICS


Eukaryotes, prokaryotes and viruses Eukaryotes – have membrane-bound

organelles. Large. Unicellular or multicellular.

Prokaryotes – no membrane-bound

organelles. Unicellular.

Viruses – no metabolism, intracellular parasites.

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VIRUSES Virus – genome enclosed in a protective coat.

Viral genomes can be either ss or ds DNA or RNA.

Viral genomes are enclosed in a protein shell called the capsid.

The capsid is made of units called capsomeres.

Some viruses have accessory structures like viral envelopes to assist

host cell invasion.


Viral reproduction


Viral reproduction Viruses are obligate intracellular

parasites.

Can only infect its host range.

Uses host cell’s machinery to reproduce.

Uses host DNA polymerase to replicate viral genome, uses host RNA polymerase to transcribe

viral RNA, uses host ribosomes to translate viral protein.


Bacteriophage Viruses infecting bacteria are called

bacteriophage, or just phage.

T1, T2, T3…

Complex head capsid containing DNA.

Tail sheath and fibres (T-even) assist attachment to host cell.


Lytic cycle


Phage life cycle leading to death of host cell. Virulent phage undergo the lytic cycle only.

Lytic cycle

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Lysogenic cycle


Lysogenic cycle Lytic cycle

Replicates phage genome without destroying the host. Temperate phage are capable of both lysogenic and lytic life cycles.

Lysogenic cycle


HIV reproductive cycle


Human immunodeficiency virus

HIV causes AIDS. It is a retrovirus – its genome is ssRNA and it uses reverse transcriptase

to convert ssRNA to dsDNA inside the host cell.

The capsid is enclosed by a viral envelope derived from the host

cell’s plasma membrane.


PRIONS & VIROIDS Viroids – tiny molecules of naked circular RNA that infect plants.

Prions – proteinaceous infections particles – misfolded proteins of proteins normally present in brain tissue.


BACTERIA

Nucleoid region Ribosomes

Plasma membrane Cell wall

Capsule

Pili

Flagella

Bacteria are prokaryotes.

Bacteria have a single circular

chromosome of dsDNA. It is referred to as the genophore.

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Binary fission

Bacteria multiply by dividing. They reproduce via binary

fission.

Bacteria can proliferate extremely rapidly in favourable

environments – up to once every 20 minutes!

Daughter cells are vegetative (asexual) clones of the parent.


Transformation

Transformation is the alteration of a bacterial cell’s genome by the uptake of naked foreign DNA from the environment.

Bacteria able to do this are termed competent.


Transduction Transduction involves phages carrying bacterial DNA

from one host cell to another.

Generalised transduction Specialised transduction


Plasmids Plasmids are small, circular, self-replicating DNA molecules

separate from the bacterial chromosome.

Some plasmids can incorporate reversibly into the cell’s chromosome

– these genetic elements are called episomes.

Plasmids contain few genes, which are not required for survival and

reproduction under normal conditions.


Conjugation Conjugation – direct transfer of genetic material

between two bacterial cells.

The DNA donor, the ‘male’, uses sex pili to attach to the DNA recipient, the ‘female’.

F factor, an episome, conveys the ability to form sex pili.

F+ cells conjugate with F- cells. ‘Females’ then turn into ‘males’.


Transposons Transposons – transposable genetic elements – pieces of DNA that can move from one location to another in a cell’s genome.

Insertion sequences – simple transposons.

Composite transposons – insertion sequences flanking 1+ other genes.

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Operons E.g. trp operon, encodes proteins needed for tryptophan synthesis.

10 eukaryotic, bacterial and viral geneticsmembers.optusnet.com.au/~dominicrb/notes/lecture...

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