2004 Biology Olympiad Preparation Program. 2. EUKARYOTIC. GENETICS.
2004 Biology Olympiad Preparation Program. 3. Recombination frequencies
again.
Recombination frequencies again
EUKARYOTIC GENETICS
Body: grey b+, black b Wings: normal vg+, vestigial vg
Mother: b+b vg+vg
Recombinant gametes Recombination frequency = # Recombinants ÷ Total offspring = (206+185) ÷(2300) = 17% Parental types 2004 Biology Olympiad Preparation Program
Recombinants
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Recombination frequencies again
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DNA packing
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.
Chromatin is DNA associated with proteins (eg histones). Euchromatin – looser than heterochromatin.
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. 2004 Biology Olympiad Preparation Program
Chromosomes are supercoiled chromatin. 4
Chromosome 2004 Biology Olympiad Preparation Program
Repetitive DNA
Retrotransposons
In eukaryotes, much of the genome (97% in humans) does not code for proteins or RNA.
Transposons are present in all organisms. They can move from one place to another within the genome – ‘jumping genes’.
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Retrotransposons move by means of an RNA intermediate.
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. 2004 Biology Olympiad Preparation Program
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Control of gene expression
DNA packing Nucleosomes & histones change shape and position to allow RNA polymerase access to naked DNA.
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2004 Biology Olympiad Preparation Program
Transcriptional regulation
Alternative splicing
Transcription factors are crucial for eukaryotic gene transcription.
Splicing occurs in eukaryotes. Introns are removed from premRNA and exons are glued together by spliceosomes.
TFs bind to specific DNA sequences to help RNA polymerase bind.
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Alternative splicing occurs when some exons are included while others are not.
TF binding sequences distant from the gene are enhancers. Activators are special TFs that bind enhancers.
Hence different proteins can result from the same gene.
Activator binding bends DNA and brings many TFs together with RNA pol. 2004 Biology Olympiad Preparation Program
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BACTERIAL & VIRAL GENETICS
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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
Viral reproduction
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. 2004 Biology Olympiad Preparation Program
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Viral reproduction
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Bacteriophage
Viruses are obligate intracellular parasites.
Viruses infecting bacteria are called bacteriophage, or just phage.
Can only infect its host range. T1, T2, T3… 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. 2004 Biology Olympiad Preparation Program
Complex head capsid containing DNA. Tail sheath and fibres (T-even) assist attachment to host cell. 16
Lytic cycle
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Lytic cycle Phage life cycle leading to death of host cell. Virulent phage undergo the lytic cycle only.
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Lysogenic cycle
Lysogenic cycle Replicates phage genome without destroying the host. Temperate phage are capable of both lysogenic and lytic life cycles.
Lysogenic cycle
Lytic cycle
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Human immunodeficiency virus
HIV reproductive cycle
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.
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2004 Biology Olympiad Preparation Program
PRIONS & VIROIDS
BACTERIA Bacteria are prokaryotes.
Viroids – tiny molecules of naked circular RNA that infect plants. Pili Cell wall
Prions – proteinaceous infections particles – misfolded proteins of proteins normally present in brain tissue.
Capsule
Plasma membrane Nucleoid region Ribosomes Flagella
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Bacteria have a single circular chromosome of dsDNA. It is referred to as the genophore.
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Binary fission
Transformation
Bacteria multiply by dividing. They reproduce via binary fission.
Transformation is the alteration of a bacterial cell’s genome by the uptake of naked foreign DNA from the environment.
Daughter cells are vegetative (asexual) clones of the parent. Bacteria can proliferate extremely rapidly in favourable environments – up to once every 20 minutes! 2004 Biology Olympiad Preparation Program
Bacteria able to do this are termed competent.
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2004 Biology Olympiad Preparation Program
Transduction
Plasmids
Transduction involves phages carrying bacterial DNA from one host cell to another. Generalised transduction
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Plasmids are small, circular, self-replicating DNA molecules separate from the bacterial chromosome.
Specialised transduction 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. 2004 Biology Olympiad Preparation Program
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Transposons – transposable genetic elements – pieces of DNA that can move from one location to another in a cell’s genome.
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’.
Insertion sequences – simple transposons.
F factor, an episome, conveys the ability to form sex pili.
Composite transposons – insertion sequences flanking 1+ other genes.
F-
cells conjugate with cells. ‘Females’ then turn into ‘males’.
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Transposons
Conjugation
F+
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Operons E.g. trp operon, encodes proteins needed for tryptophan synthesis.
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