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2B - Genetics & Evolution > week 4 > Flashcards

week 4 Flashcards

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Biology 101 flashcards
1
Q

What is genetic recombination?

A

process of breaking and re-joining DNA strands which often results in DNA molecules with new sequence.

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2
Q

What are the 4 types of recombination?

A
  1. homologous (general) recombination– extended homology
  2. site-specific recombination – limited homology
  3. illegitimate recombination– no homology
  4. replicative recombination, mainly transposition– no homology
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3
Q

What is HIV an example of?

A

illegitimate recombination (no homology)

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4
Q

Briefly describe the process of HIV

A

HIV infects T-cells by binding to their CD4 receptor, reverse-transcribing its RNA into dsDNA, and integrating into the human genome. Integration sites are random but prefer actively transcribed regions due to higher accessibility. HIV can remain silent for years, causing no symptoms. AIDS develops when HIV becomes active, producing more virus particles and infecting more T-cells, ultimately compromising the immune system. This makes even minor infections potentially lethal.

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5
Q

How does phage lambda integrate into the E. coli chromosome, and what are the lytic and lysogenic cycles?

A

Phage lambda integrates into the E. coli chromosome via site-specific recombination.

In the lytic cycle, phages hijack the host to produce viral particles, ultimately killing the host cell.
In the lysogenic cycle, the phage integrates into the bacterial chromosome as a prophage, replicating passively with the host’s DNA.
When the host experiences stress, the prophage exits the chromosome and switches to the lytic cycle, ensuring survival in its virion form before the host cell dies.

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6
Q

Homologous recombination is important for…

A

genome stability (repair of DSBs)
genetic diversity (diff alleles)

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7
Q

Homologous recombination always initiate with … and requires ____ homologous DNA to repair the break

A

DSB , unbroken

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8
Q

Explain DSB resection, strand invasion, repair synthesis, double Holliday junction and HJ resolution

A

nuclease degrade 5’ ends to make 3’ overhangs

one of the 3’ overhang invades into homologous donor sequence

DNA [olymerase extends broken strand by copying sequence across broken region

broken sequence is restored but donor and recipient are connected by 2 HJ

resolved by resolvases either crossing over or non-crossing over

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9
Q

What is the role of Rad51 and BRCA2 in homologous recombination, and what happens if BRCA2 is absent?

A

Rad51 polymerizes on ssDNA, forms a filament, finds homology, and facilitates strand invasion for repair. BRCA2 localizes Rad51 to ssDNA, enabling homologous recombination. If BRCA2 is absent, Rad51 cannot function effectively, preventing homologous recombination and leading to genome instability, increasing the risk of mutations.

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10
Q

How many ways is a Holliday junction

A

4 ways

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11
Q

What are genetically programmed double-stranded breaks in meiosis, and what enzyme is involved?

A

genetically programmed double-stranded breaks (DSBs) are deliberately created to facilitate homologous recombination. The enzyme Spo11 endonuclease generates these breaks by cleaving one strand of DNA on a sister chromatid, initiating the recombination process necessary for proper chromosome segregation.

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12
Q

What needs to be broken by separase in meiosis 1 vs 2

A

non centromeric cohesin, centromeric cohesin

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13
Q

How does the distance between genes affect recombinant frequency during crossing over?

A

When genes are far apart (e.g., A and B), crossovers outside their region don’t affect segregation, leading to a high recombinant frequency. When genes are close together (e.g., C and D), crossovers between them are rare, resulting in a low recombinant frequency as fewer recombination events occur in their proximity.

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14
Q

How to use RF to determine if genes are linked?

A

RF < 50% == linked
RF= 5-% == unlinked

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15
Q

Why does replication move and pause rather than continuous speedway?

A

diff proteins bound to diff places that slows down replication

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16
Q

Give examples of replicatgion barrier

A

RNA in the way, DNA secondary structures, tightly bound proteins

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17
Q

What is recruited to unwind DNA secondary structures?

A

specialised axillary helicases

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18
Q

What happens when a replication fork is stalled or broken, and why is the 3’ end overhang important?

A

When a replication fork is stalled or broken, a double-stranded break occurs, exposing unstable ssDNA. The 3’ end overhang is critical because:

ssDNA searches for homology in the sister chromatid and invades it.
DNA polymerase extends the 3’ end, adding nucleotides to repair the fork. Polymerase cannot add to a 5’ end, making the 3’ overhang essential for repair and continuation of replication.

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19
Q

What is the role of Rad51 in DNA repair, and how does BRCA2 affect its function?

A

Rad51 is a recombinase protein that polymerizes on ssDNA, forming filaments to repair DNA breaks by facilitating strand invasion. It dissociates once the ssDNA becomes dsDNA, as it is less stable with dsDNA. BRCA2 is essential for Rad51 binding to ssDNA; without BRCA2, strand invasion cannot occur, preventing homologous recombination and DNA repair.

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20
Q

HR recombinase are conserved from ____ to ___

A

bacteria to human

21
Q

What kinds of chromosomes are incompatible with meiotic recombination

A

Circular

22
Q

What is mendelian inheritance?

A

test for single gene inheritance, ratio of phenotype 3:1 (genotype 1:2:1) = inheritance of single gene

23
Q

What is a test cross, and why is it used?

A

A test cross involves crossing an individual with an unknown genotype (showing a dominant trait) with a homozygous recessive individual. It is used to determine whether the unknown genotype is homozygous dominant or heterozygous based on the offspring’s traits.

24
Q

What are the 4 modes of single gene inheritance?

A
  • autosomal dominant
  • autosomal recessive
  • sex-linked dominant
  • sex-linked recessive
25
Q

What is X-linked recessive inheritance, and how does it affect males and females differently?

A

In X-linked recessive inheritance, the mutated gene is on the X chromosome.

Males (XY): A single mutated allele causes the trait or disorder because they have only one X chromosome.
Females (XX): The trait manifests only if both X chromosomes carry the mutation; otherwise, they are carriers if only one is mutated.

26
Q

What does the following man pedigree symbols mean?
Circle:
Square:
Shaded:
Unshaded:
Half-shaded:
Horizontal line:
Vertical line:
Diagonal slash: Deceased individual:
Dot:

A

Circle: Female
Square: Male
Shaded: Affected individual
Unshaded: Unaffected individual
Half-shaded: heterozygotes for autosomal recessive
Horizontal line: Marriage or mating
Vertical line: Offspring
Diagonal slash: Deceased individual
Dot : carrier of sex linked recessive

27
Q

What are the key signs of autosomal dominant inheritance?

A
  • Affected individuals appear in every generation.
  • Both sexes are equally affected.
  • Only one copy of the abnormal allele is required to express the trait or disorder.
28
Q

Signs of autosomal recessive

A
  • Most generations unaffected
  • Affected siblings, both sexes
  • Parents are heterozygous and unaffected
  • But rare that both parents, if unrelated, would be heterozygous (genotype Pp)
29
Q

signs of X linked dominant

A
  • Appears in all generations
  • Affected heterozygous mother passes condition on to both daughters and sons
  • Affected father passes condition on to daughters
30
Q

signs of x linked recessive

A
  • Appears to jump a generation
  • Only males affected
  • Females can be heterozygous carriers
31
Q

What does this show: AB/ab

A

linked genes

32
Q

what does this show: AB;ab

A

unlinked genes

33
Q

What is allelic interactions vs non-allelic interactions

A
  • Allelic interactions (alleles within the same gene)
  • Non-allelic interactions (alleles within different genes)
34
Q

What are allelic interactions in heterozygotes, and how is dominance defined?

A

Dominance refers to how alleles of a single gene interact in a heterozygote to determine the phenotype. It is a property of the phenotype rather than the alleles themselves, with various forms such as complete dominance, incomplete dominance, and codominance.

35
Q

What is complete dominance, and how does it affect heterozygotes?

A

heterozygote exhibits the same phenotype as a homozygote carrying two copies of the dominant allele. This is typically due to the dominant allele producing enough functional protein, making the recessive allele’s effect unnoticeable.

36
Q

What is the difference between haplosufficiency and haploinsufficiency in mutations?

A

Haplosufficiency occurs in recessive mutations, where one functional copy of a gene produces enough protein for normal function, so the individual is unaffected.

Haploinsufficiency occurs in dominant mutations, where one copy of the gene does not produce enough protein to maintain normal function, leading to a dominant phenotype despite the presence of a normal allele.

37
Q

What is a dominant negative mutation, and how does it affect protein function?

A

A dominant negative mutation occurs when a loss-of-function allele is dominant over a functional allele. The mutant protein interferes with the function of the wild-type protein co-expressed in the cell. This is common in multimeric proteins (proteins made of multiple subunits), where the mutant subunit can disrupt the activity of the entire protein complex, leading to a dominant phenotype.

38
Q

What are lethal alleles, and how do they affect homozygotes in terms of offspring ratios?

A

Organisms can tolerate one lethal allele, as the other provides enough functional protein, but when two lethal alleles combine in a homozygote, the organism dies. This leads to a 2:1 offspring ratio instead of the expected 1:2:1 ratio.

39
Q

What is epistasis

A

Expression of one gene masked by expression of one (or more) other gene(s)

40
Q

What is recessive epistasis

A

when a recessive phenotype (e.g., white) overrides another phenotype (e.g., magenta) in a genetic pathway. The mutant phenotype of an upstream gene (W) takes precedence, no matter what happens later in the pathway.

41
Q

What is dominant epistasis

A

Dominant epistasis occurs when a dominant allele of an upstream gene inhibits the progression of a pathway. In this case, the dominant phenotype (e.g., white) overrides the expression of other phenotypes (e.g., green or yellow), meaning the dominant allele prevents the expression of other traits down the genetic pathway.

42
Q

What is duplicate gene action?

A
  • 2 genes encode same biological function (genetic redundancy)
  • One dominant allele (from either) sufficient for normal activity
43
Q

What is complementary gene action

A
  • Phenotype determined by combined, concurrent action of two genes
  • Enzymes within same biochemical pathway (but intermediate compound leads to no phenotype)
  • Loss of function of either A or B produces same phenotype as loss of both
44
Q

What is a complementation test and what does it determine?

A

A complementation test is used to determine whether two mutations causing the same phenotype are in the same gene or different genes.

Same gene: Parental mutant phenotype in F1, as both mutations are in the same gene.

Different genes: Wild-type phenotype in F1, as each parent provides a functional allele for the other parent’s mutation.

45
Q

What are modifiers

A
  • Mutations that modify (not mask) the expression of a different gene (could be TF which affects transcription of other genes)
    • Dilute locus for coat colour
46
Q

What are suppressors

A

Suppressors are mutations that reverse the effect of another mutation, restoring the wild-type (WT) phenotype.

They produce only two phenotypes: mutant and wild-type.
Suppressors can be intragenic (within the same gene) or intergenic (in a different gene).
They can act as dominant or recessive mutations.

47
Q

Give an example of codominance

A

AB blood group

48
Q

What is codominance

A

expression of both alleles. Two proteins encoded present/visible at the same time. A & B

49
Q

What is incomplete dominance

A

heterozygote has intermediate phenotype (dosage can explain it)

2B - Genetics & Evolution (9 decks)

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