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biological sciences > dna > Flashcards

dna Flashcards

1
Q

7 main people that contributed to dna

A

Griffith
Avery, McCarty and MacLeod
Hershey and Chase
Chargaff
Watson and crick
Franklin
Meselson and stahl

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2
Q

how did Griffith contribute to dna

A

demonstrations of bacterial transformation (feet up, feet down experiment)

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3
Q

explain griffiths’ feet up, feet down experiment

A
  • Streppachoccus types:
    Rough – non-pathagenic (does not cause disease)
    Smooth – evolved to stop immune systems fighting it (disease-causing)
  • Injection of live S strain: Mice died, as expected, due to the virulent bacteria.
  • Injection of live R strain: Mice survived, as the R strain is non-virulent.
  • Injection of heat-killed S strain: Mice survived because the S bacteria were dead and couldn’t cause disease
  • Injection of live R strain and heat-killed S strain: Surprisingly, the mice died, and live S bacteria were found in their bodies
  • suggested that some “transforming principle” in the dead S strain cells carried genetic information that could change the R strain’s properties, making it harmful
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4
Q

how did Avery, McCarty and MacLeod contribute to dna

A

found transforming factor is separable

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5
Q

explain how Avery, McCarty and MacLeod found transforming factor is separable

A
  • did some reductionist biology and isolated the nucleic acids, proteins and sugars
  • re-ran the Griffith experiment using individual components rather than whole bacteria
  • Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria
  • Many biologists remained skeptical, mainly because little was known about DNA
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6
Q

how did Hershey and Chase contribute to dna

A

found transforming factor is DNA

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7
Q

explain how Hershey and Chase found transforming factor is DNA

A
  • aimed to find the “transforming principle” in genetics: DNA or protein
  • used bacteriophages to investigate, as viruses inject genetic material into host cells
  • Two setups: phages with radioactive sulfur (protein) + phages with radioactive phosphorus (DNA)
  • After infecting bacteria, only radioactive DNA entered cells
  • Conclusion: DNA, not protein, carries genetic information, confirming DNA’s role in heredity
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8
Q

how did Chargaff contribute to dna

A
  • Found proportions of the 4 bases vary between species
  • Found amount of A is always equal to T + G is always equal to C (chargaffs rules)
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9
Q

how did Watson and crick contribute to dna

A
  • built a double helix model of DNA based on X-ray data and chemical knowledge
  • Franklin’s work showed 2 antiparallel sugar-phosphate backbones with bases inside
  • Watson and Crick initially thought bases paired like-with-like, but the width didn’t match X-ray data
  • They discovered purine-pyrimidine pairing (A with T, G with C) fit the structure’s uniform width
  • This model supported Chargaff’s rules: A = T, G = C.
  • The double helix’s structure suggested a copying mechanism for genetic material
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10
Q

how did Franklin contribute to dna

A

found DNA structure

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11
Q

explain how Franklin found dna structure

A
  • Not awarded nobel prize because she died (only 2 instances where nobel prize was given to dead person)
  • using a technique called X-ray crystallography to study molecular structure - produced a picture of the DNA
  • enabled Watson to deduce that DNA was helical + to deduce the width of the helix and the spacing of the nitrogenous bases
  • The width suggested that the DNA molecule was made up of two strands, forming a double helix
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12
Q

how did Meselson and stahl contribute to dna

A

found dna used semi-conservative replication

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13
Q

what were the 3 hypothesised dna replication models

A
  1. conservative models
  2. semi-conservative
  3. dispersive model
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14
Q

what is the correct model on how dna replicates itself. explain it

A

semi-conservative
- Each strand of a DNA molecule from the parent generation acts as a template for the synthesis of a new strand in the daughter generation
- each new DNA double helix in the daughter cells is a hybrid of 1 old strand + 1 new strand

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15
Q

explain how Meselson and stahl found dna used semi-conservative replication

A
  • grew bacteria in heavy nitrogen (N-15), then moved them to light nitrogen (N-14)
  • After DNA replication, they used centrifugation to analyze DNA density
  • 1st replication cycle showed DNA of intermediate density - suggest each molecule had one original and one new strand
  • 2nd cycle revealed both intermediate and light bands, confirming semi-conservative replication
  • This meant each new DNA molecule retained one old strand and incorporated one newly synthesized strand
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16
Q

explain the conservative dna replication

A
  • original DNA double helix acts as a template to create a completely new double helix - makes one daughter molecule containing the original, intact DNA strands + another daughter molecule with entirely new DNA strands
17
Q

what is helicase

A

enzyme that unwinds parental double helix at replication forks

18
Q

what is topoisomerase

A

relieves the strain of the unwinding done by helicase

19
Q

what is single-strand binding proteins

A

binds and stabilises single strands of dna at the bubble

20
Q

what is dna polymerase I

A

removes RNA nucleotides of primer from 5’ end and replaces them with DNA nucleotides

21
Q

what is primase

A

synthesises RNA primer at 5’ end

22
Q

what direction does dna polymerase extend in

A

5’ to 3’ direction

23
Q

how is dna organised

A

antiparallel

24
Q

2 sides of a replication bubble

A

leading strand - extends continuously
lagging strand - made up of lots of little fragments (Okazaki fragments)

25
Q

what is DNA ligase

A

joins 3’ end of DNA that replaces primer to rest of leading strand + joins Okazaki fragments of lagging strand

26
Q

dna replication process steps

A
  1. replication initiation
  2. replication elongation
  3. proof reading + repairing DNA
  4. replicating ends of DNA molecules
27
Q

explain replication initiation

A
  • begins at sights called ORIGINS OF REPLICATION
  • strands come apart to open a REPLICATION BUBBLE flanked by REPLICATION FORKS
  • proceeds in the both fork directions until DNA molecule is copied
  • helicase, SSBP, topoisomerase, primase
28
Q

what is replication elongation

A
  • DNA polymerase III elongates from the primer by adding nucleotides in a 5’ to 3’ direction
  • prokaryote cells - elongation proceeds at 500 bases per second
  • eukaryotic cells - elongation proceeds at 50 bases per second
  • produces both leading + lagging stands
29
Q

what is DNA polymerase III

A

elongates from the primer by adding nucleotides in a 5’ to 3’ direction

30
Q

explain proof reading + repairing DNA

A
  • PROOF READ + REPAIR - polymerases proofread newly made DNA + replace incorrect bases
  • DNA MISMATCH REPAIR - enzymes correct errors inbase pairing
  • NUCLEOTIDE EXCISSION REPAIR - sections of DNA can be cut out and replaced
  • certain chemicals, UV light, x-rays etc can cause DNA damage
31
Q

replicating the ends of DNA molecules

A
  • the replication machinery cannot replicate the ends of linear chromosomes
  • ends of chromosomes have specialised sequences called TELOMERS
  • telomere shortening is connected to aging
  • germ cells have an enzyme called TELOMERASE to maintain telomere lengths in those cells

biological sciences (13 decks)

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