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DNA Flashcards

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1
Q

What is the role of nucleic acids?

A

Nucleic acids are required for the storage and expression of genetic information

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2
Q

Identify the two types of nucleic acid

A
  • Deoxyribonucleic acid (DNA)
  • Ribonucleic acid (RNA)
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3
Q

Describe the structure of DNA

A
  • DNA is a polymer of deoxyribonucleoside monophosphates covalently linked by 3’→5’–phosphodiester bonds
  • DNA exists as a double-stranded (ds) molecule, forming a double helix
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4
Q

Describe the bonds formed in the DNA molecule

A

Phosphodiester bonds join the 3’ end of one nucleotide to the 5’ end of an adjacent nucleotide through a phosphate group

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5
Q

Describe the polarity of the overall DNA chain

A

The resulting chain has polarity, with both a 5’-end (free phosphate) and a 3’-end (free hydroxyl) that are not attached to other nucleotides

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6
Q

How are bases in DNA written?

A

Bases are written in sequence from the 5’-end of the chain to the 3’-end

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7
Q

Compare and contrast the DNA found in eukaryotes with that found in prokaryotes

A
  • Eukaryote: many chromosomes, linear molecule of dsDNA, bound to a complex of proteins to form chromatin
  • Prokaryote: single circular and supercoiled chromosome, associated with non-histone proteins that condense the DNA to form a nucleoid
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8
Q

What are plasmids?

A

Plasmids are small, circular, extrachromosomal DNA molecules

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9
Q

What do plasmids do?

A
  • Carry genes that convey antibiotic resistance to the host bacterium
  • Facilitate the transfer of genetic information from one bacterium to another
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10
Q

Describe the structure of the double helix in the DNA molecule

A
  • Two chains are coiled around an axis of symmetry in an anti-parallel manner
  • The hydrophilic deoxyribose–phosphate backbone is on the outside and the hydrophobic bases are stacked inside
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11
Q

Describe base pairing in a DNA molecule

A

Bases of one strand are paired with the bases of the second strand:

- Adenine is always paired with a thymine (two H2 bonds)

- Cytosine is always paired with a guanine (three H2 bonds)

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12
Q

Explain how the separation of the DNA strands in a double helix might occur

A
  • The two strands separate when hydrogen bonds between the paired bases are disrupted
  • Disruption occurs if pH of the DNA solution is altered or if the solution is heated
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13
Q

What are telomeres?

A

Telomeres are complexes of noncoding DNA and proteins located at the ends of linear chromosomes

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14
Q

Describe the structure of telomeres

A

The single-stranded region folds back on itself, forming a loop structure that is stabilized by protein

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15
Q

What do telomeres do?

A
  • Maintain the structural integrity of the chromosome, preventing attack by nucleases
  • Allow repair systems to distinguish a true end from a break in dsDNA
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16
Q

Telomere shortening affects eukaryotic cells.

What is this phenomenon?

A
  • After the removal of the RNA primer, there is no way to fill in the remaining gap with DNA
  • Hence, in human somatic cells, telomeres shorten with each successive cell division until the cell becomes senescent
17
Q

Identify the cells that are not affected by telomere shortening.

Explain why

A

The following cells contain telomerase, which maintains telomeric length:

  • Germ cells
  • Stem cells
  • Cancer cells
18
Q

What is telomerase and what does it do?

A
  • Telomerase is a complex that contains a protein which acts as a reverse transcriptase using the RNA template to synthesize DNA in the usual 5’→3’ direction
  • Telomerase then translocates to the newly synthesized end, and the process is repeated
19
Q

What are reverse transcriptases?

A

Reverse transcriptases are RNA-directed DNA polymerases

20
Q

Provide and example of a reverse transcriptase in human disease

A

Reverse transcriptases are involved in the replication of retroviruses e.g. HIV

21
Q

Describe the inhibition of DNA synthesis by nucleoside analogs

A
  • DNA chain growth can be blocked by the incorporation of nucleoside analogs that have been modified in the sugar portion of the nucleoside
  • These compounds slow the division of rapidly growing cells and viruses e.g. araC (cancer) and AZT (HIV)
22
Q

Describe 3 ways in which DNA damage can occur

A
  • DNA synthesis errors
  • Environmental insults alter/remove nucleotide bases
  • Bases are also altered / lost spontaneously
23
Q

Identify 2 agents which can damage DNA

A
  • Chemicals e.g. nitrous acid changes bases
  • -* Radiation e.g. UV light fuses pyramidines
24
Q

What happens when damaged DNA is not repaired?

A
  • A mutation is introduced that can result in any of a number of deleterious effects
  • Effects include loss of control over the proliferation of the mutated cell, leading to cancer
25
Q

Identify the three steps in DNA repair

A

- Recognition of the damage (lesion) on the DNA

  • Removal or excision of the damage
  • Replacement or filling the gap left by excision using the sister strand as a template for DNA synthesis and ligation
26
Q

Describe methyl-directed mismatch repair

A

Mut proteins identify the mispaired nucleotide(s) and discriminate between the correct strand and the strand with the mismatch

27
Q

Some bases experience alterations.

Describe the base alteration of cytosine

A

- Cytosine slowly undergoes deamination (the loss of its amino group) to form uracil

  • Also occurs by action of deaminating or alkylating compounds
28
Q

What can cause double-strand breaks in DNA?

A
  • Natural errors in gene rearrangements
  • High-energy radiation
  • Oxidative free radicals
29
Q

Explain the repair of double-strand breaks

A

Non-homologous end-joining repair brings together the ends of two DNA fragments by a group of proteins that effect their religation

30
Q

What are the problems with non-homologous end pair joining?

A
  • Some DNA is lost in the process and the mechanism is error prone and mutagenic
  • Defects in this repair system are associated with a predisposition to cancer and immunodeficiency syndromes
31
Q

how many chromosomes do humans have?

A

46 (diploid)

  • 22 autosomes (two of each)
  • A pair of sex chromosomes

Male – XY or Female XX

medical genetics (12 decks)

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