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IB HL BIOLOGY > DNA part 1 > Flashcards

DNA part 1 Flashcards

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1
Q

What are the two types of nucleic acids?

A

DNA and RNA

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2
Q

What is the subunit of DNA and RNA?

A

Nucleotide

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3
Q

Draw a nucleotide and label it fully

A

Covalent bond between phosphate group and c5 and between nitrogenous base and c1

Instead of pentose sugar write deoxyribose or ribose

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4
Q

How are nucleotides linked together?

A

By covalent bonds via condensation reactions. The bond forms between the phosphate of one nucleotide and the OH of the C3 pentose sugar- phosphodiester bond. This created the backbone of alternating sugar and phosphate groups

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5
Q

How do DNA run?

A

5’ to 3’

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6
Q

Draw the structure of nucleic acid and fully label it

A

The DNA strands run anti-parallel.
There are hydrogen bonds between the bases. 2 hydrogen bonds between adenine and thymine and three hydrogen bonds between guanine and cytosine

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7
Q

What is complementary base pairing?

A

Adenine always pairs with thymine and guanine with cytosine
This allows for a DNA to replicate semi-conservatively.

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8
Q

What are the three differences between RNA and DNA?

A
  1. The type of pentose is ribose in RNA but deoxyribose in DNA
  2. In both DNA and RNA there are 4 possible bases. Three are the same: adenine, cytosine and guanine. The fourth is thymine in DNA but uracil in RNA
  3. DNA is double helix, RNA is single-stranded
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9
Q

Describe the model making and the discovery of the structure of DNA by Watson and Crick

A
  1. X-ray diffraction by Rosalind Franklin had suggested that the shape of the DNA molecule is helical and that it is tightly packed
  2. Erwin Chargaff has demonstrated that DNA is composed of an equal number of purines( A and G) and pyrimidines ( C and T)

Watson and Crick combined what was known about DNA and built scale models of components of DNA. They built a model where the A-T and G-C were linked by hydrogen bonds. The base pairs were equal in length and would fit between the two backbones. They also showed that the two strands were antiparallel.

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10
Q

What did the model made by Watson and Crick cause?

A

It convinced others and immediately suggested a mechanism for DNA replication based on complementary base pairings leading to the hypothesis of semi-conservative replication

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11
Q

When does DNA replication occur in eukaryotes?

A

In the nucleus during the S phase of interphase

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12
Q

How does DNA replicate?

A

By semi-conservative replication and depends on complementary base pairing

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13
Q

What is the difference between prokaryotes and eukaryotes DNA replication?

A
  1. Prokaryotes have one origin of replication eukaryotic cells have many
  2. Prokaryotes possess one or two types of DNA polymerases whereas eukaryotes have four or more
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14
Q

Describe DNA replication in prokaryotes

A
  1. Replication begins at sites called origins of replication
  2. At the origin of replication, a replication fork forms which moves and the two strands are synthesised
  3. The enzyme helicase unwinds the DNA double helix at the replication fork by breaking the hydrogen bonds and separating the 2 strands
  4. The enzyme DNA gyrase moves ahead of helicase and relieves strains that are created when the double helix uncoils. Without it, the separated strands would form tight supercoils
  5. Single stranded binding proteins keep the strands apart long enough to allow the template strand to be copied and prevent them from re-annealing
  6. Each of the single strands serves as a template for the synthesis of a new strand based on complementary base pairings
  7. DNA polymerase III can extend a nucleotide chain, but can’t start it. RNA primer is used, a short RNA sequence attached by base pairing to the template strand
  8. DNA primase synthesises the RNA primer. DNA primase adds one primer in the leading strand and many primers in the lagging strand
  9. DNA polymerase III links nucleotides together to form new strands on each template strand according to complementary base pairing (A-T, G-C). DNA polymerase III starts replication next to the RNA primer. It covalently links the phosphate of a free nucleotide to the -OH group in C3 of the deoxyribose of the nucleotide at the end of the synthesising strand. Replication is always from 5’ to 3’
  10. The leading strand is made continuously following the form as it opens. DNA polymerase III moves in the same direction as the replication fork
  11. The lagging strand is made in fragments away from the replication fork so replication is discontinuous. Okazaki fragments are the short DNA fragments.
  12. DNA polymerase 1 removes the RNA primer and replaces it with DNA.
  13. A nick is left in the backbone where two nucleotides are still unlinked. This is sealed by DNA ligase linking them together by making a phosphodiester bond
  14. DNA polymerase III proofreads for mistakes
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15
Q

What does it mean that DNA replication is semi-conservative?

A

Each of the DNA molecules produced has one newly synthesised strand and one strand conserved from the parent molecule. Due to complementary base pairing, each of the strands is complementary to the template strand and has the same base sequence as the old strand that was separated from the template strand. The 2 DNA molecules are identical to each other and the parent DNA molecule

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16
Q

What are the two alternative theories for the replication of DNA?

A

Conservative replication- the two strands of the parent DNA remain together and another DNA molecules is produced with 2 new strands
Dispersive replication- every DNA molecule produced by replication has a mixture of old and new section in both of its strands

17
Q

Describe Meselson and Stahl expermiments

A
  1. They used 15N, an isotope of Nitrogen which is denser than 14N isotope
  2. They separated the DNA containing 15N from DNA containing 14N. This was called caesium chloride density gradient centrifugation. The more dense it is the lower it would be.
  3. They cultured E coli for many generations in a medium with 15N and so all the bases of the DNA has 15N
  4. They transferred the bacteria to a medium with 14N isotope. Suitable temperature was used so bacteria replicated the DNA every 50min. The bases incorporated 14N.
  5. They extracted DNA samples from the bacteria for several hours from the time of transfer to the 14N medium and measured the DNA density
  6. The DNA could be detected because it shows up as a dark band under UV light
  7. At time 0 they detected one band as all the DNA molecules had 15N
  8. After one generation, they detected one band that was higher in the tube and was exactly the intermediate density between 14N and 15N.
  9. After two generations, there were two bands one 14N and one 14N/15N
  10. In the following generations, the 14N band became stronger and the 14N/15N band weaker
18
Q

Where are histones present and absent?

A

Prokaryotic DNA is naked, but eukaryotic DNA is associated with proteins

19
Q

How do histones package the DNA?

A

Into globular structures called nucleosomes. A nucleosome consists of a central core of eight histone proteins with DNA wrapped around.

20
Q

What is PCR?

A

Polymerase chain reaction

Technique used to amplify/make many copies of a selected DNA sequence

21
Q

Describe PCR.

A
  1. A small quantity of DNA is needed
  2. The DNA is placed into an Eppendorf tube together with DNA polymerase, nucleotides and primers. The tube is loaded in a PCR machine
  3. The machine undergoes cycles of repeated heating and cooling a. at 95C the DNA strands separate b. at around 55C the two primers bind to the strands c. 72C Taq DNA adds nucleotides and copies both strands starting at the primer
  4. This is repeated 30 times. Each cycle doubles the quantity of DNA. In a couple of hours, millions of copies of the desired DNA are produced
22
Q

What is Taq DNA polymerase?

A

It is obtained from the bacterium Thermus aquaticus which lives in hot springs so enzymes do not denature at high temperatures

23
Q

What are uses of PCR?

A
  1. DNA profiling for paternity investigations
  2. DNA profiling for forensic investigations
  3. In gene transfer procedures
  4. Research of fossils and use of DNA sequence comparisons as an evolutionary clock
  5. Tests of genetically modified food
  6. Diagnostics eg test for a virus

IB HL BIOLOGY (25 decks)

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