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My AS level biology - OCR A > 2.3 - Nucleic Acids > Flashcards

2.3 - Nucleic Acids Flashcards

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1
Q

What is a monomer?

A

Molecule that when repeated makes up a polymer.
Amino acids are the monomers of proteins.
Nucleotides are the monomers of nuclei acids.

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2
Q

Structure of a nucleotide

A

Molecule consisting of five carbon - sugar(pentose sugar), a phosphate group, and a nitrogenous base.

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3
Q

Double helix def

A

Shape of a DNA molecule

This is due to coiling of the two sugar-phosphate backbone strands into a right-handed spiral configuration.

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4
Q

Functions of nucleotides

A
  • Form the monomers of nucleic acids
  • Help regulate many metabolic pathways, e.g. by ATP, ADP and AMP(Adenosine Monophosphate)
  • May be components of many coenzymes - used in respiration or photosynthesis.
  • become phosphorylated nucleotides when they contain more than one phosphate group, e.g. ADP or ATP
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5
Q

What is a phosphorylated nucleotide?

A

When the nucleotide contains more than one phosphate group, e.g. ATP and ADP

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6
Q

Example of nucleotide(non - phosphorylated)

A

AMP - Adenosine Monophosphate

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7
Q

Structure of DNA

A

A polymer made up of repeating units of nucleotides
A molecule of DNA consists of two polynucleotide strands
Covalent bond between sugar and phosphate group called a phosphodiester bond. Broken when polynucleotides break, and formed when polynucleotides are synthesised
DNA molecules are long and so carry a lot of encoded genetic information

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8
Q

Two types of nitrogenous bases

- info about them

A

Purines - larger, two rings

Pyrimidines - smaller, one ring

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9
Q

Which bases are purines?

A

Adenine

Guanine

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10
Q

Which bases are pyrimidines?

A

Thymine(DNA)
Uracil(RNA)
Cytosine

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11
Q

Importance of hydrogen bonds in DNA

A

H bonds form between DNA strands and nitrogenous bases

Hydrogen bonds allow the DNA molecule to unzip for transcription and replication

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12
Q

How many hydrogen bonds between each nitrogenous base?

A

Adenine and Thymine pair with two hydrogen bonds

Guanine and cytosine pair with three hydrogen bonds

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13
Q

How is DNA organised in eukaryotic cells?

A

Majority of DNA content, the genome, is in the nucleus
Each large molecule of DNA is tightly wound around special histone proteins into chromosomes.
Each chromosome is therefore one molecule of DNA.

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14
Q

How is DNA organised in prokaryotic cells?

A
  • DNA is in a a loop and is within the cytoplasm, not enclosed in a nucleus.
  • DNA not wound around histone proteins
    Instead is described as “naked”
  • Viruses that contain DNA also have it in the form of a loop of naked DNA
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15
Q

Def of DNA polymerase

A

Enzyme that catalyses formation of DNA from activated deoxyribose nucleotides, using single-stranded DNA as a a template

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16
Q

Def of Helicase

A

Enzyme that catalyses the breaking of hydrogen bonds between the nitrogenous pairs of bases in a DNA molecule

17
Q

Def of Semi-Conservative DNA Replication

A

How DNA replicated, resulting in two new molecules
Each of these two DNA molecules contains one old strand and one new strand.
One old strand is conserved in each new molecule.

18
Q

Full DNA replication mechanism

A
  • Happens during the synthesis phase of interphase
  • The DNA molecules uses a gyrase enzyme to unwind its double helix and then uses a DNA helicase enzyme to break the hydrogen bonds between the complementary base pairs on order to ‘unzip’ the molecule
  • this results in two exposed strands of nucleotides which act as a template for semi-conservative replication
  • Complementary nucleotides free in the nucleolus match up with the exposed bases on the original two strands by forming hydrogen bonds between them and DNA polymerase catalysed the formation of phosphodiester bonds between nucleotides in the 5’ to 3’ direction
  • This happens continuously for the leading strand but for the lagging strand, it is built as Okazaki fragments due to it being opposite to the leading strand.
  • The Okazaki fragments in the lagging strand are then joined by ligase enzymes
  • This process forms 2 identical DNA molecules which each contain 1 strand of the original parent DNA
  • this makes DNA replication semi-conservative
19
Q

DNA transcription mechanism

A
  1. The DNA helix is unwound by helicase enzyme which separates the hydrogen bonds between bases
  2. One of the exposed strands is the coding strand, which acts as a template for mRNA synthesis
  3. RNA Polymerase lines up the complementary nucleotides C-G and A-U and joins them together by condensation reactions to form phosphodiester bonds
  4. The mRNA is spliced before leaving the nucleus. Non-coding introns are removed and the exons are joined together.
  5. The mature mRNA leaves through a nuclear pore and enters the cytoplasm
  6. The mRNA attaches to a ribosome and is translated
  7. Pieces of the RER pinch off to form vesicles carrying the polypeptide to the Golgi body, where it is processed and packaged.
  8. The mature protein is packaged into vesicles for storage or exocytosis
19
Q

DNA translation mechanism

A
  1. The ribosome attaches to the mRNA strand
  2. The ribosome has two active sites into which tRNA molecules can enter
  3. The tRNA that enters the active site must have the correct anti-codon to match the codon being read.
  4. As the two tRNA are alongside each other, the amino acids they carry can be joined together by condensation reaction to form a peptide bond at the expense of ATP.
  5. One of the tRNA now exits the ribosome leaving a vacant active site. The ribosome moves along one codon, and then another tRNA brings the next amino acid and so on until the end of the mRNA strand, or until a STOP codon is reached.
20
Q

Differences between DNA and RNA

A

RNA:

  • single-stranded
  • contains ribose sugar
  • contains the base uracil

DNA:

  • double-stranded
  • contains deoxyribose sugar
  • contains thymine instead of uracil
21
Q

How to extract and purify DNA

A
  • You can extract from strawberries, kiwis, etc
  • macerate the tissue
  • add a strong detergent to disrupt membranes of fruit cells
  • add ethanol so that DNA precipitates out of solution
  • removal of unwanted salts purifies the DNA
22
Q

Three types of RNA

A
  • Messenger RNA(mRNA)
  • Transfer RNA(tRNA)
  • Ribosomal RNA(rRNA)
23
Q

Why is the primary structure of a protein so important?

A
  • As long as the primary structure of a polypeptide is correct, it will then fold correctly and be held in its tertiary structure or shape
  • so it will carry out its function properly
24
Q

Info about genetic code

A

-it is ‘Universal’, as almost all codons in all organisms code for the same amino acid.
- it is described as “Degenerate”, as for most of the amino acids, there is more than one codon that codes for it.
This can reduce the effect of mutations, as a change in one base of a codon could produce another codon that still codes for the same amino acid
- the genetic code is “non-overlapping”
It is read from a fixed point in groups of three bases(codons)
If a base is added or deleted then it causes a “frame shift”, as every codon after that , and hence every amino acid coded for, is changed.

25
Q

Explain how the structure of DNA is related to its functions.
(6 Marks)

A
  1. Sugar-phosphate (backbone) / double stranded / helix so provides strength / stability / protects bases / protects hydrogen bonds;
    Must be a direct link / obvious to get the mark
    Neutral: reference to histones
  2. Long / large molecule so can store lots of information;
  3. Helix / coiled so compact;
    Accept: can store a lot of genetic information in a small amount of space for ‘compact’
  4. Base sequence allows information to be stored / base sequence codes for amino acids / protein;
    Accept: base sequence allows transcription
  5. Double stranded so replication can occur semi-conservatively / strands can act as templates / complementary base pairing / A-T and G-C so accurate replication / identical copies can be made;
  6. (Weak) hydrogen bonds for replication / unzipping / strand separation / many hydrogen bonds so stable / strong;
    Accept: ‘H-bonds’ for ‘hydrogen bonds’
26
Q

Does tRNA have hydrogen bonds in its structure?

A

Yes

My AS level biology - OCR A (19 decks)

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