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Biology > M2 Nucleic Acids > Flashcards

M2 Nucleic Acids Flashcards

1
Q

What are nucleic acids?

A

Large polymers formed from many nucleotides in a long chain, containing the elements carbon, hydrogen, oxygen, nitrogen and phosphorus.

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2
Q

What are the two types of nucleic acids present in cells?

A
  • DNA (deoxyribonucleic acid) is a more stable, double stranded form that stores the genetic blueprint for cells
  • RNA (ribonucleic acid) is a more versatile, single stranded form that transfers genetic information for decoding
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3
Q

What is a nucleotide composed of?

A
  • A phosphate group
  • A pentose sugar containing 5 carbon atoms
  • A nitrogenous base (a complex organic molecule containing one or two carbon rings in its structure, as well as nitrogen)
    The subunits are joined by covalent bonds to form a nucleotide molecule.
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4
Q

Compare DNA and RNA

A
  • DNA is double stranded, RNA is single stranded
  • DNA has a deoxyribose sugar, RNA a ribose sugar
  • DNA has bases ATCG and RNA has bases AUCG
  • Both made of nucleotides
  • Both have a phosphate group, nitrogenous base and pentose sugar
  • DNA is stable, long, stores genetic code blue print for cells and RNA is versatile, short and transfers information for genetic coding.
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5
Q

How are nucleotides linked together?

A
  • Nucleotides are linked by condensation reactions to form a polymer called a polynucleotide.
  • The phosphate group at carbon 5 of one nucleotide forms a covalent bond with the hydroxyl group on carbon 3 of the pentose sugar on an adjacent nucleotide. These bonds are phosphodiester and form a long, strong sugar-phosphate backbone.
  • Hydrogen bonds form between bases, joining A and T with 3 bonds, and C and G with 2 bonds.
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6
Q

How many different DNA nucleotides are there?

A

4 as there are four different bases, they can be divided into pyrimidines and purines

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7
Q

What are pyrimidines?

A

The smaller bases which contain single carbon ring structures - thymine (T), uracil (U) and cytosine (C)

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8
Q

What are purines?

A

The larger bases which contain double carbon ring structures - adenine (A) and guanine (G)

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9
Q

How is the double helix formed?

A
  • The two strands of the double helix are held together by hydrogen bonds between the bases. Each strand has a phosphate group (5’) at one end and a hydroxyl group (3’) at the other end.
  • The two parallel stands are arranged so that they run in opposite directions - they are antiparallel.
    C - G: 3 hydrogen bonds
    A - T: 2 hydrogen bonds
    A - U: 2 hydrogen bonds
  • Base pairing rules mean that a small pyrimidine base always binds to a larger purine base. Therefore a constant distance is always kept between the DNA ‘backbones’ resulting in a constant distance between the DNA backbones creating parallel chains.
  • Complimentary base pairing means DNA always has equal amounts of A and T, and C and G.
  • The sequence of bases along a DNA strand carries the genetic information of an organism in a code.
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10
Q

What is the importance of complementary base pairing?

A
  • Complementary base pairing means there is always the same distance between each pair of bases, the pairing ensures faithful DNA replication by providing a mechanism in replication and transcription
  • A base can only pair with a specific base pair as this ensures a high fidelity of replication, this makes an accurate copy
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11
Q

Describe RNA

A
  • Free nucleotides as they exist in the nucleus and cytoplasm
  • Play an essential pole in the transfer of genetic information from DNA to proteins that make up the enzyme and tissues of the body.
  • DNA stores all of the genetic information needed by an organism, however DNA is very long and is unable to leave the nucleus in order to supply the information directly to the sites of protein synthesis.
  • Therefore DNA is transcribed into a shorter messenger RNA molecule.
  • After protein synthesis RNA molecules are degraded in they cytoplasm. The phosphodiester bonds are hydrolysed and RNA nucleotides are released and reused.
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12
Q

Describe DNA replication

A
  1. Two strands run in opposite directions, (5’ - 3’) on the sugar phosphate determines the direction of replication.
  2. The strands unwind and unzip using helicase with catalyses the breaking of hydrogen bonds. Each strand can act as a template.
  3. Primase catalyses the formation of a primer from RNA nucleotides (which marks the position that DNA replication starts)
  4. DNA polymerase binds to the primer, joining DNA nucleotides together from the 5’ end to the 3’ end.
  5. The leading strand is made continuously as nucleotides can be added one by one from 5’ to 3’.
  6. The lagging strand cannot be made continuously because of its 3’ - 5’ direction, so it is made in fragments. Each fragment begins with a primer and a small fragment make in 5’ to 3’ direction.
  7. DNA ligase joins the strand together (by hydrogen bonds) and two double stranded molecules are formed.
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13
Q

Describe DNA replication errors

A

Sequences of bases are not always matched exactly, and an incorrect sequence may occur in the newly-copied strand. These errors occur randomly and lead to a change in the sequence of bases, known as a mutation.

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14
Q

Describe the genetic code

A
  • DNA is contained within the cells of all organisms, therefore genetic information is passed on from one generation to the next.
  • DNA must carry the instructions needed to synthesise the many different proteins needed by these organisms as proteins are the foundations for living things.
  • They are made up of a sequence of amino acids, folded into complex structures. Therefore DNA must code for a sequence of amino acids. This is called the genetic code.
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15
Q

Describe the triplet code

A
  • The instructions that DNA carries are contained in the sequence of bases along the chain of nucleotides that make up the two strands of DNA.
  • The code in the base sequences is a simple triplet code. It is a sequence of three bases, called a codon. Each codon codes for an amino acid.
  • A section of DNA that contains the complete sequence of bases (codons) to code for an entire protein is a gene.
  • The genetic code is universal, all organisms use this same code, the sequence of bases differs for each protein.
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16
Q

Describe the degenerate code

A
  • There are 4 different bases, therefore 64 different base triplets or codons possible (4 x 4 x 4).
  • This includes one codon that acts as the start codon at the beginning of the gene signalling the start of a sequence that codes for a protein.
  • There are also three ‘stop’ codons that do not code for any amino acids and signal the end of the sequence.
  • Having a single codon to signal the start of a sequence ensures that the triplet of bases (codons) are read in ‘frame’ and are non-overlapping.
  • There are only 20 different amino acids that regularly occur in biological proteins, there are a lot more codons than amino acids. Therefore amino acids can be coded for by more than one codon, therefore the code is degenerate.
17
Q

What is transcription?

A

DNA to RNA
The formation of a messenger RNA molecule from a DNA template that is carried to the ribosomes in the cytoplasm, occurring in the nucleus.

18
Q

What is translation?

A

RNA to protein
The conversion of the messenger RNA codons into a specific sequence of amino acids to form a polypeptide, occurring in ribosomes and the cytoplasm.

19
Q

Describe rRNA

A

Ribosomal RNA together with proteins forms ribosomes in the nucleus

20
Q

Describe mRNA

A

Messenger RNA is made in the nucleus as complementary to the template DNA strand. Therefore is a copy of the coding strand that carries information out of the nucleus.

21
Q

Describe tRNA

A

Translation RNA is composed of a strand of DNA folded in a way that three bases, called the anticodon are at one end of the molecule. These bind to complementary codon on the mRNA and the tRNA molecules carry an amino acid corresponding to that codon.

22
Q

Describe transcription

A
  1. RNA polymerase separates the DNA strands and synthesised a complementary mRNA copy from one of the DNA strands.
  2. When the DNA strands are separated, RNA nucleotides align opposite their exposed complementary base.
  3. RNA polymerase covalently join the nucleotide to the growing sequence forming phosphodiester bonds.
  4. Once the RNA sequence has been synthesised, RNA polymerase detached from the DNA molecule and the double helix reforms.

DNA has two strands:
- Coding strand/sense strand (5’ - 3’ direction) gene, not transcribed so almost identical to RNA
- Template strand/anti-sense strand (3’ - 5’ direction) which is transcribed, so is complementary to RNA

23
Q

Describe the process of transcription

A
    • RNA polymerase binds to the DNA of the gene at a region called the promoter (which tells the polymerase where to begin transcribing)
      - Unwinding of the DNA during transcription is the reason or of the activity of a helicase enzyme downstream of the RNA polymerase. As it unzips the DNA exposes the bases.
    • RNA polymerase ‘walks along the template in the 3’ - 5’ direction
      - RNA polymerase adds a complementary RNA nucleotide to the 3’ end of the RNA strand and this continues until a mRNA strand is formed
    • Transcription stops at the end of a gene
      - This produced a STOP codon on the mRNA
      - RNA polymerase detaches and the mRNA formed detaches from the DNA template and leaves via a nuclear pore. The double helix reforms.
24
Q

Describe translation

A
  1. The ribosome binds to the mRNA and moves along the molecule in a 5’ - 3’ direction until it reaches a start codon (AUG).
  2. The first tRNA carrying the amino acid methionine is needed in order for translation to start.
  3. The mRNA is read one codon at a time. Anticodons on tRNA molecules align opposite codons according to complementary base pairing. Each tRNA molecule carries a specific amino acid.
  4. Ribosomes (peptidyl transferase) catalyse the formation of peptide bonds between adjacent amino acids (via condensation reactions).
  5. The ribosome moves along the mRNA molecule synthesising a polypeptide chain until it reaches a stop codon.
  6. At this point translation ceases and the polypeptide chain is released.
25
Q

What does translation form?

A

As the amino acids are joined together forming the primary structure is protein, they fold into secondary and tertiary structures. This folding and the bonds that are formed are determined by the sequence of amino acids in the primary structure.

26
Q

What do cells require energy for?

A
  • Synthesis eg. of large molecules such as protein
  • Transport eg. pumping molecules or ions across cell membranes by active transport
  • Movement eg. protein fibres in muscle cells that cause muscle contraction
27
Q

What is an ATP molecule composed of?

A
  • A nitrogenous base (always adenine)
  • A pentose sugar (ribose)
  • Three phosphate groups
    ATP is a nucleotide.
    ATP is used for energy transfer in all cells of living things - it is known as the universal energy currency.
28
Q

How does ATP release energy?

A
  • A small amount of energy is needed to break the weak bond holding the last phosphate group in ATP, however a large amount of energy is released when the liberated phosphate forms other bonds.
  • This is a hydrolysis reaction as water is involved of the removal of the phosphate group.
  • The hydrolysis of ATP does not happen in isolation but in association with energy-requiring reactions.
  • ATP is hydrolysed into adenosine diphosphate (ADP) and a phosphate ion, releasing energy.
29
Q

Why is ATP not a good long term energy store?

A
  • Due to the instability of phosphate bonds in ATP. Fats and carbohydrates are a better long term energy store as the energy releases in the breakdown of these molecules (in cellular respiration) is used to create ATP.
  • This occurs by reattaching a phosphate group group to an ADP molecule, called phosphorylation (a condensation reaction).
  • Due to the instability of ATP, cells do not store large amounts of it. However ATP is rapidly reformed by the phosphorylation of ADP. This interconversion of ATP and ADP is happening constantly in all living cells, meaning cells so not need a large amount of ATP in storage, but it is a good immediate energy store.
30
Q

Describe the properties of ATP

A
  • Small (moves easily in and out of cells)
  • Soluble in water (energy-requiring processes happen in aqueous environments)
  • Contains bonds between phosphates with immediate energy (large enough to be useful for cellular reactions but not so large that energy is wasted as heat)
  • Releases energy in small quantities (suitable to cellular needs, not wasted as heat)
  • Easily regenerated (can be recharged with energy)

Biology (10 decks)

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