Nucleotides & Nucleic Acids (OCR A Level Biology): Flashcards

By condensation reactions between the phosphate group of one nucleotide and the pentose sugar of the next, forming phosphodiester bonds (releasing a molecule of water each time).

The chain of alternating pentose sugars and phosphate groups, linked by phosphodiester bonds, that runs along a nucleic acid strand.

Because it consists of a phosphate group joined by two ester bonds (one to the sugar of each adjacent nucleotide).

By hydrolysis reactions that break the phosphodiester bonds between nucleotides (a molecule of water is added).

True

False — adenine and guanine are purines (double-ring); cytosine, thymine and uracil are the pyrimidines.

A nucleotide that has one or more phosphate groups attached, e.g. ADP and ATP.

A phosphorylated nucleotide that acts as the universal energy currency of cells, made of a ribose sugar, the base adenine and three phosphate groups.

A phosphorylated nucleotide made of a ribose sugar, the base adenine and two phosphate groups.

• A pentose sugar (ribose)

• A nitrogenous base (adenine)

• Three phosphate groups

ADP has two phosphate groups, whereas ATP has three. (ATP = ADP plus one more inorganic phosphate group.)

In all known forms of life, ATP from respiration is used to transfer energy in all energy-requiring processes in cells.

• Anabolic reactions (building larger molecules from smaller ones)

• Active movement of substances across membranes or within the cell

• In animals: muscle contraction and the conduction of nerve impulses

Adenine is a nitrogenous base.

Adenosine is a nucleoside – the base adenine attached to a pentose sugar (ribose).

Adenosine bonded to two phosphate groups is adenosine diphosphate (ADP); adenosine bonded to three phosphate groups is adenosine triphosphate.

The pentose sugar is ribose and the nitrogenous base is adenine.

True

False — AMP contains one phosphate group; it is ATP that has three.

A polymer made of many nucleotides joined together in a long chain by phosphodiester bonds.

Two antiparallel polynucleotide strands, each with a sugar-phosphate backbone, held together by hydrogen bonds between complementary base pairs, and twisted into a double helix.

Describes the two strands of a DNA molecule running in opposite directions – one runs 5' to 3' and the other runs 3' to 5'.

Hydrogen bonds between the complementary base pairs.

• Adenine (A) pairs with thymine (T) – 2 hydrogen bonds

• Guanine (G) pairs with cytosine (C) – 3 hydrogen bonds

The pairing of specific nitrogenous bases via hydrogen bonds: adenine with thymine, and guanine with cytosine.

In DNA, the purine adenine always pairs with the pyrimidine thymine by two hydrogen bonds.

Phosphodiester bonds.

The three-dimensional shape of a DNA molecule formed by the twisting of its two antiparallel polynucleotide strands.

They project inwards from the two sugar-phosphate backbones towards the interior of the molecule, where they pair up.

Pairing a double-ring purine with a single-ring pyrimidine keeps the width of the DNA molecule constant along its length.

Because A = T, adenine + thymine = 60%. The remaining 40% is guanine + cytosine, and G = C, so cytosine = 20%.

The numbers refer to which carbon of the pentose sugar is available to bond to the next nucleotide. The two antiparallel strands run in opposite directions: one 5' to 3' and the other 3' to 5'.

True

False — they run in opposite directions (they are antiparallel).

1. Break open (lyse) the cells and disrupt the nuclear membranes to release the DNA

1. Use enzymes to denature and remove the proteins (histones) associated with the DNA

1. Precipitate the DNA using an ice-cold organic solvent (e.g. ethanol)

Their cells contain a relatively large amount of DNA.

It disrupts the phospholipid bilayers of the cell-surface and nuclear membranes, releasing the DNA from the cells.

Heat helps disrupt the membranes and denatures enzymes that would otherwise digest the DNA.

Cooling prevents the DNA itself from breaking down, which would occur if the high temperature were maintained.

It denatures and removes the proteins (histones) associated with the DNA, leaving just the DNA.

Nucleic acids are insoluble in ice-cold ethanol, so the DNA comes out of solution and precipitates rather than staying dissolved.

The DNA precipitates as a white layer/strands at the top of the mixture.

Brief blending releases more DNA by breaking down cell walls and membranes, but blending too long would break apart the DNA strands themselves.

To remove cell debris and membrane fragments, leaving a filtrate containing the DNA and its associated proteins.

The process by which DNA comes out of solution as a solid white precipitate when ice-cold ethanol is added, because nucleic acids are insoluble in cold ethanol.

True

False — coffee filter paper is used, as liquid passes through laboratory filter paper too slowly.

A common method used to isolate DNA is known as the Marmur preparation.

The process of copying DNA in which each new DNA molecule consists of one original (conserved) strand and one newly synthesised strand.

So that both new daughter cells receive a complete set of genetic information.

1. DNA helicase unwinds the double helix and breaks the hydrogen bonds between base pairs, separating the two strands

1. Each exposed strand acts as a template; free activated nucleotides pair with their complementary bases

1. DNA polymerase joins adjacent nucleotides, forming phosphodiester bonds in the new sugar-phosphate backbone

1. Hydrogen bonds form between the complementary bases

Each new DNA molecule has one conserved (original) strand and one new strand.

It unwinds the DNA double helix by breaking the hydrogen bonds between the complementary base pairs on the two antiparallel strands.

It joins adjacent nucleotides on each template strand by catalysing condensation reactions, forming the phosphodiester bonds of the new sugar-phosphate backbone.

It catalyses the formation of phosphodiester bonds between adjacent nucleotides.

It does not form the hydrogen bonds between complementary bases – these form without an enzyme.

The enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between complementary base pairs.

Each new strand is built from an existing template, which maximises the accuracy of copying and ensures genetic continuity between generations of cells.

A change to the base sequence of DNA.

Through copying errors – e.g. bases inserted in the wrong order, an extra base added, or a base left out.

Free DNA nucleotides that carry extra phosphate groups; these are used to build the new complementary strands.

During the S (synthesis) phase of interphase.

They happen at random and are spontaneous, meaning they have no particular cause.

True

False — DNA helicase breaks the hydrogen bonds; DNA polymerase joins adjacent nucleotides to form phosphodiester bonds.

A sequence of nucleotides, forming part of a DNA molecule, that codes for the production of a specific polypeptide (protein).

The genetic code in which each sequence of three bases (a triplet) codes for one amino acid.

The sequence of bases in the gene determines the exact sequence in which amino acids are joined together when the polypeptide is synthesised, and this sequence of amino acids is the primary structure.

Describes the way the code is read so that each base is read only once, as part of a single triplet/codon.

Describes the code in which most amino acids can be coded for by more than one triplet/codon.

There are 64 possible triplets but only about 20 amino acids, so most amino acids are coded for by more than one triplet.

It can limit the effect of mutations.

Describes the fact that almost every organism uses the same triplets to code for the same amino acids.

Because the same triplets code for the same amino acids in all organisms, a gene transferred from one species to another is translated to produce the same protein.

4³ = 64 possible triplets.

A single base (4 options) or a pair of bases (4² = 16 options) could not code for all ~20 amino acids, so at least three bases per code are needed.

A triplet of bases on an mRNA molecule that codes for a specific amino acid (or acts as a start/stop signal).

A triplet of bases on a tRNA molecule that is complementary to a codon on an mRNA molecule.

They tell the cell where individual genes start and stop, so the DNA is read correctly and the correct sequence of amino acids is produced.

Because there are 64 possible triplets but only about 20 amino acids, most amino acids are coded for by more than one triplet – the code is described as degenerate.

False — the code is non-overlapping, so each base is read only once.

True

The first stage of protein synthesis, occurring in the nucleus, in which a gene in DNA is used as a template to produce a complementary mRNA molecule.

Transcription – in the nucleus: DNA is transcribed to produce mRNA.

Translation – in the cytoplasm (at a ribosome): mRNA is translated into a sequence of amino acids.

1. Part of the DNA unwinds and the hydrogen bonds between base pairs break, exposing the gene

1. Free RNA nucleotides pair (by hydrogen bonds) with their complementary bases on the template strand

1. RNA polymerase joins the RNA nucleotides by phosphodiester bonds to form the mRNA sugar-phosphate backbone

1. When complete, the mRNA detaches, the DNA re-forms its double helix, and the mRNA leaves the nucleus through a nuclear pore

It joins the RNA nucleotides together – bonding their sugar-phosphate groups with phosphodiester bonds – to build the sugar-phosphate backbone of the mRNA molecule.

A single-stranded RNA molecule, produced during transcription, that carries a complementary copy of a gene's code from the nucleus to a ribosome.

The strand of a DNA molecule that is used as the template for building the mRNA molecule during transcription (also called the transcribed strand).

The DNA strand that is not transcribed; its base sequence is the same as the mRNA (but with thymine in place of uracil). Also called the non-template strand.

mRNA is complementary to the template strand, so it has the same base sequence as the coding strand – except that uracil replaces thymine.

Uracil (U).

The hydrogen bonds between the mRNA and the DNA template break, and the double-stranded DNA re-forms its double helix.

During transcription, free RNA nucleotides pair up with the exposed bases on the template strand of the unwound DNA molecule.

RNA polymerase moves along the template strand in the 3' to 5' direction.

The mRNA molecule grows in the 5' to 3' direction.

True

False — RNA polymerase builds the mRNA; DNA polymerase is used in DNA replication.

The second stage of protein synthesis, occurring in the cytoplasm at a ribosome, in which the base sequence of mRNA is decoded to assemble amino acids into a polypeptide.

1. mRNA leaves the nucleus and attaches to a ribosome\n\n1. tRNA molecules, each with a specific anticodon and carrying a specific amino acid, bring amino acids to the ribosome\n\n1. Each tRNA anticodon pairs with the complementary codon on the mRNA; two tRNAs fit on the ribosome at a time, bringing their amino acids side by side\n\n1. A peptide bond forms between the two amino acids (catalysed by the rRNA of the ribosome)\n\n1. This continues until a stop codon is reached, and the completed amino acid chain forms the polypeptide

A single-stranded RNA molecule with an anticodon at one end and an amino acid binding site at the other; it carries a specific amino acid to the ribosome during translation.

A stable RNA molecule that makes up ribosomes; it allows mRNA to attach and be correctly aligned, and it catalyses the formation of peptide bonds during translation.

It carries a specific amino acid to the ribosome; its anticodon pairs with the complementary codon on the mRNA, ensuring amino acids are joined in the order coded for.

It holds the mRNA and two tRNA molecules in place so their amino acids are brought side by side, and the rRNA catalyses the formation of the peptide bond between them.

By a condensation reaction between two amino acids held side by side on the ribosome.

Each tRNA has a specific anticodon and carries a specific amino acid. The anticodon pairs only with its complementary codon on the mRNA, so the amino acid is placed in the correct position.

When a stop codon on the mRNA is reached, translation stops and the completed polypeptide is released.

Two, bringing their amino acids side by side so that a peptide bond can form between them.

During translation, the anticodon of a tRNA molecule pairs with a complementary codon on the mRNA molecule.

In the cytoplasm, at a ribosome.

The rRNA of the ribosome.

Translation occurs in the cytoplasm, whereas transcription occurs in the nucleus.

False — translation takes place in the cytoplasm, at a ribosome.

True