CIE A Level Biology: Structure of Nucleic Acids & Replication of DNA

Vocabulary practice flashcards covering nucleic acid structures, comparison of DNA and RNA, ATP/ADP/AMP structures, and the mechanism of semi-conservative DNA replication from CIE A Level Biology Topic 6.1.

Nucleotide

The monomer of nucleic acids, consisting of three components: a nitrogen-containing (nitrogenous) base, a pentose sugar containing $5$ carbon atoms, and a phosphate group.

Polynucleotide

A polymer made up of many nucleotide monomers joined into a long chain, such as DNA or RNA.

Adenosine

A nucleoside formed by the nitrogenous base adenine covalently attached to a pentose sugar.

Adenosine triphosphate (ATP)

An energy-carrying phosphorylated nucleotide composed of adenosine bonded to three phosphate groups that provides the energy to drive processes inside living cells.

Adenosine diphosphate (ADP)

A phosphorylated nucleotide formed when adenosine is combined with two phosphate groups.

Adenosine monophosphate (AMP)

A phosphorylated nucleotide formed when adenosine is combined with a single phosphate group.

Purines

Nitrogenous base molecules possessing a double-ring structure, specifically adenine (A) and guanine (G).

Pyrimidines

Nitrogenous base molecules possessing a single-ring structure, specifically cytosine (C), thymine (T), and uracil (U).

Phosphodiester bond

A covalent bond linking the $5$-carbon of one pentose sugar to a phosphate group, which is in turn linked to the $3$-carbon of the next pentose sugar in the polynucleotide chain.

Sugar-phosphate backbone

The structural chain formed along a polynucleotide strand by alternating pentose sugars and phosphate groups linked by phosphodiester bonds.

Antiparallel

The arrangement of the two polynucleotide strands in a DNA molecule where they lie side by side but run in opposite directions ($5'$ to $3'$ and $3'$ to $5'$).

Complementary base pairing

The specific hydrogen bonding between nitrogenous base pairs: adenine forms two hydrogen bonds with thymine ($\text{A} = \text{T}$), and guanine forms three hydrogen bonds with cytosine ($\text{G} \equiv \text{C}$).

Double helix

The three-dimensional, coiled spiral shape formed by two antiparallel polynucleotide strands of a DNA molecule.

Semi-conservative replication

The method of DNA replication in which half of the original DNA molecule is kept (conserved) in each of the two newly formed DNA molecules, each containing one original template strand and one newly synthesised strand.

Meselson and Stahl experiment (1958)

An experiment demonstrating semi-conservative replication by growing E. coli with heavy nitrogen ($^{15}\text{N}$) and normal nitrogen ($^{14}\text{N}$) isotopes to observe DNA density changes across generations.

Activated nucleotides (Nucleoside triphosphates)

Free nucleotides in the nucleus with two extra phosphate groups attached, which are cleaved to release energy for forming phosphodiester bonds during replication.

DNA polymerase

The enzyme that synthesises new DNA strands in the $5'$ to $3'$ direction by catalysing condensation reactions between the deoxyribose sugars and phosphate groups of adjacent nucleotides.

Leading strand

The DNA strand that is synthesised continuously by DNA polymerase moving towards the replication fork as the double helix unwinds.

Lagging strand

The DNA strand that is synthesised discontinuously in short segments by DNA polymerase moving away from the replication fork.

Okazaki fragments

Short segments of newly synthesised DNA formed discontinuously on the lagging strand during DNA replication.

DNA ligase

The enzyme that joins Okazaki fragments together on the lagging strand by catalysing the formation of phosphodiester bonds between sugar and phosphate groups.

Messenger RNA (mRNA)

A single-stranded polynucleotide molecule that serves as a transcript copy of a gene that encodes a specific polypeptide.

Deoxyribose

The pentose sugar found in DNA nucleotides, characterised by having a hydrogen atom ($-\text{H}$) at carbon-$2$ instead of a hydroxyl group.

Ribose

The pentose sugar found in RNA nucleotides, characterised by having a hydroxyl group ($-\text{OH}$) at carbon-$2$.