LIFE 120 Chapter 13

DNA

A nucleic acid molecule, usually a double-stranded helix, in which each polynucleotide strand consists of nucleotide monomers with a deoxyribose sugar and the nitrogenous bases adenine (A), cytosine (C), guanine (G), and thymine (T); capable of being replicated and determining the inherited structure of a cell’s proteins.

RNA

a polymeric molecule that is essential for most biological functions, either by performing the function itself or by forming a template for the production of proteins

DNA replication

The process by which a DNA molecule is copied; also called DNA synthesis.

purine

One of two types of nitrogenous bases found in nucleotides, characterized by a six-membered ring fused to a five-membered ring. Adenine (A) and guanine (G) are examples.

pyrimidine

One of two types of nitrogenous bases found in nucleotides, characterized by a six-membered ring. Cytosine (C), thymine (T), and uracil (U) are examples.

base pair

a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA and RNA.

complementary strand

a single strand of DNA or RNA that can pair with another strand through base pairing

template strand

The DNA strand that provides the pattern, or template, for ordering, by complementary base pairing, the sequence of nucleotides in an RNA transcript.

antiparallel

Referring to the arrangement of the sugar-phosphate backbones in a DNA double helix (they run in opposite 5' to 3' directions).

Chargaff’s Rule

the number of adenine (A) bases is equal to the number of thymine (T) bases, and the number of guanine (G) bases is equal to the number of cytosine (C) bases

double helix

The form of native DNA, referring to its two adjacent antiparallel polynucleotide strands wound around an imaginary axis into a spiral shape.

semiconservative model

Type of DNA replication in which the replicated double helix consists of one old strand, derived from the parental molecule, and one newly made strand.

origin of replication

Site where the replication of a DNA molecule begins, consisting of a specific sequence of nucleotides.

replication fork

A Y-shaped region on a replicating DNA molecule where the parental strands are being unwound and new strands are being synthesized.

DNA helicase

An enzyme that untwists the double helix of DNA at replication forks, separating the two strands and making them available as template strands.

topoisomerase

A protein that breaks, swivels, and rejoins DNA strands. During DNA replication, topoisomerase helps to relieve strain in the double helix ahead of the replication fork.

primase

An enzyme that joins RNA nucleotides to make a primer during DNA replication, using the parental DNA strand as a template.

DNA polymerase

An enzyme that catalyzes the elongation of new DNA (for example, at a replication fork) by the addition of nucleotides to the 3′ end of an existing chain. There are several different DNA polymerases; DNA polymerase III and DNA polymerase I play major roles in DNA replication in E. coli.

leading strand

The new complementary DNA strand synthesized continuously along the template strand toward the replication fork in the mandatory 5' to 3' direction.

lagging strand

A discontinuously synthesized DNA strand that elongates by means of Okazaki fragments, each synthesized in a 5' to 3' direction away from the replication fork

Okazaki fragment

A short segment of DNA synthesized away from the replication fork on a template strand during DNA replication. Many such segments are joined together to make up the lagging strand of newly synthesized DNA.

DNA ligase

A linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 3′ end of one DNA fragment (such as an Okazaki fragment) to the 5′ end of another DNA fragment (such as a growing DNA chain).

mismatch repair

The cellular process that uses specific enzymes to remove and replace incorrectly paired nucleotides.

nucleotide excision repair

A repair system that removes and then correctly replaces a damaged segment of DNA using the undamaged strand as a guide

nuclease

An enzyme that cuts DNA or RNA, either removing one or a few bases or hydrolyzing the DNA or RNA completely into its component nucleotides.

chromatin

The complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in its dispersed form, as a mass of very long, thin fibers that are not visible with a light microscope.

heterochromatin

Eukaryotic chromatin that remains highly compacted during interphase and is generally not transcribed.

euchromatin

the less condensed form of eukaryotic chromatin that is available for transcription.