AP Bio Chapter 16

Molecular Basis of Inheritance

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.

nucleotide excision repair

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

histone

A small protein with a high proportion of positively charged amino acids that binds to the negatively charged DNA and plays a key role in chromatin structure.

leading strand

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

model

A physical or conceptual representation of a natural phenomenon.

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.

transformation

(1) The process by which a cell in culture acquires the ability to divide indefinitely, similar to the division of cancer cells. (2) A change in genotype and phenotype due to the assimilation of external DNA by a cell.

single-strand binding protein

A protein that binds to the unpaired DNA strands during DNA replication, stabilizing them and holding them apart while they serve as templates for the synthesis of complementary strands of DNA.

ribosome

A complex of rRNA and protein molecules that functions as a site of protein synthesis in the cytoplasm; consists of a large and a small subunit. In eukaryotic cells, each subunit is assembled in the nucleolus. See also nucleolus.

telomere

The tandemly repetitive DNA at the end of a eukaryotic chromosome's DNA molecule. Telomeres protect the organism's genes from being eroded during successive rounds of replication. See also repetitive DNA.

mismatch repair

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

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.

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.

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.

nucleosome

The basic, bead-like unit of DNA packing in eukaryotes, consisting of a segment of DNA wound around a protein core composed of two copies of each of four types of histone.

primer

A short polynucleotide with a free 3′ end, bound by complementary base pairing to the template strand and elongated with DNA nucleotides during DNA replication.

virus

An infectious particle incapable of replicating outside of a cell, consisting of an RNA or DNA genome surrounded by a protein coat (capsid) and, for some viruses, a membranous envelope.

euchromatin

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

gene expression

The process by which information encoded in DNA directs the synthesis of proteins or, in some cases, RNAs that are not translated into proteins and instead function as RNAs.

heterochromatin

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

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 replication

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

helicase

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

primase

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

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).

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.