Genetics: Telomeres, Linkage Mapping, and DNA Structure & Replication

Comprehensive vocabulary flashcards covering telomeres, aging, cancer biology, chromosome mapping, linkage analysis, DNA structure, and the enzymatic mechanism of DNA replication.

Replicative senescence

A phenomenon first described by Leonard Hayflick in the 1960s where normal human fibroblasts lose their ability to divide and proliferate after about $50$ cell divisions.

Telomerase

An RNA-containing enzyme that maintains chromosome length in proliferating cells (such as embryonic cells, germ cells, and adult stem cells) by adding telomeric DNA sequences to the ends of linear chromosomes.

ALT (Alternative Lengthening of Telomeres)

An alternative mechanism utilized by human tumor cells lacking active telomerase to maintain telomeres and attain cellular immortality.

GRN163

An anti-telomerase drug developed by Geron Corporation intended to treat cancer by inhibiting telomerase to drive tumor cells into senescence.

Linked genes

Genes that are located on the same chromosome and therefore do not follow Mendel's law of independent assortment.

Interlocus distance

The physical distance separating two genes on a chromosome, where a greater distance results in a higher likelihood of meiotic crossover and recombination.

Locus

The specific, fixed physical position on a chromosome where a particular gene normally resides.

Linkage map

A chromosome diagram devised by Alfred Sturtevant that shows the linear order of gene loci and the distances between them calculated from crossover frequencies.

Centimorgan (cM)

A linkage map unit named in honor of T. H. Morgan, defined where $1\,\text{cM}$ equals $1\%$ recombinant progeny in a testcross.

Two-point testcross

A genetic cross involving an individual heterozygous at two gene loci and a homozygous recessive tester to determine linkage and calculate map distance using the formula R = \left(\frac{\text{# recombinant progeny}}{\text{# total progeny}}\right) \times 100.

DNA Helicases and Gyrases

Enzymes that facilitate the unwinding process and stimulate the separation of double-stranded DNA during replication.

Single-stranded binding proteins

Tetrameric proteins that bind to and stabilize single-stranded DNA structures formed by helicase unwinding.

Primase

An enzyme that binds single-stranded DNA and synthesizes RNA primers to provide a free 3' hydroxyl group required by DNA polymerase to begin synthesis.

DNA Polymerase III

The primary replication enzyme that carries out 5'-to-3' polymerase function, requiring a free 3' hydroxyl end supplied by an RNA primer to initiate DNA synthesis.

DNA Polymerase I

The replication enzyme responsible for removing the RNA primer so that DNA Polymerase III can proceed with synthesis.

DNA Ligase

An enzyme that seals nicks resulting from discontinuous lagging-strand synthesis by forming a covalent phosphodiester linkage between 3'-hydroxyl and 5'-phosphate groups.

Purines

A class of nitrogenous bases found in nucleic acids that includes adenine and guanine.

Pyrimidines

A class of nitrogenous bases found in nucleic acids that includes cytosine, thymine, and uracil (in RNA).

Nucleoside

A molecule composed solely of a nitrogenous base attached to a sugar.

Nucleotide

A nucleic acid building block composed of a nitrogenous base linked to a deoxyribose sugar that is attached to a phosphate group at the 5' position.

Phosphodiester linkage

A chemical bond in the DNA backbone that connects the phosphate at the 5' position of one nucleotide sugar to the 3' position of another, conferring a 5'-to-3' directionality.

Antiparallel

The structural arrangement in double-stranded DNA where the two complementary strands run in opposite orientations, one going 5' to 3' and the other 3' to 5'.

Replicative senescence

The finite life span of normal human cells in culture, first identified by Leonard Hayflick in the 1960s, where fibroblasts stop dividing after approximately 50 divisions while remaining metabolically active.

Telomerase

An RNA-containing enzyme that maintains telomere length by adding telomeric DNA sequences to the ends of linear chromosomes in proliferating cells such as embryonic, germ, and adult stem cells.

ALT (Alternative Lengthening of Telomeres)

A mechanism used by tumor cells that lack active telomerase (less than 20 percent of human cancers) to maintain their telomeres and achieve immortality.

GRN163

An anti-telomerase drug developed by Geron Corporation designed to treat cancer by inhibiting telomerase activity and forcing tumor cells into senescence.

2006 Lasker Award for Basic Medical Research

The award presented to Drs. Elizabeth Blackburn, Carol Greider, and Jack Szostak for their discoveries concerning telomerase and its roles in aging and cancer.

Linked genes

Genes located on the same chromosome that do not assort independently according to Mendel's law of independent assortment.

Interlocus distance

The physical distance between two gene loci on a chromosome; greater interlocus distance results in a higher frequency of meiotic crossover or recombination.

Sample size effect on deviation

The statistical principle that as the sample size increases, chance deviations decrease, reducing the impact of random fluctuations on predicted genetic ratios.

Locus

The fixed physical position or portion of a chromosome where a particular gene normally resides (plural: loci).

Alfred Sturtevant

An undergraduate student in T. H. Morgan's laboratory who, in 1911, recognized that crossover frequency between loci could be used as a numerical measure of physical distance to construct chromosome maps.

Centimorgan ($\text{cM}$)

A linkage map unit named in honor of T. H. Morgan, defined as equal to $1\%$ recombinant progeny in a testcross.

Map distance formula ($R$)

The formula used to calculate genetic linkage distance: $R = \left(\frac{\text{\# recombinant progeny}}{\text{\# total progeny}}\right) \times 100 = \text{cM}$.

Two-point testcross

A cross involving individuals heterozygous for alleles at two loci mated to homozygous recessive testers, used to detect linkage and determine recombination frequency.

Cis configuration

An allelic arrangement in a double heterozygote where both mutant alleles are situated on the same homologous chromosome (e.g., $y\,w/+\,+$).

DNA Helicases and Gyrases

Enzymes that assist in the unwinding process and stimulate the separation of the parental double-stranded DNA during replication.

DNA single-stranded binding proteins

Tetrameric proteins that bind to exposed single strands of DNA to stabilize them and prevent premature re-annealing during replication.

Primase

An enzyme that synthesizes short RNA primers complementary to the single-stranded DNA template, providing the free $3'$ hydroxyl group required for DNA polymerase activity.

DNA Polymerase III

The primary bacterial replication enzyme that carries out $5'\text{-to-}3'$ DNA synthesis, requiring a free $3'$ end from an RNA primer to initiate polymerization.

DNA Polymerase I

The enzyme responsible for removing the RNA primer so that DNA Polymerase III can proceed with continuous or discontinuous DNA chain elongation.

DNA Ligase

An enzyme that seals nicks along the discontinuous lagging strand by catalyzing a covalent phosphodiester linkage between adjacent $3'$-hydroxyl and $5'$-phosphate groups.

Purines

A class of double-ring nitrogenous bases found in nucleic acids consisting of adenine and guanine.

Pyrimidines

A class of single-ring nitrogenous bases consisting of cytosine, thymine (in DNA), and uracil (in RNA).

Nucleoside

A molecule consisting solely of a nitrogenous base covalently linked to a pentose sugar (such as deoxyribose or ribose).

Nucleotide

A monomeric unit of nucleic acids consisting of a nitrogenous base, a deoxyribose sugar, and a phosphate group positioned at the $5'$ carbon of the sugar.

Phosphodiester linkage

The covalent chemical bond in a nucleic acid strand that links the $5'$ phosphate of one nucleotide to the $3'$ hydroxyl group of the adjacent nucleotide, imparting $5'\text{-to-}3'$ directionality.

Antiparallel

The structural arrangement of the double helix where one DNA strand runs in the $5'\text{-to-}3'$ direction and the complementary strand runs in the opposite $3'\text{-to-}5'$ direction.