DNA Replication and Chromosome Structure Flashcards

Vocabulary flashcards covering chromatin structure, cell types, karyotypes, DNA replication mechanisms and enzymes, repair systems, and point and frameshift mutations.

Chromatin

The uncondensed form of DNA combined with structural proteins, which condenses during cell division.

Nucleosome

A structural unit of chromatin consisting of $8$ histone proteins wrapped twice by a double-stranded DNA helix.

Histones

Positively charged structural proteins around which DNA wraps to form nucleosomes.

Euchromatin

A form of chromatin that is genetically active and involved in gene expression.

Constitutive Heterochromatin

Permanently condensed and genetically inactive regions of chromatin, such as telomeres and centromeres.

Somatic Cells

Diploid ($2n$) body cells containing $46$ chromosomes in humans.

Gametes

Haploid ($n$) sex cells (egg and sperm) containing $23$ chromosomes in humans.

Homologous Chromosomes

Chromosomes that share the same shape, size, and gene types, but are not sister chromatids.

Karyotype

A photograph of all metaphase chromosomes from a single cell of an individual, used to identify numerical and structural chromosome alterations.

S Phase

The stage of interphase prior to mitosis or meiosis during which DNA replication takes place within the eukaryotic nucleus.

Meselson-Stahl Experiment

An experiment using ${}^{15}\text{N}$ and ${}^{14}\text{N}$ isotopes in E. coli that demonstrated DNA replication is semiconservative.

Antiparallel Strands

The structural arrangement of DNA where one strand runs $5' \rightarrow 3'$ (with a free phosphate at the $5'$ end) and the complementary strand runs $3' \rightarrow 5'$ (with a free hydroxyl at the $3'$ end).

Helicase

An enzyme that unwinds the DNA double helix during replication by breaking hydrogen bonds between nucleotide base pairs.

Single-Stranded Binding Protein (SSB)

A protein that binds to single-stranded DNA formed during unwinding to stabilize it and prevent re-annealing.

Topoisomerase

An enzyme that relieves supercoiling ahead of the replication fork by breaking and resealing phosphodiester bonds.

Origin of Replication

Short stretches of DNA with specific AT-rich sequences where replication begins; eukaryotes have multiple origins, whereas prokaryotes have only one.

Primase

An enzyme that synthesizes a short complementary RNA primer required to initiate DNA synthesis.

DNA Polymerase III

The main enzyme that synthesizes new DNA strands in the $5' \rightarrow 3'$ direction by adding deoxynucleotide triphosphates to a free $3'\text{-OH}$ group.

Leading Strand

The DNA strand synthesized continuously in the $5' \rightarrow 3'$ direction toward the replication fork, requiring a single RNA primer.

Lagging Strand

The DNA strand synthesized discontinuously in the $5' \rightarrow 3'$ direction away from the replication fork, requiring multiple RNA primers.

Okazaki Fragments

Short, discontinuous segments of DNA produced on the lagging strand during DNA replication.

DNA Polymerase I

An enzyme with $5' \rightarrow 3'$ exonuclease activity that removes RNA primers and replaces them with DNA nucleotides.

DNA Ligase

An enzyme that joins DNA fragments by creating phosphodiester bonds to seal nicks in the sugar-phosphate backbone.

Telomerase

An enzyme with a bound RNA template that extends the $3'$ end of parental DNA to complete lagging strand replication at chromosome ends.

Nucleotide Excision Repair (NER)

A single-stranded DNA repair process that fixes damage from exogenous sources like UV radiation using nuclease, DNA polymerase, and DNA ligase.

Silent Mutation

A point mutation where a nucleotide substitution changes a codon, but due to the degenerate property of the genetic code, it still codes for the same amino acid.

Missense Mutation

A point mutation where a nucleotide substitution changes a codon to encode a different amino acid, such as Glutamate changing to Valine in sickle cell anemia.

Nonsense Mutation

A point mutation in which a nucleotide substitution converts an amino acid codon into a stop codon, causing premature termination of protein synthesis.

Frameshift Mutation

A mutation caused by inserting or deleting a number of nucleotides not divisible by $3$, shifting the reading frame and altering all subsequent amino acids.