Chapter 3: The Chromosomal Basis of Heredity

Vocabulary flashcards covering chromosome structures, mitosis, meiosis, chromatin organization, centromeres, telomeres, sex determination, and statistical genetics.

Chromosomes complement

The complete set of chromosomes in a particular cell.

Somatic cells vs Germ cells

Somatic cells contain 46 paired chromosomes, whereas germ cells contain 23 chromosomes.

Diploid

Refers to a cell having two sets of typical chromosomes.

Haploid

Refers to a cell having half of the typical number of chromosomes.

Karyotype

An organized picture of all the chromosomes in a cell.

Homologous chromosomes

Two copies of each chromosome in a cell; they are not identical because alleles can vary on each individual chromosome.

Non-homologous chromosomes

Chromosomes that belong to different pairs, such as chromosome 1 compared to chromosome 2.

Interphase

The phase of the cell cycle comprising G1 (growth phase 1), S (synthesis), and G2 (growth phase 2).

Chromosome

DNA packaged around specific proteins called histones.

G1 phase (gap 1)

The pre-DNA synthesis phase of the cell cycle; takes about the same amount of time as S phase, together comprising about half of the cell cycle time.

S phase

The synthesis phase of the cell cycle where DNA replication occurs, resulting in 4 copies of DNA while remaining diploid.

G2 phase (gap 2)

The post-DNA synthesis phase setting up for mitosis, lasting about half the time of G1 phase.

Chromatids

The products of DNA synthesis.

Prophase (Mitosis)

Stage of mitosis where chromosomes condense to become visible, chromatids are held together by a centromere, and the nuclear membrane dissolves.

Sister chromatids

Identical copies of a chromosome held together by a centromere.

Non-sister chromatids

Chromatids from homologous chromosomes that are homologous to each other.

Metaphase (Mitosis)

Stage of mitosis where the mitotic spindle forms and chromosomes are moved to the metaphase plate (equator).

Anaphase (Mitosis)

Stage of mitosis where centromeres are degraded and sister chromatids (now referred to as chromosomes) are dragged toward opposite poles by the spindle apparatus.

Centromere

A region on chromosomes where spindle fibers attach and point of assembly for the kinetochore.

Telophase (Mitosis)

Stage of mitosis where the nuclear envelope reforms around each group of chromosomes and chromosomes relax.

Cytokinesis

The division of the cytoplasm.

Meiosis 1

Reductional division that separates homologous chromosomes and cuts the number of chromosomes in half to make haploid cells.

Leptotene

The first subtype of Prophase 1 where chromosomes begin to condense and become visible as chromomeres.

Zygotene

Subtype of Prophase 1 where the synaptonemal complex holds chromosomes together at a linear alignment.

Synaptonemal complex

A protein structure that holds chromosomes together in a linear alignment during zygotene.

Pachytene

Subtype of Prophase 1 where crossing over (physical exchange of pieces of DNA) occurs to generate genetic diversity.

Crossing over

The physical exchange of pieces of DNA during pachytene that generates genetic diversity.

Diplotene

Subtype of Prophase 1 where the synaptonemal complex breaks down and homologs remain held together by chiasmata.

Diakinesis

Subtype of Prophase 1 where chromosomes reach maximum condensation and spindle fibers begin to assemble.

Metaphase 1

Stage of Meiosis 1 where homologous chromosomes are independently positioned at the equator by the spindle apparatus.

Anaphase 1

Stage of Meiosis 1 where homologous chromosomes are separated while sister chromatids remain together.

Telophase 1

Stage of Meiosis 1 where $n$ number of chromosomes are located near each pole and the spindle breaks down; not a true cytokinesis event.

Meiosis 2

Equational division that happens faster than Meiosis 1 and separates sister chromatids.

Telophase 2

Stage of Meiosis 2 where nuclear membranes reform for cytoplasmic division, culminating in a true cytokinesis event that yields 4 daughter cells.

Nucleosome

The basic structural unit of chromatin, consisting of DNA wrapped twice (approximately 145 base pairs) around an octamer histone core with 55 base pairs of linker DNA between nucleosomes.

Chromatin

DNA wrapped around proteins.

Histones

Proteins (H1, H2A, H2B, H3, H4) with a high percentage of lysine and arginine, which are highly positively charged.

H1 histone

Histone protein that stabilizes chromatin fiber by associating with the linker DNA.

Condensins

Proteins that pack chromatin loops tightly into chromatid structure.

Heterochromatin

Tightly compacted chromatid that is less transcriptionally active, resulting in gene silencing.

Constitutive heterochromatin

Chromatin that is ALWAYS in a heterochromatic state.

Facultative heterochromatin

Chromatin that is sometimes in a heterochromatic state and sometimes not.

Satellite DNA

DNA found in regions of the chromosome that consist of constitutive chromatin.

Euchromatin

Loosely packaged chromatin that is more transcriptionally active.

CENPA

A unique histone variant used at the centromere region instead of H3 that interacts with kinetochores.

Alpha satellite DNA

A 175 base pair DNA sequence found only at the centromere on all chromosomes that specifically associates with CENPA.

Telomeres

Non-protein-coding structures at chromosome ends essential for stability that shorten with each DNA replication, acting as a "mitotic clock" for about 50 cell divisions.

Telomerase

Enzyme in germ cells and cancer cells that extends telomeres and rebuilds them after replication damage, allowing unlimited replication ability.

Heterogametic sex

The sex that possesses different sex chromosomes and produces different gametes, such as human males with X and Y chromosomes.

Wild type

The most commonly occurring genotype in the wild.

Nondisjunction

An error in meiosis 1 or meiosis 2 where chromosomes fail to separate properly, producing gametes with inaccurate chromosome numbers (e.g., trisomy 21).

Binomial probability formula

The formula $(p+q)^n$ where $p+q = 1$, used to predict progeny outcomes of genetic crosses.

Chi-square test

A goodness of fit test represented by the formula $X^2 = \frac{(\text{observed} - \text{expected})^2}{\text{expected}}$.

Degrees of freedom (DF)

A statistical parameter calculated as $n - 1$, representing one number less than the total number of categories being compared.