Cell Biology: Nucleus, Chromosomes, and Cell Division

A complete set of vocabulary flashcards covering the components of the nucleus, DNA/RNA structure, chromosome morphology, and the mechanistic stages of the cell cycle, mitosis, and meiosis based on the lecture transcript.

Karyology

The scientific study of the nucleus.

Strasburger

The scientist who stated that the "Nucleus arises from division of pre-existing nucleus only" in 1875.

Perinuclear space

The space between the two parallel membranes of the nuclear envelope, measuring $10$ to $50\,\text{nm}$.

Nuclear Pore complex (Bleb)

A structure formed by the combination of a nuclear pore and the annulus; the annulus is an octagonal discoid structure made of nucleoplasmin protein.

Nuclear lamina

A structure lining the inner side of the inner nuclear membrane formed by filaments of lamin protein.

Nucleoplasm (Karyolymph)

The ground substance of the nucleus containing chemicals like nucleotides, ATPs, and enzymes such as RNA and DNA polymerases.

Chromatin

A term given by Flemming for the network of nucleoprotein fibres in the interphase nucleus formed of DNA and histone protein complexes.

Chemical Composition of Chromatin

Consists of DNA ($31\%$), RNA ($2-5\%$), histone protein ($36\%$), and non-histone proteins ($28\%$).

Histone Proteins

Basic proteins rich in arginine and lysine, classified into five types: $H1$, $H2A$, $H2B$, $H3$, and $H4$.

Nucleosome

The repeating structural unit of chromatin consisting of a core of histone proteins around which DNA is coiled.

Linker DNA

The segment of DNA that connects two adjacent nucleosomes.

Euchromatin

The lightly stained, diffused part of chromatin that is transcriptionally or genetically more active.

Heterochromatin

The darkly stained, thick, and condensed part of chromatin containing more histone and less acidic protein, making it genetically less active.

Nucleolus

A membraneless spherical structure serving as the "ribosome factory" and the site for active ribosomal RNA synthesis.

Nucleolar Organiser Region (NOR)

The specific site on chromatin or chromosomes where the nucleolus is usually attached.

Chromosome

A term proposed by Waldeyer for the highly condensed form of chromatin material visible during cell division.

Genome

The complete set of genetic material in an organism; a single human cell has approximately $2.2\,\text{meter-long}$ thread of DNA.

Chromatid

One of the two cylindrical structures of a chromosome at the metaphase stage, joined together by a common centromere.

Centromere (Primary constriction)

The narrow region that connects two chromatids of a chromosome.

Kinetochores

Two disc-shaped proteins associated with the centromere that serve as the site for spindle attachment during cell division.

Satellite chromosome

A small fragment of the chromosome that appears beyond the secondary constriction (NOR) near the end.

Telomere

The DNA segment at the ends of eukaryotic chromosomes that protects them from binding to one another or themselves.

Metacentric chromosome

A chromosome with the centromere near the middle, resulting in two almost equal arms and looking like the English letter 'V'.

Submetacentric chromosome

A chromosome with the centromere slightly away from the middle, resulting in one shorter arm and looking like the English letter 'L'.

Acrocentric chromosome

A chromosome with the centromere near the end, resulting in one very short arm and looking like the English letter 'J'.

Telocentric chromosome

A chromosome with the centromere at the very tip and the arm on one side only, looking like the English letter 'I'.

Arm ratio

The ratio of the length of the long arm to the short arm of a chromosome, which reaches its maximum in acrocentric chromosomes.

Watson & Crick

The scientists who proposed the double helix model of DNA.

DNA Nucleotide Composition

Consists of a nitrogenous base, a phosphate group, and a pentose sugar (deoxyribose).

Chargaff's Rule

The principle of base equivalence stating that the total amount of purine is equal to the total amount of pyrimidine ($A + G = C + T$).

DNA Hydrogen Bonds

Adenine pairs with Thymine with two hydrogen bonds ($A = T$), while Cytosine pairs with Guanine with three hydrogen bonds ($C = G$).

DNA Double Helix Dimensions

The helix has a width of $20\,\text{Å}$, takes a complete turn every $34\,\text{Å}$, and contains $10\,\text{base pairs}$ per turn.

Messenger RNA (mRNA)

RNA produced by genetic DNA in the nucleus that carries coded messages for protein synthesis in the form of triplet codons.

Central Dogma

The concept illustrating the flow of genetic information from DNA to RNA to Protein.

Transcription

The process of copying genetic information from one strand of DNA into mRNA, involving the enzyme RNA polymerase.

Translation

The process by which the coded message on mRNA is converted into a sequence of amino acids with the help of tRNA and rRNA.

Cell Cycle

The sequence of events including genome duplication, constituent synthesis, and eventual division into two daughter cells; lasts approximately $24\,\text{hours}$ in human cells.

Interphase

The most active phase and interval between successive M-phases, lasting more than $95\%$ of the duration of the cell cycle.

$G_1$ - phase (1st Gap phase)

The longest phase of the cell cycle during which organelles increase, and the cell is metabolically active and grows maximum in size.

S-phase (Synthesis phase)

The phase marked by replication of nuclear DNA and synthesis of histone proteins; DNA amount doubles from $2C$ to $4C$.

$G_2$-phase (2nd Gap phase)

The pre-mitotic phase where final preparations occur, including the synthesis of tubulin protein for spindle fibres.

Quiescent stage ($G_0$)

An inactive stage where cells remain metabolically active but no longer proliferate unless required, such as heart cells.

Mitosis (Equational division)

A term proposed by Flemming for division where the chromosome number in daughter cells remains the same as in the parent cell.

Kern plasm theory

A theory by Hertwig stating mitosis occurs due to disturbance in the Karyoplasmic Index ($KI$), calculated as $KI = \frac{V_n}{V_c - V_n}$.

Prophase

The first and longest stage of mitosis where chromatin condenses into chromosomes and the nuclear envelope and nucleolus disappear.

Metaphase

The stage where chromosomes align at the equatorial plane (metaphase plate) and morphology is most easily studied.

Anaphase

The shortest stage of mitosis where centromeres split and sister chromatids separate to move toward opposite poles using approximately $30\,\text{ATP}$.

Telophase

The final stage of karyokinesis where chromosomes decondense and the nuclear membrane and nucleolus reform.

Anastral mitosis

Mitosis without asters, occurring in higher plants where centrioles are absent.

Cytokinesis in animals

Division of cytoplasm occurring by centripetal constriction and furrow formation involving a contractile ring of actin and myosin.

Cytokinesis in plants

Division of cytoplasm occurring in a centrifugal order by the formation of a cell plate made of calcium and magnesium pectate.

Meiosis (Reduction division)

A term proposed by Farmer and Moore for division that reduces the chromosome number by half to produce haploid gametes.

Leptotene

The first substage of Meiosis Prophase-I where chromosomes become visible and group like a bouquet.

Zygotene

The stage characterized by synapsis (pairing) of homologous chromosomes into bivalents and the formation of the synaptonemal complex.

Pachytene

The stage where crossing over occurs between non-sister chromatids of homologous pairs, mediated by the enzyme recombinase.

Crossing over

The exchange of genetic parts between non-sister chromatids, discovered by Morgan and Castle in Drosophila.

Diplotene

The stage recognized by the dissolution of the synaptonemal complex and the appearance of X-shaped chiasmata.

Diakinesis

The final stage of Prophase-I marked by terminalization of chiasmata and the disappearance of the nuclear membrane.

Anaphase-I

The stage in meiosis where homologous chromosomes segregate to opposite poles, but sister chromatids remain associated at their centromeres.

Interkinesis

The short-lived gap between Meiosis I and Meiosis II during which DNA replication is absent.