Unit 2: Chromosomal architecture, cell division and genetic principles - Subjective Questions

A typical metaphase chromosome is a highly condensed nucleoprotein structure composed of DNA and associated proteins.

• Sister chromatids: Each metaphase chromosome consists of two genetically identical sister chromatids formed during the S phase. They remain attached at the centromere.
• Chromonemata: Each chromatid contains a coiled chromonemal thread made of chromatin. The chromonema undergoes repeated coiling and condensation.
• Chromomeres: These are bead-like, densely staining regions observed along the chromonema, particularly in less-condensed chromosomes.
• Chromosome matrix: A proteinaceous framework historically described as surrounding or supporting the chromonemata. Modern studies recognize a chromosome scaffold composed mainly of non-histone proteins.
• Centromere: The primary constriction that joins sister chromatids. It contains the kinetochore, where spindle microtubules attach.
• Chromosome arms: The centromere divides the chromosome into a short arm, designated p, and a long arm, designated q.
• Secondary constriction: A constricted region other than the centromere. It may contain a nucleolar organizer region.
• Satellite: The chromosome segment separated from the main body by a secondary constriction.
• Telomeres: Specialized DNA-protein structures present at chromosome ends that maintain chromosome stability.

Thus, chromosome architecture represents a hierarchical organization of DNA, histones, non-histone proteins and specialized functional regions.

Chromonemata are fine, coiled, thread-like structures present within chromatids. The singular form is chromonema.

• A chromonema consists of chromatin containing DNA, histone proteins and non-histone proteins.
• During interphase, chromonemata are relatively extended and permit DNA replication and gene expression.
• During cell division, they undergo progressive coiling and condensation to produce visible chromosomes.
• The coiling may occur at several levels, beginning with nucleosomes and proceeding through higher-order chromatin loops and chromosome scaffolding.
• Chromomeres may appear as bead-like thickenings along the chromonemata.

Significance:

• Chromonemata contain genes arranged in a linear order.
• Their condensation prevents DNA entanglement and breakage during chromosome movement.
• Their duplication produces the two sister chromatids of a replicated chromosome.
• Their precise separation ensures equal transmission of genetic information to daughter cells.

The three terms describe different aspects of chromosome organization: the matrix provides structural support, chromonemata represent the genetic threads, and chromomeres are condensed regions visible along those threads.

The centromere is the primary constriction of a chromosome and is essential for chromosome segregation.

Structure and functions:

• It is composed of specialized centromeric chromatin, often containing repetitive DNA and centromere-specific proteins.
• A protein complex called the kinetochore forms on the centromere.
• Spindle microtubules attach to the kinetochore during mitosis and meiosis.
• The centromere holds sister chromatids together until their separation.
• It ensures proper chromosome orientation, movement and equal distribution.

Classification according to centromere position:

1. Metacentric: Centromere is approximately central; the two arms are nearly equal. It appears V-shaped during anaphase.
2. Submetacentric: Centromere is slightly away from the middle; one arm is shorter than the other. It appears L-shaped.
3. Acrocentric: Centromere lies near one end; the p arm is very short and the q arm is long. It appears J-shaped.
4. Telocentric: Centromere is terminal or nearly terminal, producing essentially one visible arm. It appears I-shaped.

Loss or malfunction of the centromere generally prevents normal chromosome segregation.

A secondary constriction is a narrow region of a chromosome distinct from the primary constriction or centromere. It occurs at a characteristic position and can act as a useful cytological marker.

Differences:

• The primary constriction contains the centromere and kinetochore, whereas a secondary constriction does not normally attach to spindle fibers.
• Every normal monocentric chromosome has one primary constriction, but only certain chromosomes possess secondary constrictions.
• The primary constriction is essential for chromosome segregation; a secondary constriction often has a role in nucleolus formation.

A secondary constriction may contain a nucleolar organizer region, abbreviated as NOR. The NOR carries repeated genes coding for major ribosomal RNAs. During interphase, these genes participate in the formation of the nucleolus and synthesis of ribosomal RNA.

When a small distal chromosome segment is separated from the main chromosome body by a secondary constriction, that segment is called a satellite, and the chromosome is called a satellite chromosome.

Telomeres are specialized DNA-protein structures located at the ends of linear chromosomes.

Structure:

• They contain short, tandemly repeated, usually G-rich DNA sequences.
• In vertebrates, the common repeat is .
• Telomeric DNA is associated with protective proteins, including the shelterin complex.
• The terminal DNA may form a looped structure that hides the chromosome end from DNA repair systems.

Functions:

• Prevent chromosome ends from being recognized as broken DNA.
• Protect chromosomes against degradation and end-to-end fusion.
• Prevent loss of essential genes during repeated cycles of DNA replication.
• Help maintain nuclear and chromosomal organization.

Conventional DNA polymerases cannot completely replicate the ends of linear DNA, creating the end-replication problem. Telomerase, an RNA-containing reverse transcriptase, extends telomeric DNA in germ cells, stem cells and many cancer cells. Progressive telomere shortening in many somatic cells is associated with cellular senescence.

Polytene chromosomes are exceptionally large chromosomes produced by repeated rounds of DNA replication without chromosome separation or cell division, a process called endoreduplication.

• They are commonly found in the salivary glands of dipteran larvae, such as Drosophila.
• Numerous replicated chromatids remain aligned side by side, producing a thick chromosome.
• Homologous chromosomes are usually paired somatically.
• Polytene chromosomes display a characteristic pattern of dark bands and light interbands.
• All chromosomes may meet in a densely staining region called the chromocenter.
• Localized swellings called puffs or Balbiani rings indicate regions of active transcription.

Importance:

• Their large size makes individual chromosome regions easy to observe.
• Banding patterns permit preparation of detailed cytological maps.
• Chromosomal deletions, inversions and translocations can be detected visually.
• Puffing patterns reveal changes in gene activity during development or in response to hormones.
• They helped establish correlations between cytological regions and genetic loci.

Lampbrush chromosomes are very large meiotic chromosomes characterized by numerous lateral loops extending from a central chromosome axis.

• They occur mainly in growing oocytes of amphibians, birds and some other vertebrates.
• They are most prominent during the diplotene stage of meiotic prophase I.
• Each homolog consists of two sister chromatids and remains associated with its partner at chiasmata.
• Condensed chromatin regions called chromomeres occur along the axis.
• Pairs of lateral loops extend from the chromomeres, giving the chromosome the appearance of a lamp-cleaning brush.
• The lateral loops are decondensed, transcriptionally active regions covered with nascent RNA and associated proteins.

Functions and importance:

• They support intense RNA synthesis required for oocyte growth and early embryonic development.
• Their large size makes chromosome organization and transcription directly observable.
• They are useful for studying gene activity, chromatin structure and the relationship between transcription and chromosome condensation.

B chromosomes:

• These are supernumerary chromosomes present in addition to the normal set, called A chromosomes.
• They are not essential for normal survival and may vary in number among individuals of the same species.
• They are often rich in heterochromatin and contain relatively few functional genes.
• Their inheritance may be irregular because they can show meiotic drive or nondisjunction.
• In high numbers, they may reduce fertility or vigor.

Holocentric chromosomes:

• In these chromosomes, kinetochore activity is distributed along most or all of the chromosome length rather than being restricted to one localized centromere.
• Spindle microtubules can attach at multiple sites along the chromosome.
• Chromosome fragments may retain segregation ability because centromeric activity is not confined to one point.
• They occur in some plants, nematodes and insects.

Other specialized chromosomes include polytene chromosomes, which are adapted for high gene dosage and cytological visibility, and lampbrush chromosomes, which support extensive transcription in growing oocytes.

The chromosomal theory of inheritance, proposed independently by Walter Sutton and Theodor Boveri, states that genes are located on chromosomes and that chromosome behavior during meiosis explains Mendelian inheritance.

Major postulates:

• Genes are arranged in a linear order on chromosomes.
• Chromosomes occur in homologous pairs in diploid organisms, with one homolog inherited from each parent.
• The two alleles of a gene occupy corresponding loci on homologous chromosomes.
• Homologous chromosomes separate during meiosis, explaining Mendel's law of segregation.
• Different homologous chromosome pairs orient independently at metaphase I, providing a basis for independent assortment.
• Fertilization restores the diploid chromosome number by combining maternal and paternal haploid sets.
• Genes on the same chromosome form a linkage group and may not assort independently.
• Crossing over between homologous chromosomes produces genetic recombination.

The theory connected Mendel's abstract hereditary factors with physically observable chromosomes and formed the foundation of cytogenetics.

Several observations support the chromosomal theory:

• Parallel behavior: Mendelian factors occur in pairs, just as homologous chromosomes occur in pairs in diploid cells.
• Segregation: Homologous chromosomes separate during anaphase I, paralleling the segregation of alleles into different gametes.
• Independent orientation: Different bivalents orient independently at metaphase I, explaining the independent assortment of genes on different chromosomes.
• Restoration by fertilization: Fusion of haploid gametes restores both allele pairs and homologous chromosome pairs.
• Sex-linked inheritance: Morgan's studies of white-eye inheritance in Drosophila showed that a gene could be associated with the X chromosome.
• Linkage: Genes on the same chromosome tend to be inherited together and constitute a linkage group.
• Crossing over: Recombination frequencies between linked genes correlate with distances between loci on chromosomes.
• Cytological proof: Experiments by Creighton and McClintock in maize and Stern in Drosophila demonstrated that genetic recombination was accompanied by physical exchange between homologous chromosomes.
• Nondisjunction: Exceptional inheritance patterns correlate with abnormal chromosome segregation.

Together, these findings demonstrate that chromosomes are the physical carriers of hereditary information.

The eukaryotic cell cycle consists of interphase and the M phase.

1. G1 phase:

   • Cell growth and active metabolism occur.
   • RNA, proteins and organelles are synthesized.
   • The cell evaluates whether conditions are favorable for division.

2. S phase:

   • Nuclear DNA is replicated.
   • Each chromosome becomes composed of two sister chromatids.
   • Centrosome duplication also occurs in animal cells.
   • DNA content increases from to , although chromosome number remains .

3. G2 phase:

   • The cell continues growing and produces proteins required for division.
   • Replicated DNA is checked and repaired.
   • Preparations for spindle formation are completed.

4. M phase:

   • Mitosis divides the nucleus through prophase, prometaphase, metaphase, anaphase and telophase.
   • Cytokinesis divides the cytoplasm.

Some cells exit G1 and enter G0, a quiescent state. They may remain there permanently or re-enter the cycle after receiving appropriate signals.

Cell-cycle checkpoints are control mechanisms that prevent progression until essential events have been completed correctly.

• G1/S checkpoint: Determines whether the cell has adequate size, nutrients and growth signals. It also checks for DNA damage before replication. Severely damaged cells may undergo repair, senescence or apoptosis.
• Intra-S checkpoint: Slows or stops DNA synthesis when replication forks stall or DNA damage is detected.
• G2/M checkpoint: Ensures that DNA replication is complete and damaged DNA has been repaired before mitosis begins.
• Spindle-assembly checkpoint: Operates during metaphase and prevents anaphase until every chromosome is correctly attached to spindle microtubules through both kinetochores.

Cyclins and cyclin-dependent kinases regulate transitions between phases. Checkpoint proteins inhibit these regulators when errors are detected.

Importance: Checkpoints reduce mutation accumulation, chromosome breakage and chromosome-number abnormalities. Failure of checkpoint control can produce genomic instability and contribute to cancer development.

Mitosis is an equational nuclear division that usually produces two genetically similar daughter nuclei.

1. Prophase:

   • Chromatin condenses into visible chromosomes.
   • Each chromosome consists of two sister chromatids.
   • The nucleolus disappears and the spindle begins to form.

2. Prometaphase:

   • The nuclear envelope breaks down.
   • Spindle microtubules attach to kinetochores.
   • Chromosomes begin active movement.

3. Metaphase:

   • Chromosomes align at the metaphase plate.
   • Sister kinetochores attach to opposite spindle poles.

4. Anaphase:

   • Centromeric cohesion is removed.
   • Sister chromatids separate and become daughter chromosomes.
   • Chromosomes move toward opposite poles.

5. Telophase:

   • Chromosomes reach the poles and decondense.
   • Nuclear envelopes and nucleoli re-form.
   • The spindle disassembles.

Cytokinesis then divides the cytoplasm. Animal cells form a cleavage furrow, whereas plant cells generally form a cell plate. The chromosome-number relationship is .

Biological significance of mitosis:

• Maintains chromosome number across successive cell generations.
• Produces genetically similar daughter cells under normal conditions.
• Enables growth from a single-celled zygote to a multicellular organism.
• Replaces worn-out, damaged or dead cells.
• Supports wound healing and tissue regeneration.
• Serves as the basis of asexual reproduction in many organisms.
• Preserves genetic continuity in somatic cell lineages.

Consequences of mitotic errors:

• Failure of chromosome attachment or separation can cause nondisjunction.
• Daughter cells may receive abnormal chromosome numbers, a condition called aneuploidy.
• Chromosome breakage can cause deletions, duplications or rearrangements.
• Failure of cytokinesis can produce binucleate or polyploid cells.
• Defective checkpoints may permit abnormal cells to continue dividing.

The accumulation of such errors may cause cell death, tissue dysfunction, developmental mosaicism or cancer.

Meiosis I separates homologous chromosomes and reduces the chromosome number from diploid to haploid.

Prophase I: Homologous chromosomes pair by synapsis to form bivalents or tetrads. Crossing over occurs between non-sister chromatids. Prophase I is subdivided into leptotene, zygotene, pachytene, diplotene and diakinesis.

Metaphase I: Bivalents align at the equatorial plate. Each homolog is attached to spindle fibers from an opposite pole. The orientation of each bivalent is random.

Anaphase I: Homologous chromosomes separate and move to opposite poles. Sister centromeres do not divide, so sister chromatids remain together.

Telophase I and cytokinesis: Chromosomes may partially decondense, and the cell divides to form two haploid cells. Each chromosome still consists of two sister chromatids.

It is called reductional division because homologous pairs are separated, reducing the chromosome number according to:

The DNA replicates before meiosis I but not between meiosis I and meiosis II.

Similarities:

• Both are equational divisions.
• Chromosomes align individually at the metaphase plate.
• Sister centromeres divide during anaphase.
• Sister chromatids move toward opposite poles.
• Both use spindle microtubules and pass through prophase, metaphase, anaphase and telophase.

Differences:

Thus, the mechanism of chromatid separation is similar, but the genetic context and biological consequences differ.

Prophase I is prolonged and is divided into five substages:

1. Leptotene: Chromosomes begin to condense and appear as thin threads. Each has already replicated, although sister chromatids may not be clearly distinguishable.

2. Zygotene: Homologous chromosomes pair through synapsis. The synaptonemal complex forms between homologs, producing bivalents.

3. Pachytene: Synapsis is complete, and each bivalent contains four chromatids, forming a tetrad. Crossing over occurs between non-sister chromatids at recombination nodules.

4. Diplotene: The synaptonemal complex disassembles and homologs begin to separate. They remain attached at visible points called chiasmata, which represent the cytological consequences of crossing over.

5. Diakinesis: Chromosomes reach maximum condensation. Chiasmata move toward chromosome ends through terminalization, and the nucleolus and nuclear envelope disappear.

Crossing over produces recombinant chromatids, while chiasmata help maintain homolog association until correct segregation at anaphase I.

In chromosome-number notation, mitosis generally produces , whereas meiosis produces . Meiosis promotes variation through crossing over and independent assortment.

Law of segregation:

• In a diploid organism, the two alleles of a gene occupy the same locus on homologous chromosomes.
• During prophase I, the homologs pair with each other.
• At anaphase I, the homologous chromosomes move to opposite poles.
• Consequently, the two alleles separate into different meiotic products.
• After meiosis II, each gamete normally carries only one allele of the gene.

Law of independent assortment:

• Consider two genes on different chromosome pairs, with alleles and .
• At metaphase I, each homologous pair can orient toward either spindle pole independently of the other pair.
• One orientation can produce parental combinations and , while the alternative orientation can produce and .
• For independently assorting homologous pairs, random orientation can generate chromosome combinations in gametes, excluding additional variation caused by crossing over.

Independent assortment does not strictly apply to tightly linked genes on the same chromosome. Such genes tend to be inherited together, although crossing over can produce recombinant combinations. Therefore, meiotic chromosome behavior physically explains both Mendelian laws and their linkage-related limitations.