Chromosome Structure and Composition

Chromosome Structure and Composition

• Chromosomes are threadlike structures in the cell nucleus that package DNA.
• Chromosomes consist of DNA tightly coiled around histone proteins.
• Histones support the chromosome's structure.
• Chromosomes were first described by Strasburger in 1815.
• The term "chromosome" was introduced by Waldeyer in 1888.

Visualization and Occurrence

• Chromosomes appear as rod-shaped, darkly stained bodies during the metaphase stage of mitosis.
• They are visible when cells are stained with a basic dye and viewed under a light microscope.
• Each chromosome in eukaryotic cells comprises a single DNA molecule with multiple copies of five histone types.
• Histones are rich in lysine and arginine residues, giving them a positive charge that allows them to bind tightly to the negatively charged phosphates in DNA.
• Non-histone proteins, mainly transcription factors, are also present and regulate which parts of DNA are transcribed into RNA.

Chromosome Dynamics During Cell Cycle

• Chromosomes are elongated and not observable under a microscope for much of the cell's life cycle.
• Chromosomes are duplicated during the S phase of the mitotic cell cycle.
• Duplicated chromosomes condense into shorter, stainable structures at the beginning of mitosis.
• Duplicated condensed chromosomes are called dyads.
• The centromeres hold duplicated chromosomes together.
• Human centromeres consist of approximately 1-10 million DNA base pairs.
• Centromere DNA contains mostly repetitive, short sequences in tandem arrays.
• Attached, duplicated chromosomes are known as sister chromatids.

Kinetochores and Chromosome Arms

• Kinetochores serve as attachment points for spindle fibers.
• Spindle fibers help separate sister chromatids during anaphase.
• Kinetochores are complexes made of approximately 80 different proteins.
• The shorter arm of the chromosome extending from the centromere is the p arm.
• The longer arm extending from the centromere is the q arm.

Structural Components

• Each chromosome is differentiated into three parts: the pellicle, matrix, and chromonemata.
• The pellicle is the outer envelope around the chromosome substance, very thin, and made of achromatic substances.
• The matrix is the ground substance containing the chromonemata and non-genic materials.
• Chromonemata are two identical, spirally coiled threads embedded in the matrix.
• The chromonemata appear as a single thread of about 800A thickness due to tight coiling.
• Each chromonema consists of about eight microfibrils, each formed from a double helix of DNA.

Microscopic Features

• During mitotic metaphase, the structural features visible under a light microscope include chromatids, chromonema, chromomeres, centromere, secondary constriction or nucleolar organizer, telomere, and satellite.

Centromeres

• The centromere is a small structure in the chromonema, marked by a constriction that is a permanent feature of the chromosome.
• At the centromere, the two chromonemata are joined.
• The centromere is also known as the kinetochore or primary constriction.
• It divides the chromosome into two sections, or arms, designated as the "p arm" and "q arm".
• The centromere position is constant for a given chromosome type.
• In electron microscopic sections, the kinetochore shows a trilaminar structure: a 10 nm thick dense outer protein aceous layer, a middle layer of low density, and a dense inner layer tightly bound to the centromere.
• Chromosomes attach to spindle fibers at the centromere during cell division.

Secondary Constriction or Nucleolar Organizer

• A chromosome possesses a secondary constriction at any point besides the primary constriction or centromere.
• These constrictions have a constant position and extent and are useful for identifying particular chromosomes.
• The region between the secondary constriction and the nearest telomere is known as a satellite.
• Chromosomes with secondary constrictions are called satellite chromosomes or sat-chromosomes.
• Nucleoli are associated with the secondary constriction of sat-chromosomes.
• Secondary constrictions are also known as nucleolus organizer regions (NOR).
• Sat-chromosomes are often referred to as nucleolus organizer chromosomes.

Telomeres

• Telomeres are specialized ends of chromosomes that exhibit physiological differentiation and polarity.
• Telomeres prevent chromosomal segments from fusing.
• Chromosomal ends are known as telomeres.
• Broken chromosome ends can fuse if telomeres are absent.

Types of Chromosomes: Autosomes and Sex Chromosomes

• Human chromosomes include autosomes and sex chromosomes.
• Genetic traits linked to sex are passed on through sex chromosomes, while the rest of the genetic information is in the autosomes.
• Humans have 23 pairs of chromosomes: 22 pairs of autosomes and one pair of sex chromosomes, totaling 46 chromosomes.

Classification by Centromere Number

• Monocentric chromosomes have one centromere.
• Dicentric chromosomes have two centromeres.
• Polycentric chromosomes have more than two centromeres.
• Acentric chromosomes lack a centromere and represent freshly broken segments that do not survive.

Classification by Centromere Location

• Chromosomes have two parts (p and q arms) divided by a centromere.
• Metacentric: Centromere is in the middle, with p and q arms of comparable length (e.g., chromosomes 1, 3, 16, 19, 20).
• Submetacentric: Centromere is off-center, resulting in a shorter p arm (e.g., 2, 4-12, 17, 18, X).
• Acrocentric: Centromere severely offset, leading to a much shorter p arm (e.g., 13-15, 21, 22, Y).
• Telocentric: Centromere is at the end, with no p arm; not found in humans.

Function and Significance of Chromosomes

• The constant number of chromosomes for a species is important for determining phylogeny and taxonomy.
• Chromosomes store the genetic code necessary for organism development and growth.
• DNA molecules are made of genes, which code for specific proteins.
• Humans have 23 pairs of chromosomes, with one pair being sex chromosomes.
• Females have two X chromosomes, and males have one X and one Y.
• Sex is determined by the male chromosome (X for female, Y for male).

Control and Storage

• Chromosomes ensure proper cell division during mitosis.
• Parent cell chromosomes ensure correct information is passed to daughter cells for growth.
• Chromosomes direct protein sequences and maintain DNA order.
• Proteins are stored within the coiled structure of chromosomes and aid in DNA packaging.