Chromosomes and Cell Cycle Quiz

Chromosomes: Structure, Types, and Classification

• Chromosomes are the organized genetic structures within a living organism, containing most of the DNA.
• The term "chromosome" derives from the Greek words "chroma" (color) and "soma" (body).
• Chromosomes are responsible for distinguishing species and transmitting genetic information between generations.
• They are visible under a light microscope during metaphase of mitosis, when maximally contracted.

Chromosome Structure

• A chromosome is composed of two identical sister chromatids, resulting from DNA replication during the S phase (synthesis) of the cell cycle.
• Sister chromatids are joined at a primary constriction called the centromere.
• DNA is complexed with proteins (histones) and other macromolecules. This complex is known as chromatin.
• Chromatin is packaged into nucleosomes, which are further organized into higher order structures. Different levels of chromatin packaging are visible: DNA duplex, 11 nm diameter histone fiber, 30 nm chromatin fiber, 300 nm coiled chromatin fiber and 1400 nm diameter metaphase chromatid.
• Chromatin exists differently in different cell types, with some cells lacking chromatin as part of their normal structure (e.g. erythrocytes).

Heterochromatin and Euchromatin

• Heterochromatin is more condensed, gene-poor (contains more AT sequences) and stains darker in microscopic examination.
• Euchromatin is less condensed, gene-rich (contains more GC sequences) and stains lighter.

Chromosome Tips and Middle

• Telomeres are the tips of chromosomes. They act like a biological clock, getting shorter with each cell division.
• Centromeres are the middle sections of chromosomes. They play a critical role for attaching to spindle fibers and pulling apart sister chromatids in cell division.
• Each centromere divides the chromosome into a short arm (p) and a long arm (q).

Kinetochore

• The kinetochore is a protein structure attached to each side of the centromere. It's critical for chromosome movement during cell division.

Chromosome Classification

• Classification of Chromosomes based on gene complement.

  • Autosomes: 22 pairs that control somatic characteristics.
  • Allosomes (sex chromosomes): 1 pair controlling gender determination.

• Classification based on centromere position.

  • Metacentric: centromere in the middle, equal arms
  • Submetacentric: centromere slightly off centre, unequal but similar length arms
  • Acrocentric: centromere very near one end, very short p arm
  • Telocentric: centromere at the end of the chromosome, no p arm

• Classification of chromosomes based on length.

  • Autosomes are numbered from 1-22 and classified in 7 groups (A to G) based on length, in descending order.
  • Sex chromosomes either X & Y belong to group C&G respectively.

Satellites

• Some chromosomes have satellite structures; these play a role in forming the nucleolus after cell division, a site of ribosome synthesis and assembly.

Chromosome Duplication

• In preparation for cell division, DNA replicates, creating chromosomes composed of two identical sister chromatids joined by a centromere.

Chromosome Numbers

• Members of the same species have identical chromosome numbers in somatic cells.
• All chromosomes, except the sex chromosomes, are arranged in pairs based on identical length and position of the centromere.

Telomere

• Telomeres are essential for protecting chromosome integrity and preventing fusion.

• Telomeres are composed of repeating DNA sequences (TTAGGG) telomerase is the enzyme responsible for maintaining telomere length during DNA replication.

• Telomerase is often found in stem cells and cancer cells because their division rate is high. This allows telomeres to remain long and therefore cells continually divide.

• Shortening of telomeres is linked to aging. Cancer development may occur from telomerase activity.

Telomere and Cancer

• Cancer cells often have high telomerase activity. This ensures the telomere remains long and that the cell can continue dividing. This allows cancer cells to bypass the normal mechanism for inhibiting uncontrolled cell division.
• Research explores ways to directly target telomerase as a potential cancer treatment approach.

Telomerase

• Telomerase is an enzyme that adds bases to telomeres. This maintains the length during cell division.
• Telomerase is present in germ cells, stem cells, and highly active in cancer cells; this is one of the key characteristics of cancer cells enabling them to proliferate.
• Measuring telomerase levels might become useful in cancer detection.