Biology Lecture 3: Chromosomes

Lecture 3: Chromosomes

• Walther Flemming's work in the 1880s marked the beginning of the discovery and identification of human chromosomes.
• Eukaryotic cells have a diploid number of chromosomes.
• The complete set of genetic information in a haploid chromosome is known as the genome.
• Homologous chromosomes are identical chromosome pairs appearing during meiosis.
• Homologous chromosomes have the same number and types of genes.
• Some chromosomes are non-homologous and cannot pair.

Lecture Objectives

• Differentiate between haploid (n) and diploid (2n)
• Identify key aspects of chromosomes
• Classify chromosomes
• Understand the human karyotype
• Understand the Lyon hypothesis (X-chromosome inactivation)
• Identify how DNA packs to form chromosomes

Haploid (n) and Diploid (2n) Key Concepts

• Eukaryotic cells possess a diploid number of chromosomes.
• The complete set of genetic information in a haploid chromosome is referred to as the genome.
• Homologous chromosomes are identical and pair during meiosis.
• Homologous chromosomes share similar gene numbers and types.

Key Differences Between Haploid and Diploid

Key Aspects of Chromosomes

• Chromosomes are thread-like structures within the nucleus made of DNA and proteins (primarily histones).
• The DNA is wrapped around proteins to protect it and regulate gene access.
• Chromosomes are essential for conserving space in cells.

Chromosomes Structure

• Each chromosome has a centromere (central region), telomeres (protective ends), and two identical sister chromatids (after DNA replication).
• Chromatin condenses during cell division to form visible chromosomes.

Nuclear and Mitochondrial DNA

• Nucleus chromosomes are linear and inherit from both parents (biparental inheritance).
• Mitochondrial chromosomes are circular and inherit only from the mother (maternal inheritance).

Telomeres

• Telomeres are repetitive DNA sequences at the ends of linear chromosomes.
• They act as protective caps, preventing degradation of genetic material and maintaining chromosome stability during cell division.
• In humans, the telomere sequence is 5'-TTAGGG-3' (repeated approximately 2000 times).
• Telomeres form T-loops which are maintained by the Shelterin Complex (protein shield)
• The shortening of telomeres leads to cell death.
• Telomerase, an enzyme, prevents telomere shortening.

What Is a Centromere?

• It is the point where sister chromatids join and where spindle fibers attach for accurate chromosome segregation.
• Composed of repetitive DNA sequences.

Classify Chromosomes

• Chromosomes are classified based on centromere location (metacentric, submetacentric, acrocentric, and telocentric).

Understand the Human Karyotype

• Karyotyping is the process of pairing and ordering all chromosomes for a genome-wide view of an individual's chromosomes. It uses cell staining to visualize chromosomes; allows identification of chromosome abnormalities or genetic disorders.
• Scientists use size, banding patterns, and centromere positions to identify chromosomes.
• A human karyotype shows the number and types of chromosomes; normally, 46 chromosomes in somatic cells.
• Autosomes are chromosomes not associated with sex determination.
• Sex chromosomes (X and Y) determine sex. The X chromosomes are crucial in determining sex, and the X and Y chromosomes are placed from 1 to 22 chromosomes in a human karyotype in descending order of size.

X-Chromosome Inactivation

• In female somatic cells, only one X chromosome is active and the other X-chromosome is inactivated to regulate dosage compensation between males and females.
• This inactivation happens randomly during early embryonic life
• The inactivated X chromosome is visible as a Barr body.

DNA Packing to Make Chromosomes

• DNA packaging starts as a simple 2 nm double helix, progressing through multiple levels of compaction to form a 1400 nm metaphase chromosome.
• This hierarchical structure allows nearly 2 meters of DNA to fit inside a microscopic cell nucleus.
• DNA wraps around proteins called histones to form nucleosome structures. This is a fundamental level of chromosome organization.

What Are Histones?

• Histones are basic, positively charged proteins that package DNA tightly.
• Five main histone types exist (core and linker histones).