Cell Biology: Chromosome Structure and Cell Cycle

Sexual Cells and Chromosome Structure

• Human sex cells (gametes) are haploid, containing one set of chromosomes.
• Eukaryotic chromosomes consist of a single double-helix of DNA.
• DNA wraps around proteins called histones, forming nucleosomes that compact DNA to reduce its length.
• Protein structures further coil the nucleosomes into larger configurations, resembling springs.
• During cell division, additional proteins compact chromosomes for preparation.

Cell Cycle in Eukaryotes

• Comprises two main phases: Interphase and Cell Division.
• Interphase has three stages:
  • G1 (Growth): Cells grow, some specialize and may either divide or remain specialized.
  • S (Synthesis): DNA replication occurs alongside further growth.
  • G2 (Preparation): Final preparations for mitosis.
• Mitosis predominates in eukaryotic cell divisions, with meiosis occurring mainly in reproductive cells.

Regulation of the Cell Cycle

• Controlled by cyclins and cyclin-dependent kinases (CDKs).
• Checkpoints exist at:
  • G1 to S phase
  • G2 to M phase
  • Between metaphase and anaphase
• Ensures DNA integrity and proper chromosome organization.

Mitosis

• A type of cell division producing two genetically identical daughter cells.
• Responsible for growth and repair in organisms by replacing old cells.

Genetic Understanding and History

• Mendel established the foundation of genetics through pea plant experimentation.
• Thomas Hunt Morgan studied fruit flies and identified sex chromosomes.
• Rosalind Franklin contributed critical X-ray diffraction images for DNA structure.
• Watson and Crick expanded on these findings, contributing to the Human Genome Project mapping the human genome.

DNA Double Helix Model

• Characterized by a helical structure and base complementarity: Adenine, Thymine, Cytosine, and Guanine.
• Antiparallel strands running 5’ to 3’ and 3’ to 5’.
• Comprised of nucleic acid, sugar, and nitrogenous bases.

DNA Significance

• Stores genetic information guiding protein synthesis and cellular functions.
• Replication ensures each daughter cell receives an accurate copy of DNA.

Protein Synthesis

• Involves two main processes:
  • Transcription occurs in the nucleus, where DNA is copied into mRNA.
  • Translation happens in ribosomes, where mRNA is decoded to synthesize proteins using tRNA for amino acid transport.

Genetic Code

• Organized in triplets (codons); universal across all living organisms.
• Features include redundancy, clarity (no ambiguity), and specific start (AUG) and stop signals.

Mendelian Laws of Inheritance

• Segregation: Each individual has two alleles; they segregate during gamete formation.
• Independent Assortment: Inheritance of one allele does not affect another.
• Dominance: Dominant alleles mask the effects of recessive alleles.

Types of Heredity

• Mendelian inheritance follows Mendel's principles.
• Non-Mendelian inheritance features complex gene interactions or environmental influences.

Probability Calculations in Genetics

• Punnett squares predict possible genotypes and phenotypes from parental genotypes.

Advanced Genetic Concepts

• Polygenic traits: Multiple genes affect a single phenotype, leading to continuous variation.
• Multifactorial inheritance: Interaction of multiple genes and environmental factors impacts traits.

Human Blood Groups and Inheritance

• Blood type is determined by the presence of antigens (A and B).
• Codominance occurs when both alleles express equally; blood types include A, B, AB (universal recipient), and O (universal donor).
• Rh factor determines blood type as positive (dominant) or negative (recessive).

Genetic Disorders and Chromosomal Aberrations

• Trisomy and monosomy conditions (e.g., Down syndrome, Turner syndrome).
• Structural changes in chromosomes can lead to severe genetic disorders through deletions, duplications, inversions, and translocations.

Additional Genetic Disorders

• Turner syndrome: Affects females with partial or complete absence of an X chromosome.
• Klinefelter syndrome: Affects males characterized by an additional X chromosome, leading to infertility and development issues.
• Cri du chat syndrome: Result of deletion in chromosome 5, causing distinctive crying sounds and developmental disabilities.

Learn about the structure of sexual cells, chromosome composition, and the process of cell division in eukaryotes. Explore how DNA wraps around histones, forming nucleosomes and compacting chromosomes.