Introduction to Cells

Cell Basics

• Cells are the smallest functional units of life, essential for growth, reproduction, information processing, and chemical reactions.
• Cells were discovered by Robert Hooke in 1665 and can independently carry out life-sustaining activities.
• Two main types of cells exist: plant cells and animal cells.
• Cell size varies greatly, but most are small and only visible through a microscope.

Genetic Material

• All cells can reproduce and pass on genetic information to subsequent generations.
• Prokaryotic genetic material is concentrated in the nucleoid, containing a single circular, double-stranded DNA molecule.
• Viruses can have DNA or RNA as genetic material, which may form different structures.
• Eukaryotic cells have genomic DNA enclosed in the nucleus, with genes located on chromosomes that exist as chromatin during interphase.

Chromosomes and Genes

• The term chromosome was coined by W. Waldeyer in 1888, meaning "colored bodies," referring to their organized DNA and protein arrangement.
• Each somatic cell typically contains two copies of each chromosome, defining haploid (N) and diploid (2N) numbers.
• Eukaryotic chromosomes are large and linear, contrasting with the circular chromosomes in prokaryotes.
• Ascaris megalocephalusunivalens has the fewest chromosomes, with only two per somatic cell.

Molecular Definition of Genes

• A gene is defined as the complete nucleic acid sequence necessary for synthesizing a functional product, such as proteins or RNA.
• Chromatin is composed of DNA, RNA, and proteins, categorized as histones and non-histones.

DNA and Chromosome Function

• Centromeres are condensed regions on chromosomes crucial for proper segregation during cell division, with kinetochore structures aiding spindle fiber attachment.
• Genes include both coding (exons) and noncoding (introns) regions, where most multicellular genes contain introns.

Eukaryotic Genomes Complexity

• Introns often significantly outnumber exons in eukaryotic genes, contributing to the larger genome size.
• More than 50% of mammalian DNA consists of repetitive sequences, some present in numerous copies.
• Gene families arise from duplications, with pseudogenes being non-functional due to mutations.

Composition of Eukaryotic Genomes

• Complex eukaryotic genomes contain only a small proportion of protein-coding sequences; human genome has approximately 20,000-25,000 genes.
• The human genome comprises 3 billion base pairs across 24 chromosomes, and the smallest chromosome is still larger than the entire yeast genome.

Human Chromosome Characteristics

• Human chromosomes are best observed during mitosis, showing distinct banding patterns with G-banding revealing heterochromatic and euchromatic regions.
• Karyotypes provide a numerical description of chromosomes, with Homo sapiens having 46 chromosomes (23 pairs).
• Genetic abnormalities, such as Down's syndrome, can occur due to chromosome number alterations.

Gene Structure in Eukaryotes

• Eukaryotic genes consist of coding segments (exons) interrupted by introns and require splicing to produce mature mRNA.
• Alternative splicing allows different combinations of exons, leading to varied mRNA and protein products from a single gene.

C-value

• The C-value represents the amount of DNA found in the haploid genome of an organism, reflecting the complexity and size of the genome across different species.