B6A-03: Life cycles and life histories (Week 2 p1)

Core principle of life: all life must perpetuate itself.
Cells come from cells (Latin phrase: Omnis cellula e cellula).
Cells replicate and pass on genetic information to subsequent generations.
In cell division, genetic information segregates so that each daughter cell acquires copies of DNA.

Central dogma of biology posits that DNA is the molecular basis of inheritance.
DNA molecules synthesized as precise replicas of previous molecules; necessary for copying genetic information including sequence of nucleotides.
Process of DNA segregation during cell division ensures each daughter cell receives identical genetic information.

A chromosome is defined as a double-stranded DNA molecule.
In eukaryotes, DNA is associated with proteins and organized into structures called chromatin.
Each chromosome contains hundreds or thousands of genes (units of heredity).
Genes encode specific proteins which carry out various functions in organisms.

Multicellular organisms grow and develop from a single cell (embryo) through cell division.
Critical processes include:
- replication of DNA,
- separation of genetic material,
- regulation of the cell cycle for growth and repair.
Divisions must ensure the correct number of cells (neither too few nor too many) at the right place and time for proper function.

Asexual Reproduction
- Involves replication of cells to produce genetically identical offspring (e.g., hydra budding).
- Utilizes processes such as binary fission particularly in prokaryotes (e.g., bacteria).
- Advantages include rapid population growth when conditions are stable.
Sexual Reproduction
- Involves combination of genetic material from two different individuals.
- Requires more complex processes than asexual reproduction, including the formation of gametes (sperm and egg).
- Involves the concepts of haploidy and diploidy.
- Independent assortment and crossing over during meiosis lead to genetic variation.

Prokaryotes (Bacteria)
- Reproduce asexually via binary fission; a single circular chromosome replicates and separates.
- Plasmids (small DNA molecules) may or may not be passed onto daughter cells.
Eukaryotes
- Have multiple linear chromosomes and reproduce through more complex processes, including mitosis.
- Chromosomes are packed with histones and organized into chromatin.

Cell Cycle Phases: Divided into interphase (G1, S, G2) and mitotic phase (Mitosis, Cytokinesis).
- G1: Growth stage where the cell is metabolically active.
- S Phase: DNA synthesis, where chromosomes are replicated.
- G2: Growth after DNA synthesis preparing for division.
Mitosis: The process of dividing replicated DNA into two nuclei.
Cytokinesis: Division of the cytoplasmic material and formation of two separate cells.

Stages of mitosis include:
- Prophase: Chromatin condenses to form visible chromosomes.
- Metaphase: Chromosomes align at the metaphase plate.
- Anaphase: Sister chromatids are pulled apart to opposite poles.
- Telophase: Nuclear envelopes reform around separated chromosomes.

In animal cells: cytokinesis occurs through the formation of a cleavage furrow.
In plant cells: vesicles form a cell plate that eventually develops into a cell wall separating daughter cells.

Regulation of the cell cycle is crucial for maintaining the proper number of cells and preventing uncontrolled cell growth (tumor formation).
Examples include regeneration (e.g., sea stars) and tissue repair.

Meiosis: Two rounds of division that reduce chromosome number by half to produce genetically diverse gametes (haploid).
- Meiosis I: Separates homologous chromosomes. Results in two haploid cells with duplicated chromosomes.
- Meiosis II: Separates sister chromatids, resulting in four haploid, genetically distinct cells.

Independent Assortment: Random distribution of maternal and paternal chromosomes during meiosis leads to genetic variation.
Crossing Over: Occurs during prophase I of meiosis; homologous chromosomes exchange segments leading to genetic recombination.
Fertilization: Random combining of gametes produces genetic variation in offspring.

Animals typically have a diploid life cycle, while fungi and some algae may have haploid life cycles.
Variations in life histories (e.g., haploid vs diploid bodies) occur among different groups, such as plants alternating between haploid (gametophyte) and diploid (sporophyte) stages.

Green Algae (Chlamydomonas): Exhibits haploid life history; can reproduce asexually via mitosis or sexually via gamete fusion.
Plasmodial Slime Molds: Diploid cells reproduce by mitosis without cytokinesis leading to super cells with multiple nuclei.
Ciliates (e.g., Paramecium): Exhibit unique nuclear structures with both macronucleus for everyday function and micronucleus for reproduction.
Apicomplexans (e.g., Plasmodium): Complex life cycle involving both haploid and diploid stages, parasitic behavior, and multiple hosts.

The life processes, including reproduction, vary widely among organisms, with complex mechanisms enhancing genetic diversity and adaptation to changing environments. Understanding these processes is crucial for developments in fields such as genetics, medicine, and biotechnology.