Cell Biology and Heredity

• The sharing of features among family members is explained by heredity, where traits are passed from parents to offspring
• Variation demonstrates differences among individuals
• Genetics studies heredity and variation to understand how traits are passed on and how variation arises
• All living things undergo reproduction, which requires energy from nutrients for metabolic processes, growth, development, and cell division
• Cell division is the backdrop for organismal reproduction

Chromosomes

• Chromosomes contain the genetic material that directs the activities and functions of cells
• Genetic material, or deoxyribonucleic acid (DNA), is passed on to ensure continuity of life
• In eukaryotic cells, DNA is bound with proteins and organized into chromosomes

Number Of Chromosomes

• The number of chromosomes in cells varies by species
• Humans have 46 chromosomes
• Rice has 24 chromosomes

Cell Cycle

• The cell cycle has two stages: interphase and cell division
• The chromosomes are long and extended during interphase, and referred to as chromatin
• During cell division the chromosomes condense
• The Cell Cycle is an ordered set of events, culminating in cell growth and division into two daughter cells
• Non-dividing cells are not considered to be in the cell cycle
• Eukaryotic cells divide following a process alternation
• Doubling of its genome (or DNA) in S phase (synthesis phase) of the cell cycle
• Halving of that genome during mitosis (M phase).
• Stages of the cell cycle
• G1 stage: Growth and preparation of the chromosomes for replication
• S stage: DNA replication
• G2 stage: Preparation for mitosis
• M stage: Nuclear (chromosomes separate) and cytoplasmic (cytokinesis) division

Regulation of the Cell Cycle

• Cell division is controlled, and errors in regulation can lead to cancer
• Cancer happens when normal cell growth and behavior is disregulated
• The cell cycle is controlled by
• CdK: Cyclin dependent kinase, adds phosphate to a protein
• Cyclins: Work with CdK, are major control switches causing movement from G1 to S or G2 to M
• MPF: Maturation Promoting Factor that includes the CdK and cyclins that triggers
• p53:
• A protein that functions to block the cell cycle if DNA is damaged
• If the damage is severe this protein can cause apoptosis (cell death).
• This protein allows for DNA repair when levels are high.
• Mutation of this protein is the most frequent leading cause of cancer
• p27
• A protein that binds to cyclin and cdk, blocking entry into S phase.
• Reduced levels of p27 predict a poor outcome for breast cancer patients.

Cell Division: Mitosis and Meiosis

• Mitosis
• Nuclear division plus cytokinesis
• Produces two identical daughter cells, during several stages.
• It takes place in somatic, or body cells.
• The whole process takes about an hour inside actively dividing cells

Mitosis Stages

• Interphase: Included in discussions of mitosis, but technically not part of mitosis, but encompasses stages G1, S, and G2 of the cell cycle
• Prophase:
• Takes over half of mitosis
• The nuclear membrane breaks down to form small vesicles and the nucleolus disintegrates
• The centrosome duplicates to form two daughter centrosomes, that migrate to opposite ends of the cell
• Microtubules form to create spindle fibers
• Each replicated chromosome consists of two identical chromatids (or sister chromatids) held together by a structure known as a centromere
• Chromosomes arms are labeled short 'p' arms and long 'q' arms
• Chromosomes can be stained to show alternating dar heterochromatin that are more coiled and dense and euchromatin
• Prometaphase:
• Chromosomes migrate to the equatorial plane in the midline of the cell at right-angles to the axis formed by the centrosomes, led by centromeres
• The region of the mitotic spindle, known as the metaphase plate
• Spindle fibers bind to a structure associated with the centromere of each chromosome called a kinetochore
• Metaphase: Chromosomes align at the metaphase plate.
• Anaphase:
• The shortest stage of mitosis
• Centromeres divide
• Sister chromatids of each chromosome are pulled apart, or 'disjoin'
• They move to the opposite ends of the cell, pulled by spindle fibers attached to the kinetochore regions.
• Separated sister chromatids are now referred to as daughter chromosomes
• Telophase:
• The final stage of mitosis
• The nuclear membrane reforms around the chromosomes grouped at either pole of the cell
• The chromosomes uncoil and become diffuse and spindle fibers disappear
• Cytokinesis:
• Final cellular division to form two new cells
• In plants a cell plate forms along the line of the metaphase plate
• In animals- a constriction of the cytoplasm
• The cell then enters interphase, the interval between mitotic divisions.

Meiosis

• A eukaryotic cell division that produces haploid sex cells or gametes (contain a single copy of each chromosome from diploid cells
• diploid cells (contain two copies of each chromosome)
• Process takes the form of one DNA replication followed by two successive nuclear and cellular divisions (Meiosis I and Meiosis II).
• Through meiosis, gametogenesis or the production of sperm (spermatogenesis) and eggs (oogenesis) takes place
• Is not a cycle

Meiosis 1

• Purposes:
  • Reduction division, reduces number of chromosomes in half, making daughter cells haploid
  • Genetic recombination occurs (crossing over)
• Prophase 1 sub-stages:
• Leptotene: Each chromosome made up of two long threads of sister chromatids, as a result of replication during the S phase of the cell cycle
• Zygotene: Chromosomes pair off into homologous chromosomes
• Pachytene: Chromosomes contract due to repeated coiling, crossing over takes place where a segment of a sister chromatid of one chromosome is exchanged with the same segment of the sister chromatid of the homologous chromosome through the formation of a cross-linkage of the segments called a chiasma After crossing over, sister chromatids of each chromosome may no longer be identical based on the genetic material they contain
• Diplotene: Chromosomes uncoil
• Diakinesis: Paired chromosomes disperse in the nucleus
• Metaphase I: tetrads line up at the equator and the spindle has completely formed
• Anaphase I: Spindle fibers form and attach to the centromeres of the chromosomes. The homologous chromosomes separate from each other completely and start moving toward the poles as they are pulled by spindle fibers. The sister chromatids remain together
• Telophase I: Chromosomes with two chromatids decondense and a nuclear envelope reforms. Each nucleus is now haploid.

Meiosis 2

• Function is to reduce the amount of DNA back to normal, by splitting chromosomes
• The products are thus haploid with only one chromatid per chromosome
• Prophase II: Chromosomes with two chromatids condense, a new spindle is forming while nucleus and nuclear envelope disappear
• Metaphase II: Chromosomes with two chromatids line up at the equator, spindle is fully formed
• Anaphase II: Chromosomes split so that only one chromatid heads toward each pole
• Telophase II: Chromosomes with only one chromatid decondense, surrounded by new nuclear envelopes. Now daughter cells are all haploid with the right amount of DNA. Ready to develop into sperm or eggs now.

Explore the roles of genetic material, DNA organization, and chromosomal behavior during the cell cycle. Investigate heredity, species variation, and chromosome counts in somatic and germ cells. Learn about cells that are not actively dividing according to research.