Cell Cycle and Cell Division

Phases of the Cell Cycle

The cell cycle is a series of stages that cells go through to grow and divide. It consists of interphase (G1, S, G2 phases) and the mitotic (M) phase. Some cells exit the cycle and enter a resting state called G0.

• G1 Phase: Cell grows and prepares for DNA replication.

• S Phase: DNA is replicated.

• G2 Phase: Cell prepares for mitosis.

• M Phase: Mitosis and cytokinesis occur.

• G0 Phase: Non-dividing state; cells may remain here permanently or temporarily.

Key Point: Cells that do not undergo cell division after G1 enter the G0 phase.

Cytokinesis in Plants vs. Animals

Cytokinesis is the process of cytoplasmic division following mitosis or meiosis, resulting in two daughter cells.

• Plant Cells: Division occurs centrifugally (from the center outward) by forming a cell plate.

• Animal Cells: Division occurs centripetally (from the outside inward) by forming a cleavage furrow.

Chromatin Structure and DNA Packaging

DNA is packaged in the nucleus by associating with proteins to form chromatin. The main proteins involved are histones.

• Histones: Proteins that DNA wraps around to form nucleosomes, enabling efficient packaging.

• Centromere: Region where sister chromatids are joined; not involved in packaging but in chromosome segregation.

Cancer Cells vs. Normal Cells

Cancerous cells exhibit several differences from normal cells, including:

• High nucleus-to-cytoplasm ratio

• More karyokinesis (nuclear division) than cytokinesis (cytoplasmic division)

• Presence of multiple nuclei

All of these features are characteristic of cancer cells.

Reproduction and Genetics

Asexual vs. Sexual Reproduction

Organisms can reproduce asexually or sexually, affecting their ability to colonize habitats and genetic diversity.

• Asexual Reproduction: Offspring are genetically identical to the parent; allows rapid and easy colonization of habitats.

• Sexual Reproduction: Involves genetic recombination, leading to greater diversity but slower colonization.

Mendelian Genetics

Gregor Mendel described inherited features that show variation among individuals as characters (e.g., flower color, seed shape).

• Gene: Unit of heredity.

• Allele: Variant form of a gene.

• Genotype: Genetic makeup of an organism.

• Character: Observable feature (e.g., height, color).

Sex-Linked Inheritance: Hemophilia

Hemophilia is an X-linked recessive disorder. Females can only have hemophilia if their father is affected and their mother is at least a carrier.

Probability in Genetics: Eye Color Example

Brown eyes are dominant over blue eyes. If a heterozygous brown-eyed male (Bb) marries a blue-eyed female (bb), the probability of blue-eyed offspring is 50%.

• Punnett Square: Bb x bb → 50% Bb (brown), 50% bb (blue)

Pedigree Analysis

Pedigrees use shapes to represent individuals:

• Square: Male

• Circle: Female

• Filled shape: Affected individual

Example: A filled square represents an affected male.

Molecular Biology and Biotechnology

Epigenetics and Gene Regulation

Epigenetic modifications, such as DNA methylation, can silence genes without altering the DNA sequence.

• DNA Methylation: Addition of methyl groups to DNA, often leading to gene silencing.

• Gene Silencing: Prevents gene expression.

Southern Blotting

Southern blotting is a technique used to detect specific DNA sequences. DNA probes are used as markers to identify target sequences.

Gel Electrophoresis

Gel electrophoresis separates DNA fragments based on size using an electric current.

• Smaller fragments move faster and farther through the gel.

DNA Replication

During DNA replication, both the leading and lagging strands are synthesized in the 5' to 3' direction. Okazaki fragments are short segments synthesized on the lagging strand.

• Leading Strand: Synthesized continuously.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments.

DNA Damage and Repair

UV light can cause the formation of pyrimidine dimers (especially thymine dimers), which are repaired by nucleotide excision repair mechanisms.

Hershey and Chase Experiment

This experiment demonstrated that DNA, not protein, is the genetic material that enters bacteria during viral infection.

Genetically Modified (GM) Foods

GM foods are created by altering the DNA of organisms. Advantages include improved nutrients, better yield, and resistance to pests. However, they may have unexpected effects and health risks.

Telomerase and Telomere Replenishing

Telomerase RNA acts as a template for adding telomeric repeats to chromosome ends, maintaining chromosome stability.

Gene Structure: Introns and Exons

Genes contain coding (exons) and non-coding (introns) sequences.

• Introns: Non-coding regions removed during RNA processing.

• Exons: Coding regions that remain in mature mRNA.

Posttranslational Modifications

Proteins can undergo various modifications after translation, such as acetylation, methylation, and ubiquitination. RNA splicing is not a posttranslational modification; it occurs at the RNA level.

Translation Initiation

Translation begins when the ribosome binds to the start codon on mRNA, not the promoter or terminator.

Summary Table: Key Molecular Biology Techniques

Key Equations and Concepts

• DNA Synthesis Direction:

• Punnett Square Probability (for Bb x bb):