General Biology: Cell Division, Genetics, and Molecular Biology Study Guide

Cell Division

Meiosis and Mitosis

Cell division is a fundamental process in biology, allowing organisms to grow, repair tissues, and reproduce. There are two main types: mitosis and meiosis.

• Mitosis results in two genetically identical daughter cells, each with the same chromosome number as the parent cell (diploid).

• Meiosis produces four genetically distinct daughter cells, each with half the chromosome number of the parent cell (haploid), essential for sexual reproduction.

• During telophase I of meiosis, a cleavage furrow (in animal cells) or cell plate (in plant cells) develops, dividing the cell into two haploid daughter cells, each with chromosomes consisting of two sister chromatids.

• At the end of mitotic cell division, two daughter cells that are genetically identical to the parent cell are produced.

• During meiosis II, sister chromatids are separated, forming four haploid cells.

• If a daughter cell has 30 chromosomes after mitosis, the parent cell also had 30 chromosomes, as mitosis maintains chromosome number.

Example: Human somatic cells undergo mitosis to produce identical cells for tissue growth and repair, while gametes are produced by meiosis.

Cell Cycle and Checkpoints

The cell cycle consists of several phases: G1, S, G2, and M. Checkpoints ensure proper division and prevent errors.

• 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.

• The shortest phase of the cell cycle is typically the M phase.

• If a cell does not undergo division after G1, it enters the G0 phase, a resting state.

• Checkpoints monitor for errors; if detected, the protein p53 can trigger cell repair or apoptosis (cell death).

Equation:

$\text{Cell Cycle:} \quad G1 \rightarrow S \rightarrow G2 \rightarrow M$

Cancer Cells

Cancerous cells differ from normal cells in several ways, often due to uncontrolled cell division.

• High ratio of nucleus to cytoplasm.

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

• May have more than one nucleus.

Example: Cancer cells often bypass cell cycle checkpoints, leading to tumor formation.

Genetics

Mendelian Laws and Alleles

Genetics studies how traits are inherited through genes and alleles.

• Allele: Different forms of a gene.

• Dominant allele: Expressed in heterozygous condition.

• Recessive allele: Not expressed in the presence of a dominant allele.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Alleles of different genes are sorted into gametes independently.

Example: In pea plants, the allele for tallness (T) is dominant over shortness (t).

Genetic Diseases and Inheritance

Some diseases are caused by mutated alleles. If the allele is recessive, disease manifests in individuals who are homozygous for the mutation.

• Hemophilia is an X-linked recessive disorder; females can only have hemophilia if the father is affected and the mother is at least a carrier.

• To determine if a trait (e.g., curly hair in dogs) is dominant or recessive, test crosses can be performed with individuals of known genotype.

Example: Cystic fibrosis is a recessive genetic disorder; only individuals with two mutated alleles show symptoms.

Molecular Biology

DNA Structure and Mutations

DNA is the hereditary material in cells, composed of nucleotides forming a double helix.

• Photo 51 by Rosalind Franklin revealed the double helix structure of DNA using X-ray diffraction.

• Substitution mutation: One nucleotide is replaced by another in the DNA sequence.

• The total amount of DNA in a single complete genome is referred to as genome size.

Example: Sickle cell anemia is caused by a substitution mutation in the hemoglobin gene.

Translation and Protein Modification

Translation is the process by which proteins are synthesized from mRNA templates.

• Key components: mRNA, tRNA, and ribosomes.

• Posttranslational modifications include acetylation, methylation, and ubiquitination; RNA splicing is not a protein modification but a pre-mRNA processing event.

Equation:

$\text{Protein Synthesis:} \quad \text{DNA} \rightarrow \text{mRNA} \rightarrow \text{Protein}$

DNA Fingerprinting and Genetic Engineering

DNA fingerprinting is a technique used for identification and research, but it raises privacy concerns if data is not protected.

• Genetically modified (GM) foods are created using genetic engineering; advantages include improved nutrients, better yield, and resistance to pesticides.

• Potential drawbacks: "unexpected effects" and health risks.

Summary Table: Cell Division Comparison

Process	Number of Daughter Cells	Genetic Identity	Chromosome Number
Mitosis	2	Identical	Diploid (2n)
Meiosis	4	Different	Haploid (n)

Summary Table: Mendelian Laws

Law	Description
Law of Segregation	Alleles separate during gamete formation
Law of Independent Assortment	Alleles of different genes assort independently

Additional info: Some explanations and context have been expanded for clarity and completeness, including definitions and examples relevant to General Biology.