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

Cell Division: Mitosis and Meiosis

Chromosome Configurations During Cell Division

Cell division involves the precise distribution of chromosomes to daughter cells. Mitosis and meiosis are two major types of cell division in eukaryotes, each with distinct chromosome configurations at different stages.

• Prophase of Mitosis: Chromosomes condense and become visible as distinct structures. Each consists of two sister chromatids joined at the centromere.

• Telophase of Mitosis: Chromosomes decondense in daughter cells, each receiving an identical set.

• Meiosis I: Homologous chromosomes pair and separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

Example: In humans, a diploid cell (2n = 46) will have 46 chromosomes at prophase of mitosis, 23 chromosomes in each daughter cell after meiosis II.

Chromosome Number in Human Cells

The number of chromosomes varies depending on the stage of the cell cycle and type of division.

• Interphase: Human somatic cells have 46 chromosomes (diploid).

• Metaphase I of Meiosis: 46 chromosomes (homologous pairs aligned).

• Metaphase II of Meiosis: 23 chromosomes (haploid, after homologs separate).

Comparison of Mitosis and Meiosis

Mitosis and meiosis differ in their outcomes and processes.

• Mitosis: Produces two genetically identical diploid cells.

• Meiosis: Produces four genetically unique haploid cells.

• Homologous Chromosome Pairing: Occurs in meiosis I, not in mitosis.

• Sister Chromatid Separation: Occurs in anaphase of mitosis and meiosis II.

Genetics: Mendelian Inheritance and Human Disorders

Sickle Cell Anemia: Inheritance Patterns

Sickle cell anemia is a recessive genetic disorder affecting hemoglobin. The wild-type allele (HBA) is dominant over the mutant allele (HBS).

• Genotype: The genetic makeup (e.g., HBA/HBA, HBA/HBS, HBS/HBS).

• Phenotype: The observable trait (e.g., normal, carrier, affected).

• Punnett Square: Used to predict offspring genotypes and phenotypes.

Example: If both parents are carriers (HBA/HBS), their child has a 25% chance of being affected (HBS/HBS).

Blood Type Inheritance

Blood type is determined by multiple alleles (IA, IB, i) and shows codominance and simple dominance.

• Type A: IAIA or IAi

• Type B: IBIB or IBi

• Type AB: IAIB

• Type O: ii

Example: A type A woman (IAIA or IAi) and a type B man (IBIB or IBi) can have children with type A, B, AB, or O blood, depending on their genotypes.

Molecular Biology: DNA Structure and Function

DNA Structure and Nucleotides

DNA is a double helix composed of nucleotides, each containing a deoxyribose sugar, phosphate group, and nitrogenous base.

• Nucleotide Components: Deoxyribose sugar, phosphate group, nitrogenous base (A, T, C, G).

• Covalent Bonds: Phosphodiester bonds link nucleotides in a strand.

• Hydrogen Bonds: Hold complementary bases together (A-T: 2 bonds, C-G: 3 bonds).

Example: The base pairing rule: $\text{A} = \text{T}$ and $\text{C} = \text{G}$ in double-stranded DNA.

Chargaff's Rules and DNA Discoveries

Chargaff discovered that the amount of adenine equals thymine, and cytosine equals guanine in DNA. Rosalind Franklin used X-ray diffraction to reveal the helical structure of DNA.

• Chargaff's Rule: $\%A = \%T$, $\%C = \%G$

• Franklin: Provided evidence for the double helix.

• Avery, MacLeod, McCarty: Demonstrated DNA is the genetic material.

DNA Replication

DNA replication is semi-conservative, meaning each new DNA molecule contains one old strand and one new strand.

• Enzymes: DNA polymerase adds complementary bases; DNA helicase unwinds the helix.

• Process: Each strand serves as a template for a new strand.

Equation: $\text{New DNA} = \text{Old Strand} + \text{New Strand}$

Gene Expression: Transcription and Translation

Transcription

Transcription is the process by which RNA is synthesized from a DNA template.

• Enzyme: RNA polymerase

• Product: mRNA (messenger RNA)

• Direction: 5' to 3'

Translation

Translation is the synthesis of proteins from mRNA, involving three types of RNA.

• mRNA: Carries genetic code from DNA.

• tRNA: Brings amino acids to the ribosome; has anticodon complementary to mRNA codon.

• rRNA: Forms the core of the ribosome and catalyzes peptide bond formation.

Example: Codon table is used to translate mRNA sequence into amino acids.

Cancer Genetics: p53 and Li-Fraumeni Syndrome

p53 Gene and Cancer

The p53 gene is a tumor suppressor that regulates the cell cycle and prevents cancer. Mutations in p53 are associated with many cancers.

• Function: Induces cell cycle arrest or apoptosis in response to DNA damage.

• Li-Fraumeni Syndrome: Inherited mutation in p53 leads to increased cancer risk in multiple tissues.

Pedigree Analysis

Pedigrees are used to track inheritance of genetic disorders in families.

• Symbols: Squares (males), circles (females), shaded (affected), diagonal line (deceased).

• Interpretation: Multiple cancer types in a pedigree suggest a mutation in a gene affecting cell cycle regulation (e.g., p53).

Tables

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Daughter Cells	2	4
Genetic Identity	Identical	Unique
Chromosome Number	Diploid	Haploid

DNA Base Percentages Example

Base	Percentage
Cytosine	10%
Guanine	10%
Adenine	40%
Thymine	40%

Genotype and Phenotype Ratios (Sickle Cell Example)

Genotype	Phenotype	Ratio
HBA/HBA	Normal	1
HBA/HBS	Carrier	2
HBS/HBS	Affected	1

Key Terms and Definitions

• Diploid (2n): Cell with two sets of chromosomes.

• Haploid (n): Cell with one set of chromosomes.

• Genotype: Genetic makeup of an organism.

• Phenotype: Observable traits of an organism.

• Allele: Alternative form of a gene.

• Codon: Three-nucleotide sequence in mRNA that codes for an amino acid.

• Anticodon: Three-nucleotide sequence in tRNA complementary to mRNA codon.

• Phosphodiester Bond: Covalent bond linking nucleotides in DNA/RNA.

• Pedigree: Diagram showing inheritance patterns in a family.

Formulas and Equations

• Base Pairing Rule: $\%A = \%T$, $\%C = \%G$

• Genotype Ratio (Heterozygous Cross): $1:2:1$ (homozygous dominant : heterozygous : homozygous recessive)

• Phenotype Ratio (Heterozygous Cross): $3:1$ (dominant : recessive)

Additional info:

• Some questions reference pedigree analysis and cancer genetics, which are advanced but relevant topics in general biology.

• Questions on DNA structure, replication, and gene expression are foundational for molecular biology.

• Blood type inheritance and sickle cell anemia illustrate Mendelian genetics and human disease.