BackBIOL-141 Exam 3 Study Guide: Cell Division, Inheritance, and DNA Structure
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Cell Division: Mitosis and Meiosis
Why Organisms Undergo Mitosis vs. Meiosis
Mitosis is the process by which somatic (body) cells divide to produce two genetically identical daughter cells. It is essential for growth, repair, and asexual reproduction.
Meiosis is the process by which gametes (sperm and egg cells) are produced. It reduces the chromosome number by half, resulting in genetically unique haploid cells, which is crucial for sexual reproduction and genetic diversity.
Chromosome Structure and Terminology
Sister chromatids: Identical copies of a chromosome connected by a centromere, formed during DNA replication.
Kinetochore: Protein structure on the centromere where spindle fibers attach during cell division.
Centromere: Region where sister chromatids are joined and spindle fibers attach.
Spindle fibers: Microtubule structures that separate chromosomes during cell division.
Homologous chromosomes: Chromosome pairs (one from each parent) that are similar in shape, size, and gene content but may carry different alleles.
Genetic Variation in Reproduction
Asexual reproduction (e.g., mitosis) produces genetically identical offspring; no genetic variation except by mutation.
Sexual reproduction (e.g., meiosis) introduces genetic variation through independent assortment, crossing over, and random fertilization.
Evolutionary advantage: Genetic variation increases the ability of populations to adapt to changing environments.
Cell Cycle and Mitosis
Interphase consists of three phases:
G1 phase: Cell growth and normal functions.
S phase: DNA replication (synthesis of sister chromatids).
G2 phase: Preparation for mitosis; further growth and organelle duplication.
Chronological steps of mitosis:
Prophase: Chromosomes condense, spindle forms.
Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.
Metaphase: Chromosomes align at the metaphase plate.
Anaphase: Sister chromatids separate and move to opposite poles.
Telophase: Nuclear envelopes reform, chromosomes decondense.
Cytokinesis: Division of the cytoplasm (in animals, involves actin filaments).
Role of microtubules:
Metaphase: Microtubules polymerize (grow) to align chromosomes.
Anaphase: Microtubules depolymerize (shrink) to pull chromatids apart.
Role of actin in cytokinesis (animals): Actin filaments form a contractile ring that pinches the cell into two daughter cells.
End products of mitosis in humans:
2 daughter cells
Diploid (2n)
Genetically identical to each other and to the parent cell
46 chromosomes each (in humans)
Cancer cells vs. normal cells: Cancer cells divide uncontrollably, ignore cell cycle checkpoints, and may not undergo apoptosis (programmed cell death).
Meiosis and Sexual Life Cycles
Haploid and Diploid Cells
Haploid (n): Cells with one set of chromosomes (e.g., gametes).
Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells).
Somatic cells: Body cells, diploid.
Gametes: Reproductive cells (sperm, egg), haploid.
Chromosome Number in Gametes and Somatic Cells
Gametes have half the number of chromosomes as somatic cells.
Example: Human somatic cell (2n = 46), gamete (n = 23).
Meiosis: Key Events
Haploid cells are produced at the end of meiosis II.
Separation of chromosomes:
Homologous chromosomes separate during meiosis I.
Sister chromatids separate during meiosis II.
Genetic variation arises from:
Crossing over (prophase I)
Independent assortment (metaphase I)
Random fertilization
End products of meiosis in humans (sperm):
4 daughter cells
Haploid (n)
Genetically unique
23 chromosomes each
Distinguishing mitosis and meiosis: Mitosis produces identical diploid cells; meiosis produces unique haploid cells.
Mendelian and Non-Mendelian Inheritance
Basic Genetic Terms
Dominant allele: Expressed when present (capital letter, e.g., A).
Recessive allele: Expressed only when two copies are present (lowercase, e.g., a).
Homozygous dominant: Two dominant alleles (AA).
Heterozygous: One dominant and one recessive allele (Aa).
Homozygous recessive: Two recessive alleles (aa).
Genotype: Genetic makeup (e.g., Aa).
Phenotype: Observable trait (e.g., tall or short).
Mendel’s Laws of Inheritance
Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.
Law of Independent Assortment: Genes for different traits assort independently during gamete formation (applies to genes on different chromosomes).
Alleles: Alternative versions of a gene.
Inheritance Patterns and Probability
Monohybrid cross: Cross between individuals heterozygous for a single gene.
Autosomal dominant disease: Only one dominant allele needed for trait/disease to be expressed.
Autosomal recessive disease: Two recessive alleles needed for trait/disease to be expressed.
X-linked recessive disease: Trait is carried on the X chromosome; more common in males.
Test cross: Cross between an individual with unknown genotype and a homozygous recessive individual to determine genotype.
Non-Mendelian Inheritance Patterns
Incomplete dominance: Heterozygote shows intermediate phenotype (e.g., red x white = pink).
Codominance: Both alleles are fully expressed (e.g., AB blood type).
Polygenic inheritance: Multiple genes affect a single trait (e.g., skin color).
Multifactorial inheritance: Traits influenced by genes and environment.
Blood Types and Pedigrees
Blood types: Determined by multiple alleles (A, B, O) and codominance (AB).
Pedigree analysis: Used to track inheritance patterns in families.
Identify genetic male (XY), genetic female (XX), presence/absence of trait.
Determine relationships and inheritance patterns (autosomal dominant, autosomal recessive, X-linked recessive).
Identify genotypes from pedigree information.
Karyotype analysis: Identify total chromosomes (2n), unique autosomes, gender, and gamete chromosome number.
X-linked recessive disorders: More common in males; females must inherit two copies to express the trait.
The Molecular Basis of Inheritance: DNA Structure and Replication
DNA Structure and Bonds
Hydrogen bonds occur between nitrogenous bases of the two DNA strands (A-T: 2 bonds, G-C: 3 bonds).
Hydrogen bonds allow the two strands to separate during replication.
Nucleotide Structure and DNA Polymerization
Nucleotide components: Phosphate group, deoxyribose sugar, nitrogenous base.
DNA polymerization forms phosphodiester bonds between the 3' OH of one nucleotide and the 5' phosphate of the next.
DNA strands have directionality: 5' to 3'.
Antiparallel and Semiconservative Replication
Antiparallel: The two DNA strands run in opposite directions (one 5'→3', the other 3'→5').
Semiconservative replication: Each new DNA molecule consists of one old strand and one new strand.
DNA Replication: Components and Process
Key enzymes and their functions:
Helicase: Unwinds the DNA double helix.
DNA polymerase: Synthesizes new DNA strands by adding nucleotides.
DNA ligase: Joins Okazaki fragments on the lagging strand.
Topoisomerase: Relieves tension ahead of the replication fork.
Primase: Synthesizes RNA primers needed to start DNA synthesis.
Nuclease: Removes RNA primers or damaged DNA.
Requirements for DNA replication: Template DNA, DNA polymerase, nucleotides (as nucleoside triphosphates), primers, and accessory proteins.
Nucleoside triphosphates provide energy for DNA synthesis as two phosphates are released during bond formation.
Leading strand: Synthesized continuously in the 5'→3' direction.
Lagging strand: Synthesized discontinuously as Okazaki fragments.
Prokaryotic vs. Eukaryotic DNA
Prokaryotic DNA: Circular, usually one chromosome, single origin of replication.
Eukaryotic DNA: Linear, multiple chromosomes, multiple origins of replication.
Histone Proteins and DNA Compaction
Histones: Positively charged proteins that DNA wraps around to form nucleosomes, compacting DNA into chromatin.
Histones interact with negatively charged DNA via ionic interactions.
Key Tables
Comparison of Mitosis and Meiosis
Feature | Mitosis | Meiosis |
|---|---|---|
Number of divisions | 1 | 2 |
Number of daughter cells | 2 | 4 |
Chromosome number in daughter cells | Diploid (2n) | Haploid (n) |
Genetic identity | Identical | Unique |
Role | Growth, repair, asexual reproduction | Sexual reproduction, genetic variation |
Key Enzymes in DNA Replication
Enzyme | Function |
|---|---|
Helicase | Unwinds DNA double helix |
DNA Polymerase | Synthesizes new DNA strands |
Primase | Creates RNA primers |
DNA Ligase | Joins Okazaki fragments |
Topoisomerase | Relieves supercoiling |
Nuclease | Removes RNA primers or damaged DNA |
Sample Punnett Square for Monohybrid Cross (Autosomal Recessive)
A | a | |
|---|---|---|
A | AA | Aa |
a | Aa | aa |
Key Equations
Probability of inheritance (monohybrid cross):
DNA Replication Directionality:
Additional info:
For exam preparation, practice identifying phases of mitosis and meiosis from diagrams, and interpreting pedigrees and karyotypes.
Review the differences between inheritance patterns and be able to apply them to real-world genetic problems.
