Cell Division, Genetics, and Inheritance: Study Guide Notes

The Cell Cycle and Cell Division

Purposes of Cell Division

Cell division is a fundamental process in all living organisms, serving different purposes in prokaryotes and eukaryotes.

• Prokaryotes (e.g., bacteria): Divide by binary fission for reproduction.

• Eukaryotes: Divide for growth, development, tissue repair, and reproduction (mitosis for somatic cells, meiosis for gametes).

• Key Differences: Prokaryotes lack a nucleus and have a single circular chromosome; eukaryotes have multiple linear chromosomes within a nucleus.

Key Terms in Cell Division

• Somatic cells: All body cells except gametes; diploid (2n).

• Gametes: Reproductive cells (sperm, egg); haploid (n).

• Genome: The complete set of genetic material in an organism.

• Chromosome: A DNA molecule with associated proteins, carrying genetic information.

• Chromatin: DNA-protein complex; less condensed form of genetic material.

• Duplicated chromosome: A chromosome after DNA replication, consisting of two sister chromatids.

• Sister chromatids: Identical copies of a chromosome, joined at the centromere.

• Centromere: Region where sister chromatids are attached.

Visual distinction: An unduplicated chromosome is a single DNA molecule; a duplicated chromosome consists of two sister chromatids.

Mitosis and Genetic Identity

Mitosis produces genetically identical daughter cells by equally distributing duplicated chromosomes.

• Before mitosis: Each chromosome is duplicated (DNA replication).

• During mitosis: Sister chromatids separate and are distributed to two nuclei.

• After mitosis: Each daughter cell has the same number and type of chromosomes as the parent cell.

Phases of the Cell Cycle

• Interphase: Includes G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).

• M phase: Mitosis (nuclear division) and cytokinesis (cytoplasm division).

DNA and Chromatid Changes:

• G1: Chromosomes unduplicated (one chromatid each).

• S: DNA replicates; chromosomes become duplicated (two sister chromatids each).

• G2: Chromosomes remain duplicated.

• Mitosis: Sister chromatids separate; each new cell receives unduplicated chromosomes.

Chromosome and DNA Content During the Cell Cycle

• Given a diploid number (e.g., 2n = 6):

• G1: 6 chromosomes, 6 DNA molecules.

• G2 (after S): 6 chromosomes (duplicated), 12 DNA molecules (6 pairs of sister chromatids).

• After mitosis: 6 chromosomes, 6 DNA molecules per cell.

Phases 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: Cytoplasm divides, forming two daughter cells.

Spindle Microtubules: Polymerization and depolymerization of microtubules drive chromosome movement.

Energetics and Spindle Dynamics

• Polymerization: Addition of tubulin subunits (endergonic, requires energy).

• Depolymerization: Removal of subunits (exergonic, releases energy).

• Protein Synthesis: Spindle proteins are synthesized in the cytoplasm and assembled during mitosis.

Consequences of Mitosis Inhibition

• Inhibiting spindle formation or function prevents chromosome segregation, leading to cell cycle arrest or abnormal cells.

Regulation of Cell Division

• Checkpoints: Control points (G1, G2, M) where cell cycle progression is regulated.

• G0 phase: Non-dividing state.

• Density-dependent inhibition: Cells stop dividing when crowded.

• Apoptosis: Programmed cell death.

Cancer and Cell Cycle Regulation

• Mutations in regulatory genes can lead to uncontrolled cell division (cancer).

• Cancer cells often ignore checkpoint signals and do not undergo apoptosis.

Sexual and Asexual Life Cycles

Asexual vs Sexual Reproduction

• Asexual reproduction: Offspring are genetically identical to the parent (mitosis).

• Sexual reproduction: Involves fusion of gametes (meiosis and fertilization), producing genetically diverse offspring.

Key Terms:

• Haploid (n): One set of chromosomes (gametes).

• Diploid (2n): Two sets of chromosomes (somatic cells, zygote).

• Zygote: Fertilized egg (diploid).

Homologous Chromosomes

• Pairs of chromosomes (one from each parent) with the same genes but possibly different alleles.

• Each homolog carries the same genes at the same loci, but alleles may differ.

Genes and Alleles

• Gene: A DNA segment coding for a trait.

• Allele: Different versions of a gene.

• Different alleles can result in different traits (e.g., flower color).

Karyotypes and Chromosome Structure

• Karyotype: Visual display of chromosomes.

• Key terms: homologous pair, duplicated chromosome, sister chromatids, gene, locus.

• Genetic content: Homologs have the same genes, may have different alleles; different chromosomes have different genes.

Sex Chromosomes vs Autosomes

• Autosomes: Non-sex chromosomes.

• Sex chromosomes: Determine sex (e.g., X and Y in humans).

• Humans: XX (female), XY (male).

Mitosis vs Meiosis

Comparison of Mitosis, Meiosis I, and Meiosis II

• Mitosis: Homologs do not pair; sister chromatids separate; produces two identical diploid cells.

• Meiosis I: Homologous chromosomes pair and separate; reduces chromosome number by half.

• Meiosis II: Sister chromatids separate; similar to mitosis but in haploid cells.

Chromosome and Chromatid Numbers:

• Given 2n = 6:

• After S phase: 6 chromosomes (duplicated), 12 chromatids.

• After meiosis I: 3 chromosomes (duplicated), 6 chromatids per cell.

• After meiosis II: 3 chromosomes (unduplicated), 3 chromatids per gamete.

Gene Copies: Mitosis: two copies per gene; meiosis: one copy per gene in gametes.

Genetic Similarity: Mitosis: identical cells; meiosis: genetically unique gametes.

Phases of Meiosis

• Prophase I: Homologs pair (synapsis), crossing over occurs.

• Metaphase I: Homologous pairs align at metaphase plate.

• Anaphase I: Homologs separate.

• Telophase I: Two haploid cells form.

• Meiosis II: Sister chromatids separate (similar to mitosis).

Genetic Variation in Meiosis

• Crossing Over: Occurs during prophase I between non-sister chromatids of homologous chromosomes; increases genetic diversity.

• Independent Assortment: Random orientation of homologous pairs during metaphase I; leads to different combinations of maternal and paternal chromosomes in gametes.

• Random Fertilization: Any sperm can fertilize any egg, further increasing genetic variation.

Mendelian Genetics and Inheritance

Characters, Traits, Genes, and Alleles

• Character: A heritable feature (e.g., flower color).

• Trait: Variant of a character (e.g., purple or white flowers).

• Gene: DNA sequence coding for a character.

• Allele: Variant form of a gene.

Genetic Crosses and Terminology

• True-breeding: Organisms that produce offspring identical to themselves.

• Hybridization: Mating of two different true-breeding varieties.

• P generation: Parental generation.

• F1 generation: First filial generation (offspring of P).

• F2 generation: Second filial generation (offspring of F1).

• Genotype: Genetic makeup (e.g., AA, Aa, aa).

• Phenotype: Observable trait.

• Heterozygous: Two different alleles (Aa).

• Homozygous: Two identical alleles (AA or aa).

• Dominant: Allele expressed in phenotype when present.

• Recessive: Allele masked in heterozygote.

• Law of Segregation: Alleles separate during gamete formation.

Genotype to Phenotype

• Genotype determines phenotype via gene expression (transcription and translation).

• Dominant alleles produce functional proteins; recessive alleles may result in nonfunctional proteins.

Monohybrid and Dihybrid Crosses

• Identify parent genotypes and possible gametes.

• Use Punnett squares to predict offspring genotypes and phenotypes.

Punnett Square Example (Monohybrid Cross):

	A	a
A	AA	Aa
a	Aa	aa

Independent Assortment and Dihybrid Crosses

• Genes on different chromosomes assort independently during meiosis.

• Punnett squares for two genes predict genotype and phenotype ratios.

Linked Genes and Crossing Over

• Linked genes are on the same chromosome and tend to be inherited together.

• Crossing over can separate linked genes, producing recombinant gametes.

• The closer two genes are, the less likely crossing over will occur between them.

Gamete Genotypes for Dihybrid Individual (AaBb):

Scenario	Possible Gametes
Unlinked genes	AB, Ab, aB, ab
Linked, no crossing over	AB, ab
Linked, with crossing over	AB, ab, Ab, aB (recombinants less frequent)

Dominance Patterns and Other Inheritance Modes

• Complete dominance: One allele completely masks the other.

• Incomplete dominance: Heterozygote shows intermediate phenotype (e.g., red x white = pink).

• Codominance: Both alleles are fully expressed (e.g., AB blood type).

• Multiple alleles: More than two alleles exist for a gene (e.g., ABO blood group).

• Pleiotropy: One gene affects multiple traits.

• Epistasis: One gene affects the expression of another gene.

• Polygenic inheritance: Multiple genes contribute to a single trait (e.g., skin color).

Genetics and Environment

• Both genetic makeup (genotype) and environment influence phenotype (e.g., nutrition affects height).

Pedigree Analysis

• Pedigrees trace inheritance of traits in families.

• Used to infer inheritance patterns, genotypes, and predict probabilities of traits in offspring.

Sex-Linked and Autosomal Inheritance

Sex-Linked vs Autosomal Genes

• Autosomal genes: Located on non-sex chromosomes; both sexes equally affected.

• Sex-linked genes: Usually on X chromosome; males (XY) more likely to express X-linked recessive traits.

• Females (XX) need two copies of recessive allele to express trait; males need only one.

Solving Sex-Linked Genetics Problems

• Predict genotype and phenotype ratios for male and female offspring.

• Infer parent genotypes from offspring data.

Pedigrees for Sex-Linked Traits

• Analyze inheritance patterns to determine if a trait is sex-linked.

• Predict likelihood of genotypes and phenotypes in family members.

Additional info: These notes expand on the study guide objectives by providing definitions, explanations, and examples for each concept, ensuring a comprehensive review for exam preparation.