Cell Division, Inheritance, and Gene Expression: Study Guide

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, tissue 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.

Key Terms: Sister Chromatids and Homologous Chromosomes

• Sister chromatids are two identical copies of a single replicated chromosome, connected by a centromere. They are genetically identical.

• Homologous chromosomes are pairs of chromosomes (one from each parent) that have the same genes at the same loci but may have different alleles. They are not identical but are similar in structure and gene content.

End Products of Mitosis in Humans

• Number of daughter cells: 2

• Ploidy: Diploid (2n)

• Genetic relationship: Genetically identical to each other and to the parent cell

• Chromosome number: 46 chromosomes per cell (in humans)

Meiosis and Sexual Life Cycles

Gametes vs. Somatic Cells

• Gametes are reproductive cells (sperm and eggs) that are haploid (n), containing one set of chromosomes.

• Somatic cells are all other body cells and are diploid (2n), containing two sets of chromosomes.

• Example: In humans, somatic cells have 46 chromosomes (2n), while gametes have 23 chromosomes (n).

Separation of Chromosomes During Meiosis and Mitosis

• In meiosis I, homologous chromosomes separate.

• In meiosis II, sister chromatids separate.

• In mitosis, sister chromatids separate (no homologous chromosome separation).

Genetic Variation: Crossing Over and Independent Assortment

• Crossing over is the exchange of genetic material between homologous chromosomes during prophase I of meiosis, increasing genetic diversity.

• Independent assortment refers to the random orientation of homologous chromosome pairs during metaphase I, leading to varied combinations of chromosomes in gametes.

End Products of Meiosis in Humans (Sperm)

• Number of daughter cells: 4

• Ploidy: Haploid (n)

• Genetic relationship: Genetically unique due to crossing over and independent assortment

• Chromosome number: 23 chromosomes per cell (in humans)

Mendelian Genetics and Chromosomal Inheritance

Dominant and Recessive Traits

• Dominant allele: Expressed in the phenotype even if only one copy is present (represented by a capital letter, e.g., A).

• Recessive allele: Expressed only when two copies are present (represented by a lowercase letter, e.g., a).

Genotype and Phenotype

• Genotype: The genetic makeup of an organism (e.g., AA, Aa, or aa).

• Phenotype: The observable traits or characteristics (e.g., brown eyes, blue eyes).

Homozygous and Heterozygous

• Homozygous dominant: Two dominant alleles (AA)

• Heterozygous: One dominant and one recessive allele (Aa)

• Homozygous recessive: Two recessive alleles (aa)

Monohybrid Crosses and Probability of Inheritance

• Monohybrid cross (autosomal dominant): Cross between two individuals for a single gene with dominant inheritance.

• Monohybrid cross (autosomal recessive): Cross between two individuals for a single gene with recessive inheritance.

• Punnett square is used to calculate probabilities.

Blood Type Inheritance

• Blood types are determined by multiple alleles (A, B, O) and show codominance (A and B).

• Possible genotypes and phenotypes:

  • IAIA or IAi: Type A

  • IBIB or IBi: Type B

  • IAIB: Type AB

  • ii: Type O

Pedigree Analysis

• Autosomal dominant: Trait appears in every generation; affected individuals have at least one affected parent.

• Autosomal recessive: Trait can skip generations; affected individuals may have unaffected parents.

• X-linked recessive: More common in males; affected males often have carrier mothers.

Molecular Basis of Inheritance

Semi-Conservative DNA Replication

• Each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

• Purpose: To accurately copy genetic information for cell division.

Recognizing Complementary DNA Sequences

• Base pairing rules: Adenine (A) pairs with Thymine (T), Cytosine (C) pairs with Guanine (G).

• Example: If the template strand is 5'-ATCG-3', the complementary strand is 3'-TAGC-5'.

Gene Expression: From Gene to Protein

Genes and Genome

• Gene: A segment of DNA that codes for a functional product (usually a protein).

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

Order of Steps in Gene Expression

1. Transcription (DNA to RNA)

2. RNA processing (in eukaryotes)

3. Translation (RNA to protein)

Redundancy in Codons

• Multiple codons can code for the same amino acid, which is called redundancy or degeneracy of the genetic code.

Transcription: Process Overview

• Starting product: DNA template

• Ending product: Messenger RNA (mRNA)

• Main components: RNA polymerase, promoter region, transcription factors (in eukaryotes)

Translation: Process Overview

• Starting product: mRNA

• Ending product: Polypeptide (protein)

• Main components: Ribosome, transfer RNA (tRNA), amino acids

Mutations and Their Impact on Proteins

• Mutations in DNA can alter the mRNA sequence, which may change the amino acid sequence of the resulting protein.

• Types of mutations:

  • Silent mutation: No change in amino acid

  • Missense mutation: Changes one amino acid

  • Nonsense mutation: Introduces a stop codon, truncating the protein

• Use a codon dictionary to determine the effect of a specific mutation.