Cellular Reproduction: Cells from Cells

Purpose of Cell Division

• Growth: Cell division allows multicellular organisms to grow by increasing the number of cells.

• Repair: Damaged or dead cells are replaced through cell division.

• Reproduction: Cell division is essential for both asexual and sexual reproduction.

Asexual vs. Sexual Reproduction

• Asexual Reproduction: Offspring are genetically identical to the parent; involves a single parent (e.g., binary fission in bacteria, mitosis in plants).

• Sexual Reproduction: Offspring inherit a combination of genes from two parents, resulting in genetic variation.

Structure of Chromosomes

• DNA: The molecule that stores genetic information.

• Chromatid: Each of the two identical halves of a duplicated chromosome.

• Sister Chromatids: Two identical chromatids joined at a region called the centromere.

• Centromere: The region where sister chromatids are attached.

The Cell Cycle

• G1 Phase: Cell grows and carries out normal functions.

• S Phase: DNA is replicated.

• G2 Phase: Cell prepares for division.

• M Phase (Mitosis): Division of the nucleus and cytoplasm.

Stages of Mitosis

• Prophase: Chromosomes condense, spindle forms, nuclear envelope breaks down.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids separate and move to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes decondense.

Control of the Cell Cycle and Cancer

• Cell Cycle Control: Regulated by checkpoints and proteins (e.g., cyclins).

• Cancer: Uncontrolled cell division due to loss of cell cycle control.

• Benign Tumors: Non-cancerous, do not spread.

• Malignant Tumors: Cancerous, can invade other tissues (metastasis).

• Metastasis: Spread of cancer cells to other parts of the body.

Diploid vs. Haploid Cells

• Diploid (2n): Cells with two sets of chromosomes (e.g., somatic cells).

• Haploid (n): Cells with one set of chromosomes (e.g., gametes).

Mitosis vs. Meiosis

Genetic Variation in Meiosis

• Independent Assortment: Random distribution of homologous chromosomes during meiosis I.

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I.

Nondisjunction

• Definition: Failure of chromosomes to separate properly during meiosis.

• Result: Gametes with abnormal chromosome numbers (e.g., trisomy 21 causes Down syndrome).

Patterns of Inheritance

Mendel’s Experiments and Principles

• Gregor Mendel: Studied inheritance using pea plants.

• Principles: Traits are inherited as discrete units (genes); established laws of segregation and independent assortment.

Key Genetic Terms

• Gene: A segment of DNA that codes for a trait.

• Allele: Different forms of a gene.

• Dominant Allele: Expressed if present (represented by uppercase letter).

• Recessive Allele: Expressed only if both alleles are recessive (lowercase letter).

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

• Heterozygous: Two different alleles (e.g., Aa).

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

• Phenotype: Observable traits (e.g., purple flowers).

Mendel’s Law of Segregation

• Alleles separate during gamete formation and reunite during fertilization.

Punnett Squares

• Used to predict possible genotypes and phenotypes of offspring from genetic crosses.

• Example: Crossing Aa x Aa yields genotypes AA, Aa, and aa.

Mendel’s Law of Independent Assortment

• Inheritance of one trait is generally independent of another trait (applies to genes on different chromosomes).

Pedigrees

• Diagrams used to study inheritance patterns in families.

• Can identify carriers of genetic disorders.

Genetic Disorders

• Recessive Disorders: Require two copies of the mutant allele (e.g., cystic fibrosis).

• Dominant Disorders: Require only one copy of the mutant allele (e.g., Huntington's disease).

• Carrier: Heterozygous individual who carries one recessive allele but does not show symptoms.

Exceptions to Mendelian Inheritance

• Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).

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

• Polygenic Inheritance: Multiple genes affect a single trait (e.g., skin color).

• Sex-linked Inheritance: Genes located on sex chromosomes (e.g., color blindness).

The Structure and Function of DNA

DNA Structure

• Nucleotides: Building blocks of DNA, each with a phosphate group, deoxyribose sugar, and a nitrogenous base.

• Sugar-Phosphate Backbone: Alternating sugars and phosphates form the sides of the DNA ladder.

• Nitrogenous Bases: Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

DNA Double Helix and Base-Pairing Rules

• DNA is a double helix with two strands held together by hydrogen bonds between bases.

• Base-Pairing: A pairs with T, C pairs with G.

DNA vs. RNA

DNA Replication

• DNA makes a copy of itself before cell division.

• DNA Polymerase: Enzyme that adds complementary nucleotides to the new strand.

• Complementary Base Pairing: Ensures accurate copying of genetic information.

Central Dogma: DNA → RNA → Protein

• Transcription: DNA is used as a template to make messenger RNA (mRNA).

• Translation: mRNA is decoded by ribosomes to build a protein.

Genetic Code and Codons

• Codon: A sequence of three mRNA bases that codes for a specific amino acid.

• The genetic code is universal and redundant.

Roles in Protein Synthesis

• mRNA: Carries genetic information from DNA to ribosomes.

• tRNA: Brings amino acids to the ribosome during translation.

• Ribosomes: Sites of protein synthesis.

Mutations

• Mutation: Any change in the DNA sequence.

• Substitution: One base is replaced by another.

• Insertion: Extra base is added.

• Deletion: Base is removed.

• Frameshift Mutation: Insertion or deletion that shifts the reading frame, often altering the entire protein sequence.

• Effects: Can be silent, harmful, or beneficial depending on the change.