Cell Division, Chromosomes, and Inheritance: Study Notes for General Biology

5.1 Cell Division Provides for Reproduction, Growth, and Repair

Cell Theory and Cell Division

Cell division is a fundamental biological process that enables organisms to grow, repair damage, and reproduce. It underlies the formation of gametes, development from embryo to adult, and maintenance of bodily tissues.

• All living things are made of cells.

• All cells come from preexisting cells.

• Cell division allows organisms to grow, repair damage, and reproduce.

Sexual Reproduction

• Involves two parents (male and female).

• Gametes (sperm and egg) are produced in the gonads through cell division.

• Fertilization joins male and female gametes to form a zygote containing chromosomes from each parent.

• Humans produce many rounds of cell division to form an embryo, then a fully developed adult.

• Growth and repair continue throughout life as new cells replace old or damaged ones.

• Offspring are genetically unique (mix of both parents' DNA).

Asexual Reproduction

• Involves one parent only—no sperm or egg.

• Offspring are genetically identical to the parent.

• Common in unicellular organisms, some plants, and animals.

Examples of Asexual Reproduction

1. Binary Fission: Single-celled organisms (e.g., Amoeba proteus) divide into two identical offspring.

2. Plant Asexual Reproduction:

   • Sprouting: e.g., potatoes growing new plants from "eyes".

   • Runners: e.g., strawberry plants sending out horizontal stems to grow new plants.

3. Regeneration: Some animals can regrow body parts or even regenerate as entire organisms from a part.

5.2 Chromosomes Are Associations of DNA and Protein

DNA and Chromosomes

Chromosomes are structures within the nucleus of eukaryotic cells that contain DNA and proteins. They carry genetic information from one generation to the next.

• All life forms use DNA to store and share information.

• Each chromosome is a long piece of DNA combined with proteins called chromatin.

• During cell division, chromatin is condensed into visible chromosomes.

Chromosome Number

• All eukaryotic cells have a nucleus where chromosomes are stored.

• Each species has a specific, consistent number of chromosomes in its cells.

• Example: Human body cells contain 46 chromosomes (23 pairs).

• Members of the same species have nearly identical chromosomes.

• The number of chromosomes does not indicate an organism’s size or complexity.

• The DNA of two humans is about 99.5% identical; only about 0.5% accounts for unique traits.

Chromosome Structure

• Each chromosome is a long DNA molecule combined with proteins for organization.

• The combination of DNA and protein is called chromatin.

• Before cell division, chromosomes duplicate and condense, forming two sister chromatids joined at a centromere.

5.3 Cells Have Regular Cycles of Growth and Division

The Cell Cycle

The cell cycle is the ordered sequence of events from the creation of a cell to its division into two daughter cells. It consists of two main stages: interphase and the mitotic phase.

• Interphase: Cell grows, performs normal functions, and duplicates its chromosomes.

• Mitotic Phase: Cell divides its nucleus and cytoplasm, forming two genetically identical cells.

Interphase

• Makes up about 90% of the cell cycle.

• Cell performs normal life functions (e.g., producing digestive enzymes).

• Cell grows in size, builds organelles, and duplicates chromosomes.

• As the cell prepares to divide, it duplicates its chromosomes within the nucleus.

Chromosome Duplication

• Occurs near the end of interphase.

• Each chromosome is copied, forming two sister chromatids joined at the centromere.

5.4 During Mitosis, the Nucleus of the Cell Divides

Mitosis and Cytokinesis

Mitosis is the process by which the duplicated chromosomes are organized, aligned, and separated into two new nuclei. Cytokinesis divides the cytoplasm, resulting in two genetically identical cells.

• Mitosis: The nucleus divides, and duplicated chromosomes are distributed evenly into two nuclei.

• Cytokinesis: The cytoplasm divides, separating the cell into two distinct offspring cells.

Result: Two genetically identical cells, each with a complete set of chromosomes.

5.5 During Cytokinesis, the Cell is Split into Two

Cytokinesis in Animal and Plant Cells

• In animal cells, cytokinesis occurs through a process called cleavage:

  • Cleavage furrow forms, pinching the cell into two.

  • Ring of protein filaments contracts, dividing the cytoplasm.

• In plant cells, a cell plate forms along the center line of the cell:

  • Vesicles containing membrane and cell wall materials collect in the middle.

  • New cell wall separates the two offspring cells.

5.6 Nuclear Transfer Can Be Used to Produce Clones

Cloning and Nuclear Transplantation

Cloning produces genetically identical offspring from a single parent. Nuclear transplantation is used in animal cloning and involves transferring the nucleus from a donor cell into an egg cell.

• Cloning can be applied to plants, animals, and medical research.

• Therapeutic cloning uses stem cells to treat diseases and injuries.

Steps in Animal Cloning (Nuclear Transplantation)

1. Nucleus is removed from an adult donor cell.

2. Nucleus is injected into an egg cell that has had its own nucleus removed.

3. Egg is stimulated to divide and grow into an embryo.

4. Embryo may develop into a complete organism (clone).

Plant Cloning

• Small pieces of plant tissue or single cells are placed in a growth medium.

• Cells divide and grow into new plants genetically identical to the parent.

Reproductive Cloning

• Embryo is placed into the uterus of a surrogate mother to complete development.

• Example: Dolly the sheep (first cloned adult mammal, 1997).

5.7 Gametes Have Half as Many Chromosomes as Body Cells

Sexual Reproduction and Genetic Variety

• Gametes (sperm and egg) are haploid (n), containing half the chromosomes of body cells (diploid, 2n).

• Fertilization restores the diploid number in the zygote.

The Human Life Cycle

1. Adults: Every somatic (body) cell is diploid (2n = 46 chromosomes).

2. Gamete Formation: Testes produce sperm; ovaries produce eggs. Each gamete has 23 chromosomes (haploid).

3. Fertilization: Sperm and egg fuse, forming a diploid zygote (46 chromosomes).

4. Zygote and Development: Zygote divides repeatedly, forming a diploid embryo, then a diploid adult.

An Inventory of Chromosomes (Karyotype)

• A karyotype is a photographic inventory of all chromosomes in a cell.

• Each chromosome appears as two sister chromatids joined at a centromere.

• Chromosomes exist in matching pairs called homologous chromosomes.

5.8 Meiosis Produces Gametes

Overview of Meiosis

Meiosis is a type of cell division that produces haploid gametes from diploid cells. It involves two rounds of division, resulting in four unique haploid cells.

• Purpose: To produce haploid gametes for sexual reproduction.

• Key difference from mitosis: Mitosis produces two identical diploid cells; meiosis produces four unique haploid cells.

Haploid vs. Diploid Cells

• Diploid (2n): Two sets of chromosomes (e.g., human body cells: 46 chromosomes).

• Haploid (n): One set of chromosomes (e.g., human gametes: 23 chromosomes).

5.9 Mitosis and Meiosis: Similarities and Differences

Comparison of Mitosis and Meiosis

• Both involve duplication and distribution of chromosomes.

• Mitosis: One cell division → 2 identical diploid cells.

• Meiosis: Two cell divisions → 4 unique haploid cells.

Summary Table: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Cells Produced	2	4
Genetic Identity	Identical	Unique
Chromosome Number	Diploid (2n)	Haploid (n)
Function	Growth, repair, asexual reproduction	Sexual reproduction

5.10 Several Processes Produce Genetic Variation Among Sexually Reproducing Organisms

Sources of Genetic Variation

1. Independent Assortment: Homologous chromosomes line up randomly during meiosis I, creating new combinations.

2. Random Fertilization: Any sperm can fertilize any egg, multiplying possible combinations.

3. Crossing Over (Recombination): Homologous chromosomes exchange segments during meiosis I, creating hybrid chromosomes.

Summary Table: Sources of Variation

Source of Variation	When It Occurs	How It Increases Diversity
Independent Assortment	Meiosis I	Random alignment of chromosomes
Crossing Over	Meiosis I	Exchange of DNA between homologous chromosomes
Random Fertilization	After Meiosis (Fertilization)	Combines DNA from two parents

5.11 Mistakes During Meiosis Can Produce Gametes with Abnormal Numbers of Chromosomes

Nondisjunction and Chromosome Abnormalities

• Nondisjunction occurs when chromosomes fail to separate properly during meiosis.

• Results in gametes with abnormal chromosome numbers (e.g., n + 1 or n – 1).

• Fertilization with abnormal gametes can produce zygotes with extra or missing chromosomes.

• Example: Trisomy 21 (Down syndrome) – three copies of chromosome 21.

• Sex chromosome abnormalities include Turner syndrome (XO) and Klinefelter syndrome (XXY).

5.12 Mendel Deduced the Basic Principles of Genetics by Breeding Pea Plants

Heredity and Genetics

• Heredity: Transmission of traits from one generation to the next.

• Genetics: Scientific study of heredity.

• Founded by Gregor Mendel (1822–1884) using pea plants.

Characters and Traits

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

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

Alleles

• Alleles: Different versions of a gene.

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

• Heterozygous: Two different alleles (Aa).

Dominant vs. Recessive Alleles

• Dominant allele (uppercase letter) determines appearance.

• Recessive allele (lowercase letter) has no visible effect unless both alleles are recessive.

5.13 A Punnett Square Can Be Used to Predict the Results of a Genetic Cross

Punnett Squares and Genetic Crosses

• Punnett squares predict the probability of offspring genotypes and phenotypes.

• Monohybrid cross: Parents heterozygous for one trait (e.g., Bb x Bb).

• Testcross: Used to determine unknown genotype by crossing with a homozygous recessive.

Law of Segregation

• During meiosis, paired alleles separate into different gametes.

• Each gamete gets only one allele from each pair.

5.14 Mendel’s Law of Independent Assortment Accounts for the Inheritance of Multiple Traits

Dihybrid Crosses

• Involves two traits (e.g., seed color and seed shape).

• Law of independent assortment: Inheritance of one trait does not affect the inheritance of another.

• Produces four kinds of gametes and varied offspring combinations.

5.15 Pedigrees Can Be Used to Trace Traits in Human Families

Pedigree Analysis

• Pedigree: Family genetic history chart showing inheritance patterns.

• Used to track inheritance and determine genotypes.

• Symbols: Square = male, Circle = female, shaded = expresses trait, half-shaded = carrier.

• Most genetic disorders are recessive; carriers have one normal and one disease allele.

5.16 The Inheritance of Many Traits is More Complex Than Mendel’s Laws

Complex Patterns of Inheritance

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

• Multiple alleles and codominance: More than two alleles exist (e.g., human blood type).

• Pleiotropy: One gene affects multiple traits (e.g., sickle-cell disease).

• Polygenic inheritance: Many genes influence one trait (e.g., height, skin color).

• Environment vs. genetics: Some traits are influenced by both genes and environment.

5.17 Linked Genes May Not Obey the Law of Independent Assortment

Linked Genes and Genetic Recombination

• Genes located close together on the same chromosome are inherited together (linked genes).

• Crossing over during meiosis can shuffle linked genes, creating new combinations.

• Recombination frequency gives clues to gene distance on chromosomes.

5.18 Sex-Linked Genes Display Unusual Inheritance Patterns

Sex Chromosomes and Sex-Linked Genes

• Humans have 44 autosomes and 2 sex chromosomes (XX or XY).

• Genes on the X chromosome (not Y) are X-linked.

• X-linked recessive traits are more common in males (e.g., color blindness, hemophilia).

• Females can be carriers; males with the allele express the trait.

Key Takeaways: Sex is genetically determined by X and Y chromosomes. Sex-linked traits follow unique inheritance patterns due to differences in male and female chromosome composition.