Biology Chapter on Genetics

Learning Objectives

• Understand the functions of cellular division and the organization of genetic material in chromatin/chromosomes
• Define key chromosome/genetic material terminology
• Understand the major phases and processes occurring in the cell cycle during mitosis
• Explain how the mitotic spindle contributes to chromosomal movement in all phases of mitosis
• Understand the difference between animal and plant cell cytokinesis
• Understand why cellular checkpoints are important and what happens when they malfunction
• Explain how genetic material is passed through generations
• Understand chromosomal numbers in humans, haploid vs diploid cells, and examples in the body, including the human life cycle
• Describe the differences between asexual and sexual reproduction
• Explain the major outcomes of meiosis, major phases of meiosis I and meiosis II, and how genetic diversity occurs
• Compare and contrast mitosis and meiosis

Cellular Division

• Organisms reproduce through cell division
• Cell division is the basis of the continuity of life

Functions of Cellular Division

• In unicellular organisms, cell division reproduces the entire organism
• In multicellular organisms, cell division is responsible for: -Development from a fertilized egg -Growth -Repair

Introducing the Cell Cycle

• Cell division is an integral part of the cell cycle
• The cell cycle describes the life of a cell from its formation to its own division
• Most cell division results in two daughter cells with identical genetic information
• The exception is meiosis, which creates non-identical daughter cells, including sperm and egg cells

Cellular Organization of the Genetic Material

• Genome: all the DNA in a cell
• A genome can consist of:
  • A single DNA molecule (common in prokaryotic cells)
  • A number of DNA molecules (common in eukaryotic cells)
• DNA molecules are packaged into chromosomes

Cellular Organization of the Genetic Material (Eukaryotic Chromosomes)

• Chromatin: a complex of DNA and protein (histones) that condenses during cell division
• Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus
• Eukaryotic chromosomes consist of chromatin

Distribution of Chromosomes During Eukaryotic Cell Division

• In preparation for cell division, DNA is replicated, and the chromosomes condense
• Each duplicated chromosome has two sister chromatids joined together at the centromere
• Cohesins: protein complexes that attach sister chromatids
• Centromeres are located on each sister chromatid

The Two Major Phases of the Cell Cycle

• Interphase (G1, S, and G2): -Cell growth and copying of chromosomes in preparation for cell division
  • About 90% of cell cycle
  • G1 phase: First gap, cells increase in size
  • S phase: Synthesis, DNA replication occurs
  • G2 phase: Second gap, cells continue to grow, ready for mitosis
• Mitotic phase (mitosis and cytokinesis): -Mitosis: the division of the genetic material in the nucleus
  • Cytokinesis: the division of the cytoplasm

Mitosis Consists of 5 Stages

• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase and Cytokinesis

G2 of Interphase

• A nuclear envelope encloses the nucleus
• Two centrosomes have formed
• Chromosomes duplicated in S phase cannot be seen because they have not condensed

Prophase

• Chromatin fibers tightly coil into discrete chromosomes, becoming visible
• Duplicated chromosomes appear as two sister chromatids
• Mitotic spindle begins to form
• Centrosomes move away from each other

Prometaphase

• Nuclear envelope fragments, chromosomes are even more condensed
• Microtubules from each centrosome invade the nuclear area
• Kinetochore: protein structure at the centromere on each sister chromatid
• Some microtubules become kinetochore microtubules, jerking chromosomes back and forth
• Non-kinetochore microtubules elongate the cell

Metaphase

• Centrosomes now at opposite poles of the cell
• Chromosomes arrive at the metaphase plate
• An imaginary plane at the middle of the cell
• Centromeres lie at the metaphase plate
• Kinetochore of each sister chromatid attaches to kinetochore microtubules

Anaphase

• Shortest phase of mitosis
• Cohesins between chromatids are cleaved, allowing each pair to separate
• Two new daughter chromosomes move towards opposite poles
• Centromeres lead because microtubules are attached to kinetochores (at the centromere)

Telophase and Cytokinesis

• Telophase:
  • Two daughter nuclei form
  • Nuclear envelope reappears
  • Chromosomes start to decondense
  • Mitosis is now complete, creating two genetically identical nuclei
• Cytokinesis:
  • Division of the cytoplasm
  • Involves formation of a cleavage furrow, pinching cell in two (animal cells)

Animal Cell Mitosis (time-lapse)

BioFlix Animation: Mitosis

Plant cell cytokinesis in onion root cells

The Mitotic Spindle:

• A structure made of microtubules that controls chromosome movement
• In animal cells, assembly of spindle microtubules begins in the centrosome
• The microtubule-organizing center (MTOC)
  • The centrosome replicates during interphase
  • Two centrosomes migrate to opposite ends of the cell during prophase and prometaphase

The Mitotic Spindle

• During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes
• Kinetochores are protein complexes associated with centromeres
• At metaphase, all chromosomes line up at the metaphase plate

The Mitotic Spindle

• In anaphase, cohesins are cleaved by an enzyme called separase
• Sister chromatids separate and move along the kinetochore microtubules towards opposite ends of the cell
• Microtubules shorten by depolymerizing at their kinetochore ends

The Mitotic Spindle

• Non-kinetochore microtubules from opposite poles overlap and push against each other, elongating the cell
• At the end of anaphase, duplicate groups of chromosomes arrive at opposite ends of the elongated cell
• Cytokinesis beings during anaphase or telophase, and the spindle eventually disassembles

Comparing Cytokinesis in Animal and Plant Cells

• In animal cells, cytokinesis occurs by a process known as cleavage, forming a cleavage furrow
  • Contractile ring of microfilaments
• In plant cells, a cell plate forms during cytokinesis
  • Vesicles contain cell wall material derived from the Golgi apparatus

Comparing Cytokinesis in Animal and Plant Cells (Diagram)

How Do Cells Know When to Divide?

• Cell cycle is tightly regulated
• Some cells divide frequently (e.g., intestinal cells) and others rarely (e.g., neuronal cells)
• Cancerous cells can escape usual controls and proliferate

Molecular Clocks

• The cell cycle is driven by specific chemical signals present in the cytoplasm
• The cell cycle is directed by a distinct cell-cycle control system, which is similar to a clock
• The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

Cellular Checkpoints

• Changes in regulatory protein concentrations drive the cell cycle
• Three important checkpoints are those in the G1, G2, and M phases
• If the cell does not receive the go-ahead signal, it will exit the cycle and switch to a non-dividing state called the G0 phase

Cancer Cells

• Have lost their cell cycle checkpoints (due to internal and external factors)
• Can become immortal, dividing continuously provided they have enough nutrients
• HeLa cells: Harvested from a woman named Henrietta Lacks in 1951

Chapter 13: Meiosis

Meiosis

• A special type of cell division that produces sperm and egg cells (gametes)
• Produces non-identical daughter cells
• Heredity: the transmission of traits from one generation to the next
• Variation is demonstrated by the differences in appearance that offspring show from parents and siblings
• Genetics is the scientific study of heredity and variation

Inheritance of Genetic Material

• Offspring inherit genetic material from parents → genes
• Genes are the units of heredity made up of segments of DNA
• Locus (plural, loci): A gene's specific position along a chromosome
• Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs)

Inheritance of Genetic Material

• Offspring acquire genes from parents by inheriting chromosomes
• Most DNA is packaged into chromosomes
• Human somatic cells have 23 pairs of chromosomes (total of 46)
• Karyotype: an ordered display of the pairs of chromosomes from a cell

Comparison of Asexual and Sexual Reproduction

• In asexual reproduction, a single individual passes all its genes to its offspring without the fusion of gametes
• A clone is a group of genetically identical individuals from the same parent
• In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents

Cellular Organization of the Genetic Material (Somatic and Gametes)

• Somatic cells: non-reproductive cells with two sets of chromosomes (diploid)
• Gametes: reproductive cells (sperm and eggs) with half the number of chromosomes (haploid)
• In an unfertilized egg, the sex chromosome is X
• In a sperm cell, the sex chromosome may be either X or Y

Sets of Chromosomes in Human Cells

• Human somatic cells have 23 pairs of chromosomes
• The sex chromosomes are called X and Y
• Human females have a homologous pair of X chromosomes (XX)
• Human males have one X and one Y chromosome
• The remaining 22 pairs of chromosomes are called autosomes
• Aneuploidy: abnormal number of chromosomes

Sets of Chromosomes in a Cell

• The two chromosomes in each pair are called homologous chromosomes or homologs
• Chromosomes in a homologous pair are the same length and shape and carry similar genes
• Each pair includes one chromosome from each parent
• Non-sister chromatids: chromatids in a homologous pair, one from each parent

Sets of Chromosomes in a Cell

• The 46 chromosomes in a human somatic cell are two sets of 23
• Each replicated chromosome consists of two identical sister chromatids
• A diploid cell has two sets of chromosomes

Behavior of Chromosome Sets in the Human Life Cycle

• A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism
• Fertilization: the union of gametes
• The fertilized egg is called a zygote, having one set of chromosomes from each parent
• The zygote develops into an adult form by mitosis in somatic cells

Behavior of Chromosome Sets in the Human Life Cycle

• At sexual maturity, the ovaries and testes produce haploid gametes
• Gametes are the only human cells produced by meiosis, not mitosis
• Meiosis results in one set of chromosomes in each gamete
• Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number, a common feature of sexually reproducing organisms

Animal Life Cycles

• Gametes are the only haploid cells in animals
• Produced by meiosis and don't divide again before fertilization
• Gametes fuse to form a diploid zygote, which develops into a multicellular organism by mitosis
• Only diploid cells can undergo meiosis

Meiosis

• Meiosis reduces the number of chromosome sets from diploid (2n) to haploid (n)
• Like mitosis, meiosis is preceded by the replication of chromosomes
• Meiosis takes place in two consecutive cell divisions called meiosis I and meiosis II
• Meiosis I and II follow steps similar to mitosis (PMAT)

Meiosis

• The two cell divisions result in four daughter cells, rather than two in mitosis
• Each daughter cell has only half as many chromosomes as the parent cell
• Crossing over during meiosis allows for genetic recombination

Meiosis

Events Unique to Meiosis, Occurring in Meiosis I

• Synapsis and crossing over in prophase I
• Homologous pairs at the metaphase plate
• Separation of homologs during anaphase I
• Telophase I creates two haploid cells

Meiosis II

• Meiosis II is similar to mitosis
• During another round of cell division, the sister chromatids separate
• Produces four haploid daughter cells

Results of Meiosis

• Chromosomes duplicate before meiosis
• Pairs of homologs and crossover at chiasma allows for genetic recombination
• The chromatids are sorted into four haploid daughter cells

Comparing Mitosis and Meiosis

• Mitosis:
  • Conserves the number of chromosome sets
  • Produces cells that are genetically identical to the parent cell
• Meiosis:
  • Reduces the number of chromosomes sets from two (diploid) to one (haploid)
  • Produces cells that differ genetically from each other and from the parent cell

Comparing Mitosis and Meiosis (Diagram)

Comparing Mitosis and Meiosis (Summary)

• Mitosis:
  • DNA replication occurs during interphase before mitosis begins
  • One division; including prophase, prometaphase, metaphase, anaphase, and telophase
  • No synapsis of homologous chromosomes
  • Two daughter cells; genetically identical to parent cell
• Meiosis:
  • DNA replication occurs during interphase before meiosis I begins
  • Two divisions; each includes prophase, metaphase, anaphase, and telophase
  • Synapsis of homologous chromosomes occurs during prophase I
  • Four daughter cells; genetically different from each other and the parent cell
  • Enable multicellular animal or plant (gametophyte or sporophyte) to arise from a single cell; produces cells for growth, repair, and sexual reproduction; produces gametes in the gametophyte plant