Genetics: Mitosis and Genetic Information lecture 14

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Hamish Spencer

• Sesquicentennial Distinguished Professor of Zoology & Genetics
• Contact him at [email protected]
• Research interests include population-genetic theory, genomic imprinting, phylogenetics, phenotypic plasticity, NZ Molluscs, and the history of eugenics

Lecture 14 Objectives

• Interpret a karyotype
• Identify the structures of mitosis
• Summarize the order of events in mitosis
• Explain the mechanics and function of mitosis in the context of the cell cycle

Genetics Impacts

• Genetics has impacts on medicine, agriculture, law, ecology, sociology, and philosophy

How Organisms Store Genetic Information

• Acellular microbes, such as viruses, store genetic information as linear, circular, or segmented genomes, with either double-stranded DNA/RNA or single-stranded DNA/RNA
• Prokaryotic cells, like bacteria and archaea, have circular, double-stranded DNA genomes
• Eukaryotic cells (protists, fungi, plants, and animals) store genetic information as linear, double-stranded DNA
• Diploid human cells contain about 2 meters of DNA and 3,000 bp = 1 μm
• Genome sizes and gene estimates vary across organisms

Karyotype

• Karyotype is an ordered, visual representation of the chromosomes in a cell
• Made using an image of cell division stopped at metaphase
• Chromosomes are cut out and arranged to create karyotype display
• All chromosomes but X and Y are present in homologous pairs
• Homologous chromosomes are a pair of chromosomes with the same genes, one inherited from the father and one from the mother
• Locus - the location of a gene
• Gene - a defined region of DNA that produces a functional RNA molecule and is a unit of hereditary information
• Allele - An alternative version of a gene.
• Homologous pairs 1-22 are called autosomes, X & Y are sex chromosomes

Cell Division

• Multicellular organisms rely on cell division for development, growth to adult size (10^14 cells), and repair

Eukaryotic Cell Division

• A eukaryotic cell divides its DNA equally

Cell Cycle

• In dividing cells, the mitotic phase (M) alternates with interphase, which is a growth period
• Mitotic Phase involves
  • Mitosis: distribution of chromosomes into two daughter nuclei
  • Cytokinesis: division of cytoplasm, producing two daughter cells

Duplicated Chromosome

• Comprises two genetically identical sister chromatids, which separate during mitosis
• A duplicated G2 chromosome is made up of two chromatids, and each chromatid is a double-stranded DNA molecule

G2 of Interphase

• Nucleus envelope is intact
• Nucleolus is visible with one or more nucleoli
• Two centrosomes form
• Duplicated chromosomes cannot be individually seen as they are not condensed yet

Mitosis Phases

• Mitosis produces two genetically identical daughter cells
• Consists of five phases: prophase, prometaphase, metaphase, anaphase, and telophase/cytokinesis

Mitosis - Prophase

• Nucleoli disappear
• Duplicated chromosomes condense and appear as two identical sister chromatids joined at their centromeres
• Mitotic spindle begins to form
• Microtubules lengthen and centrosomes move to opposite poles.

Mitosis - Prometaphase

• Nuclear envelope breaks down and chromosomes fully condense
• A kinetochore (protein structure) forms at the centromere of each chromatid
• Microtubules that attach to the kinetochores are called kinetochore microtubules
• Nonkinetochore microtubules lengthen the cell by interacting with those from the opposite pole of the spindle

Mitosis - Metaphase

• Centrosomes are now at opposite poles of the cell
• Kinetochore microtubules are attached to the kinetochores of all sister chromatids
• Duplicated chromosomes align at the metaphase plate
• Homologous pairs do not interact
• Centromeres lie on the metaphase plate, at an equal distance between the spindle's two poles

Mitosis - Anaphase

• Sister chromatids disjoin at the centromeres and each chromatid becomes an independent daughter chromosome
• Daughter chromosomes move toward opposite poles as their kinetochore microtubules shorten
• The nonkinetochore microtubules lengthen, and the cell elongates
• Anaphase ends when the two poles of the cell contain identical and complete collections of chromosomes

Mitosis - Telophase & Cytokinesis

• Chromosomes become less condensed
• Spindle microtubules breakdown
• Two daughter nuclei (with nuclear envelope) form in the cell, and nucleoli reappear
• Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete
• Cytoplasm divides, resulting in two daughter cells
• In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell into two
• In plant cells, cytokinesis involves the formation of a cell plate between the daughter cells
• Each daughter cell has one copy of each duplicated chromosome

Practice Questions

• Sister chromatids in mitosis each have 2 double-stranded copies of their DNA molecule
• During mitotic anaphase, daughter chromosomes move to opposite ends of the cell and also nonkinetochore microtubules shorten

Lecture 14 Summary

• A karyotype is an organized visual display of a cell's chromosomes, arranged in homologous pairs
• Multicellular eukaryotes rely on mitosis for growth, development, and repair, enabling humans to develop from a single fertilized cell with trillions of cells
• Dividing cells cycle between interphase (cell growth and duplication of chromosomes) and the mitotic phase, where mitosis distributes chromosomes and cytokinesis splits the cytoplasm to form two daughter cells
• Mitosis produces two genetically identical daughter cells through five phases: prophase, prometaphase, metaphase, anaphase, and telophase/cytokinesis
• During mitosis, duplicated chromosomes comprised of two sister chromatids align at the metaphase plate
• During anaphase, sister chromatids separate into daughter chromosomes that move to opposite poles
• Events in telophase and cytokinesis result in the formation of two identical diploid daughter cells

Objective Questions

• What are the different elements of a karyotype?
• Outline what happens during the different phases of the cell cycle
• List the 5 phases of mitosis in order, and draw a simple labeled diagram of each phase to show how the chromosomes are organized; include the mitotic spindle in the diagrams
• During what phase are sister chromatids separated?
• During what phase does the mitotic spindle form?
• During what phase do the replicated chromosomes line up along the metaphase plate?
• Are the daughter cell products of mitosis genetically different or identical?