Genetics--Nuclear Division & Inheritance

Mitosis & Meiosis

Introduction

• What is Genetics?

• What level of organization is being examined?– molecular, cellular, species, or population

• What is the purpose of this lab?

Somatic versus Reproductive Cells

• Somatic cells:– all except the reproductive

cells

– 2 copies of each chromosome

– diploid

– example: in humans, the diploid number is 46

• Reproductive or sex cells:– haploid

– 1 copy of each chromosome

Nuclear Division

• Mitosis– prior to mitosis chromosomes are duplicated– nuclear division in somatic cells– genetic info from “parent” to daughters– cancer: uncontrolled cell division

Nuclear Division

• Meiosis– prior to meiosis: chromosomes are duplicated– nuclear division in the sex cells– genetic diversity

Cytokinesis

• division of the cytoplasm• occurs at the end of mitosis• occurs at the end of meiosis I• occurs at the end of meiosis II

Interphase

Mitosis

Interphase

• no nuclear or cell division

• DNA replication, chromosome duplication, and cell growth

• “chromosomes,” not visible inside nucleus

• but can see nucleolus inside nucleus

Mitosis

• Four phases of mitosis:

– prophase

– metaphase

– anaphase

– telophase

• End result of mitosis: 2 daughter cells

Prophase

• Duplicated chromosomes condense and become visible as sister chromatids joined at the centromere

• nuclear membrane disintegrates; neither the nucleus nor the nucleolus is distinct

• spindle apparatus of the cell is formed

Early prophase

Prophase

Prophase

• Chromatin condenses into discrete chromosomes– two identical “sister

chromatids”

– joined at the centromere

Metaphase

• Chromosomes are aligned along the cell’s equator at their centromeres

• spindle fibers are visible

MetaphaseChromosomes convene on the metaphase plate, an imaginary “equator” located between the two poles of the spindle. Centromeres are aligned, and sister chromatids of each chromosome straddle the metaphase plate.

Metaphase plate

Spindle fibers

Anaphase

• Migration of the chromosomes: – centromeres split – move along the spindle fibers towards opposite

poles – pulling the sister chromatid

• Each chromatid -- a chromosome

AnaphaseThe sister chromatids separate from each other and move along the spindle fibers towards opposite poles. Each chromatid is now considered a chromosome.

Early anaphase

Anaphase

Spindle poles

Anaphase

Late anaphase

By the end of anaphase, each pole has an equivalent and complete set of chromosomes.

Telophase

• Chromosomes are aggregated at the poles and begin to thin out and extend in length

• new nuclear membrane forms; nucleolus and nucleus begin to reappear

• spindle disintegrates

• cytoplasm divides (cytokinesis occurs); daughter cells begin to form

Telophase

Late telophase

Cytokinesis begins, separating the cytoplasm into 2 “daughter cells”.

Telophase

• The “daughter” nuclei begin to form at the two poles

• Nuclear membranes begin to form

• chromosomes lose their distinct appearance.

Daughter Cells

Telophase

• formation of 2 daughter cells

• Each daughter cell has same genetic material as parent

Meiosis: Reproductive or Sex Cells• Gametes

– female gametes: “eggs” or “ova” (ovum, singular)

– male gametes: “sperm”

– HAPLOID, have 1 copy of each chromosome (i.e., 1/2 as many chromosomes as somatic cells)

– example, in humans, the haploid number is 23.

Meiosis in the Lily Anther

• Lily anther:

– male reproductive organ in flowering plants.

– meiosis takes place

– result: haploid pollen grains

Meiosis in the Lily Anther• A very thin cross-section of a lily anther

• stain it

• examine with a compound light microscope:

Meiosis in the Lily AntherEach anther contains 4 pollen sacs where meiosis takes place.

Meiosis in the Lily Anther

• Study the various phases of meiosis in prepared slides of the lily anther.

• look at the cells inside the pollen sacs

• locate the different phases.

Stages of Meiosis

Meiosis consists of meiosis I and meiosis II and both have 4 stages leading up to the actual cell division.

Meiosis I: Meiosis II:

Prophase I Prophase II

Metaphase I Metaphase II

Anaphase I Anaphase II

Telophase I Telophase II

Meiosis: Prophase I

Chromatin condenses into discrete chromosomes. Chromosome is actually composed of 2 sister chromatids.

Homologous chromosomes come together as pairs.

Meiosis: Metaphase I

Metaphase plate

Spindle fibers

• Chromosomes are arranged on the metaphase plate, still in homologous pairs.

• Spindle fibers from each pole attach to one chromosome of each pair of the homologue.

Meiosis: Anaphase I

• each homologous pair of chromosomes separate and move toward opposite poles.

• each chromosome still consists of 2 sister chromatids joined together at the centromere.

Meiosis: Telophase I

Haploid

Haploid

• Chromosomes have moved to opposite poles.

• Cytokinesis separates the cytoplasm into 2 daughter cells.

• Each cell is now haploid.

Meiosis: Prophase IIIn prophase II, the chromatin again condenses so the chromosomes have a distinct shape.

Meiosis: Metaphase II

In metaphase II, chromosomes align on the metaphase plate.

Meiosis: Anaphase II

• sister chromatids separate, and move towards opposite poles

• sister chromatids are now individual chromosomes.

Meiosis: Telophase II

Haploid

Haploid Haploid

Haploid

• nuclei begin to form at opposite poles, and cytokinesis occurs

• 4 daughter cells, each with the haploid number of chromosomes.

Meiosis I: Review

Prophase I Metaphase I

Telophase IAnaphase I

Review: Meiosis II

Prophase II Metaphase II

Anaphase II Telophase II

Chromosomes and Genes• one long strand of

DNA• associated proteins

– “chromatin”

• thousands of genes• gene

– discrete unit of heredity

• locus– physical location of a

gene

Homologous Chromosomes, Genes, and Alleles

• Chromosome pair:– “homologous

chromosomes”

• pair has genes at the same loci– “alleles”

– may be the same or different

Human Genetics

• Genotype– heterozygous– homozygous

• Phenotype

• Observe traits and identify which form you possess

• Make a table

Extra 21st chromosome

Heredity and Genetic Disorders

• Video, “Healthy, Heredity, and Genetic Disorders”

Mechanics of Mitosis

• Answer questions– use instructional materials available

• video: Mechanics of Mitosis

• posters

• Photo Atlas of Biology

Mitosis in the Onion Root Tip

• Today’s lab exercises

– examine the tip of an onion root

– find and identify the 4 different phases of mitosis.

• Root tips are useful material because the cells are frequently dividing as the root grows.

Growth and Mitosis in Onion Root Tips

A) Examine onion (Allium) root tip under low power objective (10x)– answer questions

B) Examine same slide under high power objective (40x)– follow instructions

Meiosis: The basis of Genetic Inheritance and Sexual

Reproduction

• Mechanics of Meiosis– Answer questions

• use instructional materials available– video: Mechanics of Meiosis

– posters

– Photo Atlas of Biology

Meiosis and Gamete Formation in Lily Anthers

• Observe first and second meiotic division in demonstration slides

Note:

•DO NOT move the slides out of position

•Use only FINE focus