Lecture 2

Cell Structure, Mitosis and Meiosis

Study Questions - Lecture 2

1) Describe the major components of a typical animal cell and theirfunction(s).

2) What is the cell cycle? At which stage is the genetic material replicated? What events take place at each phase?

3) Discuss the concepts of homologous chromosomes, diploidy, and haploidy. What characteristics are shared between two chromosomes considered to be homologous?

4) What is mitosis? Describe the events that characterize each stage of mitosis.

5) Describe how chromosomes are named on the basis of their centromere placement.

6) What is meiosis? Describe the events that characterize each stage of meiosis.

7) Contrast mitosis and meiosis. Explain why meiosis leads to significantgenetic variation while mitosis does not.

CELLS

Cells - “fundamental unit of life”

Prokaryotes

-single celled

-no nucleus

-no organelles

-DNA = single, circular molecule

- e.g., bacteria

Eukaryotes

-single or multicellular

-nucleus with nuclear membrane

-DNA packaged into chromosomes

-organelles present

Biological Kingdoms: Monera, Protista, Fungi, Plantae, Animalia

Cellular Components

Cell membraneEndoplasmic reticulum

RibosomesGolgi apparatus

LysosomesMitochondria

Microtubules (centriole)Nucleus (nucleolus)

Chromatin/Chromosomes

Fluid-Mosaic Model of Membrane Structure - the thingsurrounding the internal components of the cell

Bilayer (2 layers)- of proteins and lipids

Endoplasmic Reticulum-set ofinterconnected membranes. 2types.

Rough ER (studded with Ribosomes) - involved inproduction and modificationof proteins

Smooth ER- (no ribosomes) -involved in production oflipids, detoxification of molecules,and calcium storage in muscle cells

*Ribosomes formed in nucleolus*

Golgi ApparatusAfter proteins andlipids are made by theEndoplasmic Reticulumthey may be modifiedfurther and/or stored by the GA.

Lysosome - formed by GA, contain enzymes that break downcellular debris and foreign substances brought into the cell

MitochondriaAerobic respiration produces ATPHas own genome

MicrotubuleCell shape, motion,and growth.

Microtubule- composed of tubulin. Both cilia and flagella have the sameStructure of microtubules: 9+2.Forms spindle during mitosis & meiosis

Nuclear Membrane or Envelope - two membranes which formthe nucleus, is porous. Allows RNA to leave nucleus.

Chromosome = DNA (deoxyribonucleic acid) + associatedproteins (mainly histones) = “packaged” DNA

Cell Cycle: replication of genetic material (=DNA) and nucleardivision (= mitosis + cytokinesis).

Cell Cycle: Interphase

• Before mitosis• Time of high metabolic activity• DNA replicated and synthesized• Three phases: G1, S, and G2• G1(gap 1)- longest stage of cell cycle, RNA,

protein sysnthesis• S (synthesis)- DNA replicated , 2 chromatids per

chromosome, chromatids genetically identical• G2(gap 2)- RNA synthesis, not well understood

Cell Cycle: Mitosis

• Process of cell division(nuclear division) which produces daughter cells genetically identical to the parent cell

• Four Phases (P-M-A-T): prophase, metaphase, anaphase, and telophase.

• Upon completion of the phases of mitosis (nuclear division) the cell “officially” divides into two by a process called cytokinesis - division of cytoplasm

InterphaseNot part of mitosisDNA is replicatedchromosomes start to condense

Chromosome duplication

Prophase*Chromosomes coil and condense further.*Nucler membrane breaks down/ disappears.*Microtubules increase in number, spindle apparatusforms.

Metaphase*Nuclear membrane completely disappeared*Chromosomes move to equator of cell - begin to line up*Chromosomes attach to spindle via kinetochore

Centromeric Region

Anaphase*Movement of chromosomes via microtubulesto opposite sides of the cell. One chromatid to one end the otherChromatid to the opposite end

Telophase*Genetically identical info at each pole*Spindle fibers disappear*Chromosomes uncoil*Nuclear envelope reforms around

Cytokinesis - is separate from mitosis,= pinching of cell/divison of cytoplasm.

Mitosis + Cytokinesis result in twoidentical daughter cells.

Mitosis in a plant cell

Mitosis in an onion root

Chromosomes and Chromatids During Mitosis

Begin

Interphase

After

Interphase

After

Prophase

After

Metaphase

After

Anaphase

After

telophase

# of

Chromosomes4 4 4 4 8 4

# of

Chromatids4 8 8 8 8 4

Meiosis

• Cell division which results in halpoid “sex” cells (i.e., egg and sperm)

• One replication of the genetic material (DNA) during interphase, but two nuclear divisions (meiosis I and meiosis II).

• Results in haploid (N) cells (= gametes in animals) from an initial diploid (2N) cell

• Very similar to mitosis except that the cells produced are not genetically identical.

The human life cycle

Overview of meiosis: how meiosis reduces chromosome number

Independent Assortment

The stages of meiotic cell division: Meiosis I

Crossing-Over/Synapsis

A Tetrad from the Grasshopper

The stages of meiotic cell division: Meiosis I

The stages of meiotic cell division: Meiosis II

Oogenesis, 2n = 4

Spermatogenesis, 2n = 4

SEM of sea urchin sperm fertilizing egg

Chromosomes and Chromatids During Meiosis I

Begin

Interphase

After

Interphase

After

Prophase

I

After

Metaphase

I

After

Anaphase

I

After

Telophase

I

# of

Chromosomes4 4 4 4 4 2

# of

Chromatids4 8 8 8 8 4

Chromosomes and Chromatids During Meiosis II

After

Prophase

II

After

Metaphase II

After

Anaphase

II

After

Telophase

II

# of

Chromosomes2 2 4 2

# of

Chromatids4 4 4 2

A comparison of mitosis and meiosis

A comparison of mitosis and meiosis: summary