BSC 2010 Integrated Principles of Biology I - Genetics Lecturer: Dr. J. C. Herrera, Zoology Office:...

BSC 2010 Integrated Principles of Biology I - Genetics

• Lecturer: Dr. J. C. Herrera, Zoology

• Office: 3175 McCarty Hall

• Office Hours: TWTh 3rd and 5th periods, and by appointment

• Phone: 213-2498, Email: [email protected]

• Course Home Page: http://nersp.nerdc.ufl.edu/~herrera/bsc.html

• Lectures

• Tuesday, Wednesday, Thursday, McCarty C 100 (MCC 100)

• Discussions

• Friday, MCC 100; May include lectures as needed.

Exam II Lectures and Text Pages

• I. Cell Cycles

– Mitosis (218 – 228)

– Meiosis (238 – 249)

• II. Mendelian Genetics

• III. Chromosomal Genetics

• IV. Molecular Genetics

– Replication

– Transcription and Translation

• V. Microbial Models

• VI. DNA Technology

The Key Roles of Cell Division• The continuity of life

– Is based upon the reproduction of cells, or cell division

Figure 12.1

Unicellular organisms• reproduce by cell division

100 µm

(a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM).Figure 12.2 A

Multicellular organisms• depend on cell division for

– Development from a fertilized cell (zygote)

– Growth

– Maintenance and repair

20 µm200 µm

(b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM).

(c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM).

Figure 12.2 B, C

The Cell Cycle• The cell cycle is the life of the cell from its

formation to its own division.

• Cell division is just a portion of the cell’s lifecycle (cell cycle)

– Some cells go through repeated cell cycles.

– Other cells never or rarely divide once they are formed (e.g., vertebrate nerve and muscle cells).

Mitotic cell division• produces genetically identical daughter cells

• Cells precisely duplicate their genetic material

• They allocate the two copies to opposite ends of the cell

• Then they divide into two new daughter cells

Cellular Organization of Genetic Material• A cell’s total endowment of genetic information

– Is called its genome

50 µm

Figure 12.3

• The DNA in a cell

– Is packaged into chromosomes

Eukaryotic chromosomes• Eukaryotic chromosomes

– Are supercoils of chromatin, a complex of DNA and protein that condenses durnig cell division

– Allow duplication and distribution of large genomes

• In animals

– Somatic cells have two haploid sets of chromosomes

– Gametes have one haploid set of chromosomes

Chromosomes• Each chromosomes has:

– A single, long, molecule of DNA, segments of which are genes.

– Proteins which maintain the structure of the chromosome or are involved with the expression of genes, DNA replication, and DNA repair

• Each species: has a characteristic number of chromosomes

• The chromosomes: are in different states at different stages of the cell cycle.

– Interphase: loosely folded; not visible.

– Mitotic phase: highly folded and condensed; visible with a light microscope (basis of karyotype)

Distribution of Chromosomes During Cell Division

• During S-phase of interphase, DNA is replicated and the chromosomes condense

0.5 µm

Chromosomeduplication(including DNA synthesis)

Centromere

Separation of sister

chromatids

Sisterchromatids

Centromeres Sister chromatids

A eukaryotic cell has multiplechromosomes, one of which is

represented here. Before duplication, each chromosome

has a single DNA molecule.

Once duplicated, a chromosomeconsists of two sister chromatids

connected at the centromere. Eachchromatid contains a copy of the

DNA molecule.

Mechanical processes separate the sister chromatids into two chromosomes and distribute

them to two daughter cells.

Figure 12.4

• Each duplicated chromosome

– Has two sister chromatids, attached at the centromere

Important Note

• chromosome, chromatid, chromatin

• The course includes several sets of words which are very similar – be sure you know the differences among each set of similar words.

• Another example: centromere, centrosome, centriole

Two Types of Eukaryotic Cell Division• Mitotic cell division (occurs only in eukaryotes) consists of

– Mitosis - the division of the nucleus - sister chromatids pulled apart into two sets of chromosomes, one at each end of the cell.

– Cytokinesis - the division of the cytoplasm into two separate daughter cells, each containing 1 nucleus with 1 diploid set of single-copy chromosomes.

• Animal cells - cytokinesis = cleavage

• Plant cells - cytokinesis = cell plate formation

• Not all cells undergo cytokinesis following mitosis.

• Meiotic cell division

– Gametes are produced after a reduction in chromosome number

– Gametes each have 1 haploid set of chromosomes

Human Life Cycle • Adult inherits 46 chromosomes (2n)

• Meiosis in gonads halves chromosome number (n) – Sperm cell (23 chromosomes)– Ovum (23 chromosomes)

• Fertilization restores the chromosome number to 46 and results in a zygote

• Mitosis produces genetically identical daughter cells. Mitosis is responsible for growth and development.

Figure 13.5

Key

Haploid (n)

Diploid (2n)

Haploid gametes (n = 23)

Ovum (n)

SpermCell (n)

MEIOSIS FERTILIZATION

Ovary Testis Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

Phases of the Mitotic Cell Cycle• The cell cycle consists of

– Interphase

– The mitotic phase (m-phase)INTERPHASE

G1

S(DNA synthesis)

G2Cyto

kines

is

Mito

sis

MITOTIC(M) PHASE

Figure 12.5

Interphase• Interphase is 90% of the cell cycle and can be

divided into 3 sub-phases

– G1 phase – Growth 1 (Gap 1) cell grows

– S phase – Synthesis phase – DNA replicates (cell continues to grow)

– G2 phase - Growth 2 (Gap 2) cell grows

The Mitotic (M) Phase• The mitotic phase - 10% of the cell cycle, and

– Is made up of mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm)

• Mitosis is unique to eukaryotes and thought to be an evolutionary adaptation to handle large amounts of genetic material

• Mitosis is a very reliable process, only 1 error in about 100,000 cell divisions

• Mitosis is a continuous process, and cytokinesis usually begins during telophase

Mitosis• Mitosis begins with a cell just leaving G2 of

Interphase

G2 OF INTERPHASE

PROPHASE PROMETAPHASE

Centrosomes(with centriole pairs) Chromatin

(duplicated)

Early mitoticspindle

Aster

CentromereFragmentsof nuclearenvelope

Kinetochore

Nucleolus Nuclearenvelope

Plasmamembrane

Chromosome, consistingof two sister chromatids

Kinetochore microtubule Figure 12.6

Nonkinetochoremicrotubules

Nucleus: well-defined

Nucleoli: visible

Centrosomes: two, adjacent to nucleus

A pair of centrioles: in each centrosome

An aster: a microtubular array around each centrosome

Chromosomes: duplicated, but cannot be distinguished individually.

The Five Phases of Mitosis - Prophase

In the cytoplasm:

Mitotic spindle (microtubules) forms.

Centrosomes move apart due to lengthening of spindle fibers between them.

G2 OF INTERPHASE



(duplicated)


Aster


Kinetochore


Plasmamembrane




In the nucleus:

Nucleoli disappear

Chromatin fibers condense into visible doubled chromosomes

The Five Phases of Mitosis - PrometaphaseNucleus: envelope disintegrates

Spindle fibers: extend from centrosome at each pole toward the cell’s equator.

Each chromatid: has a kinetochore, at the centromere region = spindle attachment site.

By the end of prometaphase the chromosomes are aligned at the center of the cell.

G2 OF INTERPHASE



(duplicated)


Aster


Kinetochore


Plasmamembrane




Spindle microtubules:

Kinetochore microtubules: become attached to the kinetochores.

Nonkinetochore microtubules: radiate from each centrosome toward the equator without attaching to chromosomes. They overlap with those from opposite pole. May be attached by protein bridges.

The Five Phases of Mitosis - Metaphase

Centrosomes: already at opposite poles.

Chromosomes: already lined up at the metaphase plate.

Centromeres: aligned on metaphase plate.

The long axis of each chromosome: at a right angle to the spindle axis

Kinetochores: (structures of proteins and DNA that are part of the centromere) of sister chromatids face opposite poles.

Centrosome at one spindle pole

Daughter chromosomes

METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS

Spindle

Metaphaseplate Nucleolus

forming

Cleavagefurrow

Nuclear envelopeforming

Figure 12.6

The Five Phases of Mitosis - Anaphase

Paired centromeres: of each chromosome separate and move apart.

Sister chromatids: split apart - move towards opposite poles.

Kinetochore microtubules: shorten at the kinetochore end.

Nonkinetochore microtubules: move cell ends further apart.




Spindle


forming

Cleavagefurrow


Figure 12.6

The Five Phases of Mitosis - TelophaseNonkinetochore microtubules further elongate cell.

Daughter nuclei form, from fragments of the parent nucleus, at poles.

Nucleoli reappear

There is a set of chromosomes at each pole of the cell, and the chromatin uncoils and chromosomes no longer visible.




Spindle


forming

Cleavagefurrow


Figure 12.6

By the end of telophase: Mitosis is complete. Cytokinesis has begun.

The Mitotic Spindle• Events of mitosis depend on the spindle,

which forms in the cytoplasm from microtubules and other proteins.

• Microtubules of the cytoskeleton: partially disassembled during spindle formation.

– alpha- and beta-tubulin

– Fibers: elongate by adding tubulin subunits at the end away from the centrosome

• Assembly begins in the centrosome (microtubule organizing center).

The Mitotic Spindle• The spindle arises

from the centrosomes

– And includes spindle fibers and asters

CentrosomeAster

Sisterchromatids

MetaphasePlate

Kinetochores

Overlappingnonkinetochoremicrotubules

Kinetochores microtubules

Centrosome

ChromosomesMicrotubules0.5 µm

1 µm

Figure 12.7

The Mitotic Spindle• The kinetochore microtubules of the spindle attach to the kinetochores of

chromosomes and move the chromosomes to the metaphase plate

• In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell

• Current model: Kinetochore microtubules shorten by depolymerizing at the

kinetochore ends, and motor proteins “walk” the chromosomes poleward.

The microtubules of a cell in early anaphase were labeled with a fluorescent dye that glows in the microscope (yellow).

Spindlepole

Kinetochore

Figure 12.8

The Mitotic Spindle• Nonkinetechore microtubules from opposite

poles

– Overlap and push against each other, elongating the cell in preparation for cytokinesis. Probably attached to each other by protein bridges.

• Cytokinesis usually begins during telophase

Cytokinesis• In animal cells

– Cytokinesis occurs by a process known as cleavage, first forming a cleavage furrow

– Actin microfilaments form a ring just under the plasma membrane. They constrict and pinch the cell in two.

Cleavage furrow

Contractile ring of microfilaments

Daughter cells

100 µm

(a) Cleavage of an animal cell (SEM)Figure 12.9 A

Cytokinesis• In plant cells

– Vesicles from the Golgi apparatus carrying cell wall material converge at the center of the cell

– The vesicles fuse forming the cell plate

– The plate grows and connects to the existing cell wall forming new cell wall.

Daughter cells

1 µmVesiclesforming cell plate

Wall of patent cell Cell plate

New cell wall

(b) Cell plate formation in a plant cell (SEM)Figure 12.9 B

Mitosis in a plant cell

1 Prophase. The chromatinis condensing. The nucleolus is beginning to disappear.Although not yet visible in the micrograph, the mitotic spindle is staring to from.

Prometaphase.We now see discretechromosomes; each consists of two identical sister chromatids. Laterin prometaphase, the nuclear envelop will fragment.

Metaphase. The spindle is complete,and the chromosomes,attached to microtubulesat their kinetochores, are all at the metaphase plate.

Anaphase. Thechromatids of each chromosome have separated, and the daughter chromosomesare moving to the ends of cell as their kinetochoremicrotubles shorten.

Telophase. Daughternuclei are forming. Meanwhile, cytokinesishas started: The cellplate, which will divided the cytoplasm in two, is growing toward the perimeter of the parent cell.

2 3 4 5

NucleusNucleolus

ChromosomeChromatinecondensing

Figure 12.10

Binary Fission – Prokaryotic Cell Division• In binary fission

– The single circular bacterial chromosome replicates– The two daughter chromosomes actively move apart

Origin ofreplication

E. coli cell BacterialChromosome

Cell wall

Plasma Membrane

Two copiesof origin

OriginOrigin

Chromosome replication begins.Soon thereafter, one copy of the origin moves rapidly toward the other end of the cell.

1

Replication continues. One copy ofthe origin is now at each end of the cell.

2

Replication finishes. The plasma membrane grows inward, andnew cell wall is deposited.

3

Two daughter cells result.4Figure 12.11

The Evolution of Mitosis• Since prokaryotes are simpler and preceded

eukaryotes by billions of years

– It is likely that mitosis evolved from bacterial cell division

• Certain protists

– Exhibit types of cell division that may represent intermediates between binary fission and the type of mitosis carried out by most eukaryotic cells

A hypothetical sequence for the evolution of mitosis

Most eukaryotes. In most other eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down during mitosis. Microtubules separate the chromosomes, and the nuclear envelope then re-forms.

Dinoflagellates. In unicellular protists called dinoflagellates, the nuclear envelope remains intact during cell division, and the chromosomes attach to the nuclear envelope. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a fission process reminiscent of bacterial division.

Diatoms. In another group of unicellular protists, the diatoms, the nuclear envelope also remains intact during cell division. But in these organisms, the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei.

Prokaryotes. During binary fission, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism is not fully understood, but proteins may anchor the daughter chromosomes to specific sites on the plasma membrane.

(a)

(b)

(c)

(d)

Bacterialchromosome

Microtubules

Intact nuclear envelope

Chromosomes

Kinetochore microtubules

Intact nuclearenvelope

Kinetochore microtubules

Fragments ofnuclear envelope

Centrosome

Figure 12.12 A-D

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