Cell Reproduction 1

How Cells Reproduce

Chapter 8

Part 1

8.1 Impacts/IssuesHenrietta’s Immortal Cells

Runaway cell divisions killed Henrietta Lacks, but her cells live on in research laboratories

Fig. 8-1a, p. 136

Fig. 8-1b, p. 136

Guide Questions

• Why do they call Henrietta’s cells immortal?

• How did they immortalize Henrietta?• Do you think Henrietta will still be alive at

this time because of this technology?• Do you think the Johns Hopkins Hospital

did the right thing? Why or why not?• Do you think they can still clone Henrietta

using HeLa cells?

Video: Henrietta's immortal cells

8.2 Multiplication by Division

A cell reproduces by dividing in two

Each descendant cell receives a full set of chromosomes and some cytoplasm

Nuclear division mechanisms partition chromosomes of a parent cell into new nuclei

The cytoplasm divides by a separate mechanism

Divisions of a Fertilized Egg

Third division of an early frog embryo

Two Methods of Nuclear Division: Mitosis

Mitosis • Nuclear division process that maintains the

chromosome number• Basis of body growth, tissue repair and

replacement in multicelled eukaryotes; also asexual reproduction in some plants, animals, fungi, and protists

Two Methods of Nuclear Division: Meiosis

Meiosis • Nuclear division process that halves the

chromosome number • Basis of sexual reproduction in multicelled

eukaryotes; precedes the formation of gametes or sexual spores

Comparison of Division Mechanisms

8.3 The Life of a Cell

A cell cycle starts when a new cell forms, and ends when the cell reproduces

Cell cycle • A series of events from the time a cell forms until

its cytoplasm divides• Includes three phases: interphase, mitosis, and

cytoplasmic division

Interphase

Most of a cell’s activities, including DNA replication, occur in interphase

Interphase • In a eukaryotic cell cycle, the interval between

mitotic divisions when a cell grows, roughly doubles the number of its cytoplasmic components, and replicates its DNA

Three Stages of Interphase

G1• 1st interval (gap) of growth before DNA replication

S• Interval of synthesis (DNA replication)

G2• 2nd interval (gap) when the cell prepares to divide

Controls of Cell Division

Different types of cells proceed through the cell cycle at different rates

Gene expression controls regulate the cell cycle at different points

Loss of cell cycle controls results in cancer

Chromosomes During the Cell Cycle

Human body cells have 23 chromosome pairs

Except for male sex chromosomes, the chromosomes of each pair are homologous

Homologous • Two members of a pair of chromosomes with the

same length, shape, and genes

Chromosomes During the Cell Cycle

Mitosis maintains the chromosome number• Each chromosome is duplicated• Each new cell receives identical chromosomes

Fig. 8-4, p. 139

A An unduplicated pair of chromosomes in a cell in G1.

B By G2, each chromosome has been duplicated.

C Mitosis and cytoplasmic division package one copy of each chromosome into each of two new cells.

C Mitosis and cytoplasmic division package one copy of each chromosome into each of two new cells.

Fig. 8-4, p. 139

A An unduplicated pair of chromosomes in a cell in G1.

B By G2, each chromosome has been duplicated.

Stepped Art

Eukaryotic Cell Cycle

Fig. 8-3, p. 138

mitosis ends, interphase begins

interphase ends, mitosis begins

12

3

4

56

Animation: The cell cycle

8.4 Mitosis

At the end of interphase, a cell’s duplicated chromosomes condense tightly in preparation for nuclear division (mitosis)• Each chromosome consists of two sister

chromatids

Mitosis has four stages: prophase, metaphase, anaphase, and telophase

Prophase

Prophase • Chromosomes condense and spindle forms• Nuclear envelope breaks up• Spindle microtubules attach to chromosomes

Spindle • Dynamically assembled and disassembled array

of microtubules that moves chromosomes during nuclear division

Metaphase

Metaphase • Duplicated homologous chromosomes line up at

the spindle equator (halfway between spindle poles)

• Sister chromatids begin to move apart toward opposite spindle poles

Anaphase

Anaphase • Microtubules separate the sister chromatids of

each chromosome and pull them toward opposite spindle poles

• Each DNA molecule is now a separate chromosome

Telophase

Telophase • Two clusters of chromosomes arrive at the

spindle poles and decondense; new nuclei form

End of mitosis• Nuclear envelopes form around the two clusters

of chromosomes, forming two new nuclei with the parental chromosome number

Mitosis

Fig. 8-5a, p. 140

Fig. 8-5b, p. 141

Fig. 8-5b (1), p. 141

Fig. 8-5b (1), p. 141

centrosome

1 Early ProphaseMitosis begins. In the nucleus, the DNA begins to appear grainy as it organizes and condenses. The centrosome is duplicated.

Fig. 8-5b (2), p. 141

Fig. 8-5b (2), p. 141

2 Prophase The chromosomes become visible as distinct structures as they condense further. Microtubules assemble and move one of the two centrosomes to the opposite side of the nucleus, and the nuclear envelope breaks up.

Fig. 8-5b (3), p. 141

Fig. 8-5b (3), p. 141

pole pole3 Transition to MetaphaseThe nuclear envelope is gone, and the chromosomes are at their most condensed. Spindle microtubules assemble and attach sister chromatids to opposite spindle poles. microtubule of

spindle

Fig. 8-5b (4), p. 141

Fig. 8-5b (4), p. 141

4 MetaphaseAll of the chromosomes are aligned midway between the spindle poles. Microtubules attach each chromatid to one of the spindle poles, and its sister to the opposite pole.

Fig. 8-5b (5), p. 141

Fig. 8-5b (5), p. 141

5 Anaphase

Motor proteins moving along spindle microtubules drag the chromatids toward the spindle poles, and the sister chromatids separate. Each sister chromatid is now a separate chromosome.

Fig. 8-5b (6), p. 141

Fig. 8-5b (6), p. 141

6 TelophaseThe chromosomes reach the spindle poles and decondense. A nuclear envelope forms around each cluster. Mitosis is over.

Animation: Mitosis-step-by-step

Fig. 8-5b (6), p. 141

Stepped Art

6) TelophaseThe chromosomes reach the spindle poles and decondense. A nuclear envelope forms around each cluster. Mitosis is over.

centrosome 1) Early ProphaseMitosis begins. In the nucleus, the DNA begins to appear grainy as it organizes and condenses. The centrosome is duplicated.

2) Prophase The chromosomes become visible as distinct structures as they condense further. Microtubules assemble and move one of the two centrosomes to the opposite side of the nucleus, and the nuclear envelope breaks up.

pole pole 3) Transition to MetaphaseThe nuclear envelope is gone, and the chromosomes are at their most condensed. Spindle microtubules assemble and attach sister chromatids to opposite spindle poles.

microtubule of spindle

4) Metaphase All of the chromosomes are aligned midway between the spindle poles. Microtubules attach each chromatid to one of the spindle poles, and its sister to the opposite pole.

5) Anaphase Motor proteins moving along spindle microtubules drag the chromatids toward the spindle poles, and the sister chromatids separate. Each sister chromatid is now a separate chromosome.

Animation: Spindle apparatus

Animation: Mechanisms for chromosome movement

3D Animation: Mitosis

Animation: Mitosis

8.5 Cytoplasmic Division Mechanisms

A cell’s cytoplasm usually divides after mitosis, forming two cells, each with its own nucleus

Mechanisms of cytoplasmic division differ between animal cells and plant cells

Cytoplasmic Division in Animal Cells

In animal cells, a contractile ring pinches the cytoplasm in two

Contractile ring • Band of actin and myosin filaments that contracts

to form the cleavage furrow

Cleavage furrow • In a dividing animal cell, the indentation where

cytoplasmic division will occur

Cytoplasmic Division of an Animal Cell

Fig. 8-6a, p. 142

A After mitosis is completed, the spindle begins to disassemble.

Fig. 8-6b, p. 142

B At the midpoint of the former spindle, a ring of actin and myosin filaments attached to the plasma membrane contracts.

Fig. 8-6c, p. 142

C This contractile ring pulls the cell surface inward as it shrinks.

Fig. 8-6d, p. 142

D The ring contracts until it pinches the cell in two.

Animation: Cytoplasmic division

Cytoplasmic Division in Plant Cells

In plant cells, a cell-plate forms midway between the spindle poles and partitions the cytoplasm when it connects to the parent cell wall

Cell plate • After nuclear division in a plant cell, a diskshaped

structure that forms a cross-wall between the two new nuclei

Cytoplasmic Division of a Plant Cell

Fig. 8-7a, p. 143

A The plane of division was established before mitosis began. Vesicles cluster here when mitosis ends.

Fig. 8-7b, p. 143

Fig. 8-7b, p. 143

cell plate forming

B As the vesicles fuse with each other, they form a cell plate along the plane of division.

Fig. 8-7c, p. 143

C The cell plate expands outward along the plane of division. When it reaches the plasma membrane, it attaches to the membrane and partitions the cytoplasm.

Fig. 8-7d, p. 143

D The cell plate matures as two new cell walls that join with the parent cell wall, so each descendant cell becomes enclosed by its own cell wall.