How Cells Reproduce

Chapter 9

Hsueh-Fen Juan

Oct. 16, 2012

Video: Henrietta's immortal cells

Impacts, IssuesHenrietta’s Immortal Cells

Henrietta Lacks died of cancer at age 31, but her cells (HeLa cells) are still growing in laboratories

9.1 Overview of Cell Division Mechanisms

Individual cells or organisms produce offspring by the process of reproduction

When a cell reproduces, each descendent receives information coded in DNA, and enough cytoplasm to begin operating

細胞分裂包含： 1. 細胞核分裂 (mitosis or meiosis) 2. 細胞質分裂

Mitosis, Meiosis, and the Prokaryotes

Eukaryotic cells• Mitosis copies DNA and divides a nucleus,

producing two identical nuclei• Meiosis is a nuclear division that produces

haploid gametes for sexual reproduction• ( 有絲、減數都是在討論細胞核的分裂，非細胞

質 ) Prokaryotic cells reproduce asexually by

prokaryotic fission ( 原核裂變，和有絲、減數分裂相比，機制上完全不同 )

Comparison of Cell Division Mechanisms

Key Points About Chromosome Structure

Each species has a characteristic number of chromosomes that differ in length and shape• Each consists of one double strand of DNA• After duplication, each consists of two double strands

(sister chromatids) that remain attached to each other at a centromere until late in nuclear division

• Chromatid: 染色分體 (chromosome: 染色體 )

Centromere: 著絲點 (centrosome: 中心體 ) ，其上 的 binding sites 稱 kinetochore ( 著絲粒 )

A Chromosome and Sister Chromatids

Key Points About Chromosome Structure

A chromosome consists of DNA that is wrapped around proteins (histones 組織蛋白 ) and condensed

Each histone and the DNA wrapped around it make up a nucleosome ( 核小體 ), the smallest unit of structural organization in eukaryotic chromosomes

Fig. 9-3 (b-e), p. 143

C When the coiled coils unwind, a molecule of chromosomal DNA and its associated proteins are organized as a cylindrical fiber.fiber

D A loosened fiber shows a “beads-on-a-string” organization. The “string” is the DNA molecule; each “bead” is one nucleosome.

beads on a string

DNA double helix core of

histonesE A nucleosome consists of part of a DNA molecule looped twice around a core of histone proteins.

nucleosome

B When a chromosome is at its most condensed, the DNA is packed into tightly coiled coils.

multiple levels of coiling of DNA and proteins

centromere

A Duplicated human chromosome in its most condensed form. If this chromosome were actually the size shown in the micrograph, its two DNA strands would stretch out about 800 meters (0.5 miles).

Stepped Art

Chromosome Structure

Animation: Chromosome structural organization

Fig. 9-4, p. 144

G1

Interval of cell growth and activity before DNA replication (chromosomes unduplicated)

S

Interval of cell growth when the DNA is replicated (all chromosomes are duplicating)

G2

Interval after DNA replication; the cell prepares to divide (Cells make the proteins that will drive mitosis)

Interphase ends for parent cellcytoplasmic division;

each descendant cell enters interphase

Telophase

Anaphase Metaphase Prophase

Stepped Art

The cell cycle

Animation: The cell cycle

9.1 Key Concepts:Chromosomes and Dividing Cells

Individuals have a characteristic number of chromosomes in each of their cells

The chromosomes differ in length and shape, and they carry different portions of the cell’s hereditary information

Division mechanisms parcel out the information into descendent cells

9.2 Introducing the Cell Cycle

Cell cycle• A sequence of three stages (interphase,

mitosis, and cytoplasmic division) through which a cell passes between one cell division and the next

Interphase

Interphase consists of three stages, during which a cell increases in size, doubles the number of cytoplasmic components, and duplicates its DNA• G1: Interval of cell growth and activity ， (DNA

未複製 )• S: Interval of DNA replication (synthesis ， DNA

複製中 )• G2: Interval when the cell prepares for division.

Cells make the proteins that will drive mitosis. (DNA 已完成複製 )

Interphase and the Life of a Cell

Most cell activities take place during G1

Control mechanisms work at certain points in the cell cycle; some can keep cells in G1( 例：神經細胞大多永遠停於 G1 ，因此不分裂 )

Loss of control may cause cell death or cancer ( 例：強制使神經細胞跳脫 G1 會使其死亡，而非細胞分裂，因為它有控制機制使其在 cell cycle 失控時 self-destruct)

Mitosis and the Chromosome Number

Mitosis produces two diploid nuclei with the same number and kind of chromosomes as the parent

Chromosome number• The sum of all chromosomes in a type of cell• Human cells have 46 chromosomes paired in 23

sets (diploid number ，二倍體 ) • Pairs have the same shape and information about

the same traits (except sex chromosomes XY)

Mitosis and the Chromosome Number

Bipolar spindle (雙極紡錘絲 )• A dynamic network of microtubules that forms

during nuclear division• Grows into the cytoplasm from opposite poles of

the cell and attaches to duplicated chromosomes• Microtubules from opposite poles attach to

different sister chromatids and separate them

Bipolar Spindle Separates Sister Chromatids

Mitosis Maintains Chromosome Number

9.2 Key Concepts:Where Mitosis Fits in the Cell Cycle

A cell cycle starts when a new cell forms by division of a parent cell, and ends when the cell completes its own division

A typical cell proceeds through intervals of interphase, mitosis, and cytoplasmic division

9.3 A Closer Look at Mitosis

When a nucleus divides by mitosis, each new nucleus has the same chromosome number as the parent cell

There are four main stages of mitosis: prophase, metaphase, anaphase, and telophase

Prophase Prophase• Chromosomes condense• Microtubules form a bipolar spindle with motor

proteins traveling along them ( 消耗 ATP)• Nuclear envelope breaks up• Microtubules attach to the chromosomes• 中心體在前期之前 (early prophase) 即完成複製• 植物細胞無中心體，但有其他構造控制紡錘絲

Centrosome• A region near the nucleus that organizes spindle

microtubules; usually includes two centrioles ( 中心體通常含兩個中心粒 )

Metaphase and Anaphase

Metaphase• All duplicated chromosomes line up midway

between the spindle poles by adding and losing tubulin subunits to grow or shrink microtubules.

Anaphase• Microtubules separate the sister chromatids of

each chromosome and pull them to opposite spindle poles

• 3 cell activities bring this about:

1.連接在染色分體上的紡錘絲微管縮短2.機動蛋白把染色分體拖往紡錘絲微管縮短的方向3.在赤道板上重疊的微管也彼此滑動縮短

Telophase

Telophase• Two clusters of chromosomes reach the spindle

poles and decondense• A new nuclear envelope forms around each

cluster• New plasma membrane may assemble

Two new nuclei are formed, each with the same chromosome number as the parent cell

Mitosis

Fig. 9-6 (2), p. 147

Fig. 9-6 (2a), p. 147

Fig. 9-6 (2b), p. 147

Fig. 9-6 (2c), p. 147

Fig. 9-6 (2d), p. 147

Fig. 9-6 (2e), p. 147

Fig. 9-6 (2f), p. 147

Animation: Mitosis-step-by-step

Animation: Mechanisms for chromosome movement

9.3 Key Concepts:Stages of Mitosis

Mitosis divides the nucleus, not the cytoplasm

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

A bipolar spindle forms; it moves the cell’s duplicated chromosomes into two parcels, which end up in two genetically identical nuclei

9.4 Cytoplasmic Division Mechanisms

In most kinds of eukaryotes, the cell cytoplasm divides between late anaphase and the end of telophase, but the mechanism of division differs

Cytokinesis (aka cytoplasmic division)• The process of cytoplasmic division

Cytoplasmic Division in Animal and Plant Cells Animal cells• A contractile ring partitions the cytoplasm• A band of actin filaments rings the cell midsection,

contracts, and pinches the cytoplasm in two (The indentation is called Cleavage furrow 注意 )

Plant cells• The plane of division is established by

microtubules and actin filaments that formed and broke up before mitosis began

• A cell plate forms midway between the spindle poles; it partitions the cytoplasm when it reaches and develop into primary cell wall merging with the parent cell wall

Cytoplasmic Division in Animal and Plant Cells

Fig. 9-7a, p. 148

Fig. 9-7b, p. 148

Animation: Cytoplasmic division

9.4 Key Concepts:How the Cytoplasm Divides

After nuclear division, the cytoplasm divides

One nucleus ends up in each of two new cells

In animal cells, the cytoplasm pinches in two

In plant cells, a cross-wall forms in the cytoplasm and divides it

9.5 When Control is Lost

Sometimes, controls over cell division are lost• Cancer may be the outcome

Cell Cycle Controls

Checkpoints in the cell cycle allow problems to be corrected before the cycle advances

Proteins produced by checkpoint genes interact to advance, delay, or stop the cell cycle (when DNA’s not copied completely, damaged, or the nutrient is not sufficient to support cell growth)• Kinases can activate other molecules to stop

the cell cycle or cause cells to die• Growth factors can activate kinases (by

binding to receptors on target cell) to activate genes that stimulate cells to start mitosis

Protein Products of Checkpoint Genes in Action

53BP1 BRCA1

Checkpoint Failure and Tumors

When all checkpoint mechanisms fail, a cell loses control over its cell cycle and may form a tumor (abnormal mass) in surrounding tissue

Usually one or more checkpoint gene products are missing in tumor cells• Checkpoint gene products (protein) that inhibit

mitosis are called Tumor suppressors ( 腫瘤抑制物 )

• Checkpoint genes encoding proteins that stimulate mitosis are called Protooncogenes ( 原癌基因 )

Neoplasms

Neoplasms (腫瘤 )• Abnormal masses of cells that lack control over

how they grow and divide• Benign (noncancerous 良性 ) neoplasms, such

as ordinary skin moles, stay in one place and are not cancerous

• Malignant (cancerous 惡性 ) neoplasms are cancerous

Characteristics of Cancer Cells

Cancers (malignant neoplasms) 1. Cells grow and divide abnormally; capillary

blood supply to the cells may increase abnormally ( 微血管暴量，供血異常暴增 )

2. Cells may have altered plasma membrane and cytoplasm: Membrane may be leaky and missing proteins; Cytoskeleton may be shrunken or disorganized; Metabolism may shift toward fermentation from aerobic respiration ( 超級瘋狂 )

3. Cells have altered recognition proteins and weakened adhesion; may break away and invade distant tissues (metastasis 轉移 )

Fig. 9-11, p. 151

benign tumor malignant tumor

A Cancer cells break away from their home tissue.

B The metastasizing cells become attached to the wall of a blood vessel or lymph vessel. They release digestive enzymes onto it. Then they cross the wall at the resulting breach.

C Cancer cells creep or tumble along inside blood vessels, then leave the bloodstream the same way they got in. They start new tumors in new tissues.

Benign and Malignant Tumors

Animation: Cancer and metastasis

Skin Cancers

A basal cell carcinoma

Squamous cell carcinoma

Malignant melanoma

ABC video: Blood test for lung cancer

ABC video: Genetically modified cancer fighters

9.5 Key Concepts:The Cell Cycle and Cancer

Built-in mechanisms monitor and control the timing and rate of cell division

On rare occasions, the surveillance mechanisms fail, and cell division become uncontrollable

Tumor formation and cancer are the outcome