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Chapter 5 The Cell Cycle and Mitosis: The Basis for Asexual Reproduction - KEY

You started off as a single cell and are now composed of trillions of cells – How?

Cell division, in which 1 cell divides into 2

Cells are constantly dying and being replaced by cell division

Why do an organism’s cells divide? For growth, repair & development

How does a cell know when to divide? DNA

Different cells are replaced at various intervals ( from 2 days to 200 days ) depending on the specific type of cell

o Which cells would need to be replaced more frequently? Skin, RBC’s, Cells lining digestive tract

o Which cells would need to be replaced less frequently? Brain cells

The Cell Cycle

Vocabulary: interphase, mitosis, nucleus, nucleolus, cytokinesis, daughter cells, chromatin, chromosomes, centromere, duplicated chromosome, daughter chromosome, prophase, metaphase, anaphase, telophase, spindle fibers, centrioles

During cell division a cell goes through a set series of stages, which is referred to as the cell cycle:

Objectives

I can identify and describe the events of the 3 main stages of the cell cycle (interphase, mitosis, cytokinesis) and identify these stages on a diagram

I can identify what form DNA is during the different stages of the cell cycle (i.e. chromatin or chromosomes) and when it is replicated.

I can describe the key events that take place during the 3 stages within interphase.

I can identify and describe the 4 main phases of mitosis.

I can compare cytokinesis in animal and plant cells.

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Consists of 3 stages:

o Interphase - cells carry out normal functions and prepare for reproduction o Mitosis - after duplication of contents, the cell’s nucleus divides into 2 equal & identical parts o Cytokinesis- the two nuclei and cell contents separate into 2 “daughter cells”

Interphase

Longest stage in the cell cycle

During this phase, DNA is in the form of chromatin

Divided into 3 stages:

Growth & Preparation

DNA Replication

Continued Growth & Preparation

Growth & Preparation o cell increases in size & doubles its organelles (while performing its usual functions)

DNA Replication o DNA replicates, making an exact copy of itself:

o DNA replication results in duplicated chromosomes during mitosis

Step 1: Enzyme unzips DNA by separating base pairs

Step 2: New bases pair with bases on original DNA strand

Step 3: Two new identical DNA molecules are produced.

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Because DNA replication occurs before mitosis, this means that when the chromatin condenses, it is in the form of duplicated chromosomes, which are composed of two sister chromatids:

Continued Growth & Preparation

o After DNA replicates, the cell continues to grow and prepare for division while performing its usual functions

o In particular, it synthesizes a lot of proteins needed for cell division

Mitosis

Shortest stage in the cell cycle

Contents of cell nucleus divide, resulting in 2 daughter nuclei that are identical to the original parent cell:

2 important structures are involved in cell division:

1) Spindle fibers: protein structures that move the chromosomes during cell division

2) Centrioles: organelles that control the spindle fibers during cell division; not found in plant cells

Mitosis is divided into 4 stages that are each characterized by specific events: (PMAT)

o Prophase (which is further subdivided into early prophase& late prophase)

o Metaphase

o Anaphse

o Telophase

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Phases of Mitosis (dividing of nucleus)

Early Prophase:

o Duplicated chromosomes condense (coil up) into X-shape

o Nucleolus disappears

o Nuclear membrane breaks down

o Spindle fibers form & spread across the cell

o Centrioles move to opposite ends of the cell

Late Prophase:

o Spindle fibers complete forming

o Spindle fibers attach to chromosomes at their centromeres

o By the end of prophase, the nuclear membrane & nucleolus

have disappeared

Metaphase:

o Spindle fibers pull chromosomes into a line across the metaphase plate along the center of the cell

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Anaphase:

o Spindle fibers contract & shorten to pull the sister chromatids apart toward opposite poles of the cell

o The sister chromatids are now considered to be daughter chromosomes

Telophase:

o Final stage of mitosis

o A complete set of chromosomes is at each end of the cell

o Nuclear membrane reforms around each set of chromosomes

o Nucleolus reappears

o Spindle fibers disappear

o Now there are two nuclei in the one cell

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Cytokinesis

Final stage of cell cycle

The cell with 2 nuclei splits into 2 daughter cells

The new cells are identical to each other and to the original parent cell

Cytokinesis is different in animal and plant cells due to the presence of the cell wall in plants:

o Animal cells: cell membrane pinches together to separate nuclei & organelles

o Plant cells: cell plate forms along center of cell

Practice Questions

1) Identify two differences between the cell cycle in plant cells and in animal cells.

2) Use the terms chromatin, duplicated chromosomes, and daughter chromosomes to describe how DNA is organized during:

a. Early interphase ________________________________

b. Late interphase ________________________________

c. Prophase ________________________________

d. Anaphase ________________________________

3) Identify the 3 main stages of the cell cycle.

4) Identify and briefly describe what occurs during the 3 stages of interphase.

5) Identify the stages shown on the following diagram: