A Guide to the Natural World

David Krogh

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Chapter 9 • Lecture OutlineThe Links in Life’s Chain: Genetics and Cell Division

Biology

Fifth Edition

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9.1 An Introduction to Genetics

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An Introduction to Genetics

• DNA is an information-bearing molecule that plays a critical role in the reproduction, development, and everyday functioning of living things.

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DNA

• DNA contains the information for the production of proteins, which carry out a wide variety of tasks in living things.

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DNA

• One series of bases contains information for the production of one protein, while a different series of bases specifies a different protein.

• Each series of protein-specifying bases is known as a gene.

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Genome

• Most of the cells in an organism contain a complete copy of that organism’s genome, meaning its collection of genetic information.

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Genome

• Before cells divide, their genome must first be copied and the resulting copies apportioned evenly into what will become two daughter cells.

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9.2 An Introduction to Cell Division

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Cell Division

• Cell division takes place because: • Cells die and need to be replaced.• Cells can only grow so large before they

become dysfunctional.• There are times in which an organism needs

quantities of new cells above “replacement” level.

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Cell Division

• Cell division includes:• The duplication of DNA (replication)• The apportioning of the copied DNA into two

quantities in a parent cell (mitosis)• The physical splitting of this parent cell into

two daughter cells (cytokinesis)

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cellnucleus

1. Replication DNA is duplicated.

2. Mitosis The two quantities of DNA are moved to opposite sides of the parent cell.

3. Cytokinesis The parent cell splits into two daughter cells.

Cell Division

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DNA Replication

• In DNA replication, the two strands of the double helix unwind, after which each single strand serves as a template for construction of a second, complementary strand of DNA.

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DNA Replication

• The result is a doubling of the original quantity of DNA.

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1. Original DNA molecule unwinds.

2. New DNA strands are synthesized from the two original strands.

DNA Replication

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9.3 DNA in Chromosomes

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DNA is Packaged in Chromosomes

• DNA comes packaged in units called chromosomes.

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Chromosomes

• Chromosomes are composed of DNA and its associated proteins—a combined chemical complex called chromatin.

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Chromosomes and DNA Replication

• Chromosomes exist in an unduplicated state until such time as DNA replicates, prior to cell division.

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Chromosomes and DNA Replication

• DNA replication results in chromosomes that are in duplicated state, meaning one chromosome composed of two identical sister chromatids.

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(a) DNA is packaged in units called chromosomesDNA wraps around

protein to makechromatin

Chromatin foldsup to make

chromosomes

duplicatedchromosome

cell

DNA chromatin

(b) DNA replication at two levelsDNA replication . . . . . . has this effect at

the chromosomal level.

unduplicated chromosome(not actual shape)

duplicated chromosome

sister chromatidsFigure 9.6

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Matched Pairs

• Chromosomes in human beings (and many other species) come in matched pairs, with one member of each pair inherited from the mother, and the other member of each pair inherited from the father.

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Matched Pairs

• Such homologous chromosomes have closely matched sets of genes on them, although many of these genes are not identical.

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Homologous Chromosomes

• A given paternal chromosome may have genes that code, for example, for different hair or skin color than the counterpart genes on the homologous maternal chromosome.

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Chromosomes

• Human beings have 46 chromosomes.

• 22 matched pairs and either a matched pair of X chromosomes (in females) or an X and a Y chromosome (in males).

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Karyotype

Figure 9.7

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The Cell Cycle

• Cell division fits into the larger framework of the cell cycle, meaning a repeating pattern of growth, genetic replication, and cell division.

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The Cell Cycle

• The cell cycle has two main phases: interphase and mitotic phase.

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The Cell Cycle

• In interphase, the cell carries out its work, grows, and duplicates its chromosomes in preparation for division.

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The Cell Cycle

• In mitotic phase, the duplicated chromosomes separate (mitosis) and the cell splits in two (cytokinesis).

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1. Interphase—G1

In this gap-1 phase of the cell cycle, the cell is growing and carrying out its normal functions.

2. Interphase—S In this synthesis phase, the cell is replicating its DNA (duplicating its chromosomes) in preparation for mitosis and cytokinesis.

3. Interphase—G2

In this gap-2 phase, DNA replication has been completed, and the cell continues with its normal functions, even as it prepares for mitosis and cytokinesis.

4. Mitosis The cell begins the process of apportioning its DNA into two opposite sides of itself.

5. Cytokinesis With mitosis nearly complete, the cell begins the process of splitting into two daughter cells. Once this is finished, each of the two cells moves back into G1 of interphase.

Cellcycle

mitosis cytokinesis

S

G2

G1

Figure 9.9

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9.4 Mitosis and Cytokinesis

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Mitosis and Cytokinesis

• There are four stages in mitosis: prophase, metaphase, anaphase, and telophase.

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Mitosis

• The essence of the process is that duplicated chromosomes line up along an equatorial plane of the parent cell, called the metaphase plate, with the sister chromatids that make up each duplicated chromosome lying on opposite sides of the plate.

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Mitosis

Suggested Media Enhancement:

Mitosis

To access this animation go to folder C_Animations_and_Video_Filesand open the BioFlix folder.

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Mitosis

• Attached to fibers called microtubules, the sister chromatids are then pulled apart, to opposite poles of the parent cell.

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pair ofcentrosomes

nucleus

replicated,uncondensed

DNA

spindle fibers(microtubules)

mitotic spindle

metaphaseplate

chromosomes(each a pair ofsister chromatidsjoined together)

End ofinterphase

DNA has already duplicatedback in S phase. Centrosomehas doubled.

Prophase

Mitosis begins:Chromosomes take shape;the two centrosomes beginto move toward the cellularpoles, sprouting microtubulesas they go

Metaphase

Attachment and alignment:Microtubules attach to sisterchromatids and align them atthe metaphase plate.

Mitosis and Cytokinesis

Figure 9.10 (1 of 2)

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separatingchromatids

spindle fibersshortening

cleavagefurrow

Anaphase

Separation:Sister chromatids aremoved to opposite polesin the cell, each chromatidnow becoming a full-fledgedchromosome.

Telophase andcytokinesis

Completionof cytokinesis

Exit from mitosis:Chromosomes decondense; nuclear envelopes formaround the two separatecomplements of chromo-somes. Cleavage furrowbegins to form.

One cell becomes two:The cell membrane pinchestogether completely;membranes on either sidefuse together, creating twocells.

Beginning ofinterphase

These two cells now enterthe G1 phase of interphase.

Figure 9.10 (2 of 2)

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Mitosis and Cytokinesis

• Once cell division is complete, sister chromatids that once formed a single chromosome will reside in separate daughter cells, with each sister chromatid now functioning as a full-fledged chromosome.

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Mitosis and Cytokinesis

• Cytokinesis in animal cells works through a ring of protein filaments that tightens at the middle of a dividing cell.

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Cytokinesis in Animals

Figure 9.11

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Cytokinesis

• Membranes on the portions of the cell being pinched together then fuse, resulting in two daughter cells.

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9.5 Cell Division in Plants and Bacteria

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Variations in Cell Division

• Because of their cell walls, plant cells must carry out cytokinesis differently from animal cells.

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Plant Cell Division

• The plant’s solution is to grow new cell walls and plasma membranes near the metaphase plate, thus dividing the parent cell into two daughter cells.

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Cytokinesis in Plants

Figure 9.12

cell wall

vesicles

plasmamembrane

1. Membrane-lined vesicles accumulate near the metaphase plate. The vesicles contain precursors to the cell wall.

2. Vesicles fuse together, forming a cell plate that grows toward the parent cell wall.

3. The newly formed plasma membrane and cell wall fuse with the parent plasma membrane and cell wall, forming two distinct daughter cells.

twodaughtercells

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Binary Fission

• Prokaryotes such as bacteria employ a process called binary fission.

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Binary Fission

• They double their single, circular chromosome, with the two resulting chromosomes attaching to different sites on the plasma membrane.

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Binary Fission

• Then, an outgrowth of plasma membrane and cell wall, called a septum, begins growing from opposite sides of the cell, in between the two chromosomes.

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Binary Fission

• When the two septum extensions join in the middle, they divide the one cell into two.

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cell wall

chromosome

cell membrane

parentalbacterial

cell

twodaughtercells

1. Bacterial cell starts with a single, circular chromosome attached to its plasma membrane.

2. The chromosome replicates and the daughter chromosomes attach to different sites on the plasma membrane.

3. The cell membrane and wall grow an extension between the attachment points of the two chromosomes.

4. The cell wall and membrane join together in the middle, resulting in two new cells.

Binary Fission in Bacteria

Figure 9.13

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Cell Division in Bacteria

Animation 9.1: Cell Division for Bacteria