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Learning Objectives – Chapter 8

• Brief overview of prokaryotic cell replication

• The three main phases of eukaryotic cell division:

Interphase, M phase, C phase

• Interphase is broken down into three sub-phases (know

what is happening during these phases)

• M phase (mitosis) is broken down into phases (know what

happens during mitosis)

• Cells divide a finite number of times

• Uncontrolled cell division = cancer

• Meiosis allows organisms to reproduce sexually while

maintaining the same number of chromosomes

Learning Objectives – Chapter 9

• Meiosis allows organisms to reproduce sexually while

maintaining the same number of chromosomes

Become familiar with gametes and how they package hereditary

information

• What is reduction division

• Understand key differences between meiosis I and mitosis

• Understand key differences between meiosis II and

meiosis I

• Learn the sequence of events that occurs from the

beginning of meiosis I to the end of meiosis II

• What are the mechanisms by which meiosis introduces

genetic diversity?

Prokaryotes Have a Simple Cell Cycle

• Cell division in prokaryotes takes place in two

stages, which together make up a simple cell

cycle

1. copy the DNA

this process is called replication

2. split the cell in two to form daughter cells

this process is called binary fission

Does this process introduce

genetic diversity?

DNA Replication in Prokaryotes

the prokaryotic chromosome is

a single circle of DNA

DNA replication begins with the

unzipping of the double-

stranded DNA at the origin of

replication

a new double helix is formed by

adding complementary

nucleotides to the exposed DNA

strands

At the end the cell possesses

two complete copies of the

chromosome

Prokaryotic Cell

Division

• After replication, the cell grows in order to partition the replicated DNA molecules when the cell reaches an

appropriate size, the cell splits into two equal halves

new plasma membrane and cell wall are added at a point between the partitioned DNA

eventually the cell constricts in two to form two daughter cells

• each daughter cell is a complete, living cell with its own DNA

The Eukaryotic Cell Cycle

Eukaryotic cells contain

more DNA than

prokaryotic cells and the

DNA is also packaged

differently

DNA in eukaryotic cells

is linear and packaged

into a compact

chromosome

• there is more than one

chromosome in a

eukaryotic cell

Mitosis vs. Meiosis

Eukaryotic cells have two different mechanisms to divide up the DNA

mitosis is cell division that occurs in nonreproductive cells

• these cells are called somatic cells

meiosis is cell division that occurs in cells of sexual reproduction

• these cells are called germ line cells

Eukaryotic Cell Cycle: 3 Phases

The eukaryotic cell cycle is divided into distinct

phases (each further broken down into sub-

phases)

Interphase (G1,S, and G2 phases)

Mitosis (M phase)

Cytokinesis (C phase)

Interphase

Interphase is comprised of three phases • G1 phase (“Gap 1”)

– the primary growth phase of the cell following division

– most cells spend the majority of their lifespan in this phase

• S phase (Synthesis) – DNA replication occurs in preparation for cell division

• G2 phase (“Gap 2”) – further preparation for cell division, including replication

of mitochondria and synthesis of microtubules

M Phase and C Phase

• Mitosis (M phase)

a microtubular apparatus binds to the

chromosomes and moves them apart

• Cytokinesis (C phase)

the cytoplasm divides, creating two daughter

cells

Essential Biological Process 8A:

The Cell Cycle

Animation: How the Cell Cycle Works

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Chromosomes

• Chromosome number varies among organisms most eukaryotes have between 10 and 50

chromosomes in their somatic cells

• Chromosomes exist as pairs in somatic cells these pairs are called homologous chromosomes,

or homologues

homologues contain information about the same traits but the information may vary

cells that have two of each type of chromosome are called diploid cells

• one chromosome of each pair is inherited from the mother and the other is inherited from the father

Chromatids

• Prior to cell division, each of the homologous chromosomes replicates, forming two identical copies called sister chromatids

the sister chromatids are joined together by a structure called a centromere

humans have 23 pairs of homologous chromosomes

• when each chromosome in the pair is replicated, this makes for a total of 92 chromatids

Figure 8.2 The difference between homologous

chromosomes and sister chromatids

How many chromosomes?

Karyotype

• A karyotype is an arrangement of chromosomes

• Chromosomes can be compared based on size, shape, and centromere location

• The karyotype at right shows the 23 pairs of human chromosomes

Figure 8.4 The 46

chromosomes of a human Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Homologous pair

© Andrew S. Bajer

Chromatin

Chromosomes are comprised of chromatin, a complex of

DNA and protein

there is also some RNA associated with chromosomes

the DNA in a chromosome is one very long double-stranded fiber

that extends unbroken for the length of the chromosome

the DNA is coiled in order to allow it to fit into a small space

despite being very long

Why?

Figure 8.5 Levels of eukaryotic

chromosomal organization

DNA is coiled around histones

Cell Division - Interphase

Interphase sets the stage for cell division

chromosomes are first duplicated

although not visible, chromosomes begin to wind up tightly in a process called condensation

Mitosis

The cell division that follows interphase is a division of the nuclear contents, known as mitosis mitosis is a continuous

process but it is divided, for ease of study, into four distinct stages 1. prophase

2. metaphase

3. anaphase

4. telophase

Mitosis: 1st Phase

• Prophase in prophase, the condensed

chromosomes first become visible with a light microscope

the nuclear envelope begins to disintegrate

centrosomes (centrioles in animal cells) begin to assemble a network of protein cables called the spindle

• each cable in the spindle is made of microtubules

• some of the microtubules attach to the chromosomes

• when the process is complete, the sister chromatids of a chromosome are attached by microtubules to opposite poles of the cell

Mitosis: 2nd Phase

Metaphase

the chromosomes attached to

microtubules of the spindle are

aligned in the center of the cell

• the centromeres are aligned

along an imaginary plane that

divides the cell in half, known as

the equatorial plane

Mitosis: 3rd Phase

Anaphase

centromeres split

sister chromatids separate

the microtubules of the spindle

are dismantled starting at the

poles

• this pulls the chromatids

toward the poles

Mitosis: 4th Phase

Telophase

the spindle is dismantled

a nuclear envelope forms

around the set of

chromosomes at each pole

the chromosomes begin to

decondense

the nucleolus reappears

Cytokinesis

End of mitosis – division of

the cytoplasm into halves

In animals, cytokinesis

occurs by actin filaments

contracting and pinching the

cell in two

• this action is evident as a

cleavage furrow that appears

between the daughter cells

In plants, a new cell wall is

laid down to divide the two

daughter cells

• the cell wall grows at right

angles to the mitotic spindle and

is called the cell plate

Essential Biological Process 8B: Cell Division

Animation: Mitosis

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Animation: Mitosis and Cytokinesis













What Is Cancer?

• Cells are programmed undergo only so many times and

then die

human cells divide about 50 times

• Cancer is a growth disorder of cells

begins when apparently normal cells grow uncontrollably

the result is a growing cluster of cells called a tumor

• malignant tumors are invasive

– cells from malignant tumors can metastasize, spreading to

different areas of the body to form new tumors

Lung Cancer

Figure 8.9 Lung cancer cells (300X) Figure 8.10 Portrait of a tumor

Mutation and Cancer

• Cell division is regulated by proteins called

growth factors

• Cancer is caused by damage to genes the

encode growth factors

mutation causes damage to genes

• may result from chemical or environmental

exposure, such as UV rays

viral exposure may also alter DNA

Cancer Genetics

• There are two general classes of growth factor

genes that are usually involved in cancer

proto-oncogenes

• these genes encode proteins that stimulate cell division

• mutations to these genes can cause cells to divide

excessively

– when mutated, these genes become oncogenes

tumor-suppressor genes

• these genes normally turn off cell division in healthy cells

• when mutated, these genes allow uncontrolled cell division

Animation: How Tumor Suppressor

Genes Block Cell Division













Inquiry & Analysis

Why Do Human Cells Age?

Animation: Telomerase Function

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Sexual Reproduction

Gametes are reproductive cells (eggs and

sperm) that contain half the complement of

chromosomes found in somatic cells

(haploid or diploid?)

The gametes fuse to form a new cell called a

zygote, which contains two complete copies

of each chromosome (haploid or diploid?)

• the fusion of gametes is called fertilization, or

syngamy

Meiosis

The formation of gametes must involve

some mechanism to halve the number of

chromosomes found in somatic cells

If not the number of chromosomes would

double with each fertilization

Meiosis is a process of reduction division in

forming gametes

• this ensures a consistent chromosome number

across generations

Haploid Gametes

• Meiosis and fertilization

constitute a cycle of

sexual reproduction

• Somatic cells have two

sets of chromosomes

making them diploid

• Gametes have only one

set of chromosomes,

making them haploid

Figure 9.1 Diploid cells carry

chromosomes from two parents

What is the diploid number in humans?

Sexual vs. Asexual Reproduction

• Some organisms reproduce by mitotic division and do not involve gametes this is called asexual reproduction

an example is binary fission in prokaryotes

• Other organisms are able to reproduce both sexually and asexually • For example,

strawberry plants flower (sexual reproduction) and send out runners (asexual reproduction)

• Many fungi have sexual and asexual forms

The Sexual Life Cycle

• In sexual reproduction, haploid cells or

organisms alternate with diploid cells or

organisms

Figure 9.4 Three types of sexual life cycles

Germ-Line Cells

In animals, the cells that will eventually

undergo meiosis are reserved early on for

the purpose of reproduction

these cells are referred to as germ-line cells

and are diploid like somatic cells

but only the germ-line cells will undergo

meiosis to produce haploid gametes

Figure 9.3 The sexual life cycle in animals

Animation: How Meiosis Works


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The Stages of Meiosis

Meiosis involves two divisions, meiosis I and meiosis II

DNA is replicated only before meiosis I

meiosis I separates the homologous chromosomes

meiosis II separates the replicate sister chromatids

when meiosis is complete, the result is that one diploid cell

has become four haploid cells

How many chromosomes are present in a human cell after meiosis I?

How many chromatids?

After meiosis II?

Meiosis I

Meiosis I is traditionally divided into four stages

1. Prophase I • homologues pair up and exchange segments

2. Metaphase I • the paired homologous chromosomes align on a central plane

3. Anaphase I • homologues separate and move to opposite poles

4. Telophase I • chromosomes gather at each of the two poles

What’s the big difference between meiosis I and mitosis?

Animation: Meiosis I













Meiosis I: Prophase

During prophase I, homologous

chromosomes line up together as

a pair

crossing over occurs between

nonsister chromatids of

homologous chromosomes

• the chromatids break in the same

place and sections of

chromosomes are swapped

• the result is a hybrid

chromosome

the pairing is held together by the

cohesion between sister

chromatids and the crossovers

What is the difference between mitosis and meiosis

in terms of how chromosomes are held together?

Figure 9.5 Crossing over

Animation: Meiosis with Crossing Over


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Meiosis I: Metaphase

During metaphase I, the orientation of

the homologous chromosomes on the

metaphase plate is random

each possible orientation of which

homologue faces which pole results in

gametes with different combinations of

parental chromosomes

this process is called independent

assortment

• In humans, produces over 8 million different

chromosome combinations!

How is this different from metaphase in

mitosis?

Figure 9.6 Independent assortment

Figure Independent assortment

increases genetic variability

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Paternal gamete

Diploid offspring

Maternal gamete

Potential gametes

Homologous pairs

Meiosis I: Anaphase

• In anaphase I, the

chromosome pairs separate

and individual homologues

move to each pole

Meiosis I: Telophase

• In telophase I, the

chromosomes gather at their

respective poles to form two

chromosome clusters

How many copies of each

chromosome is present after

meiosis I?

Meiosis II

• After meiosis I, a brief interphase occurs

where there is no replication of DNA

• Meiosis II follows and is basically a mitotic

division of the products of meiosis I

except that the sister chromatids are non-

identical because of crossing over in meiosis I

The Stages of Meiosis II

Meiosis II is also divided into four stages

1. Prophase II

• new spindle forms to attach to chromosome clusters

2. Metaphase II • spindle fibers bind to both sides of the centromere and individual

chromosomes align along a central plane

3. Anaphase II • sister chromatids are pulled to opposite poles

• Non-disjunction of chromatids at this stage can result in trisomy

4. Telophase II • the nuclear envelope is reformed around each of the four sets of

daughter chromosomes

Animation: Meiosis II



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Meiosis I


Meiosis II

Animation: Stages of Meiosis


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How Meiosis Differs from Mitosis

Meiosis has two unique features not found in

mitosis

synapsis

• homologous chromosomes pair along their entire

lengths and are held together by cohesin proteins;

this close association permits crossing over

reduction division

• because meiosis involves two nuclear divisions but

only one replication of DNA, the final amount of

genetic material passed to the gametes is halved

Animation: Comparison of

Meiosis and Mitosis

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Figure 9.8 Unique features of meiosis

How do sister

chromatids entering

meiosis II differ from

each other?

Animation: The Function of

Cohesin


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Animation: Unique Features of

Meiosis


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Figure 9.9 A comparison of meiosis and mitosis

learning objectives chapter 8 - linn-benton community...

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