The division of a unicellular organism reproduces...

• The division of a unicellular organism reproduces an

entire organism, increasing the population. Here’s

one amoeba dividing into 2.

1. Cell division functions in 3 things : reproduction,

growth, and repair

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 12.1

• Cell division is also central to the development of a

multicellular organism that begins as a fertilized

egg or zygote.

• Multicellular organisms also use cell division to

repair and renew cells that die from normal wear

and tear or accidents.


Fig. 12.1b Fig. 12.1c

• Cell division requires the distribution of identical

genetic material - DNA - to two daughter cells.

• What is remarkable is the accuracy with which DNA is

passed along, without changing, from one generation to

the next.

• A dividing parent cell duplicates its DNA, moves

the two identical copies to opposite ends of the

cell, and then splits into two daughter cells, each

with the same DNA.


• A cell’s entire set of genetic information, packaged

as DNA, is called its genome.

• In prokaryotes, the genome is often a single, long, circular

DNA molecule.

• In eukaryotes, the genome consists of several shorter,

linear DNA molecules. How many in yours?

• A human cell must duplicate about 3 m of DNA and

separate the two copies such that each daughter cell

ends up with a complete genome.

2. Genome


• DNA molecules are packaged into chromosomes.

• Every eukaryotic species has a characteristic number of

chromosomes in the nucleus.

• Human somatic cells (body cells) have 46

chromosomes.

• Human gametes

(sperm or eggs)

have 23 chromosomes,

half the number in

a somatic cell.


Fig. 12.2

• Each duplicated chromosome consists of two sister chromatids which contain identical copies of the chromosome’s DNA, plus proteins.

• As they condense, the region where the strands connect shrinks to a narrow area, the centromere.

• Later, the sister chromatids are pulled apart and repackaged into two new nuclei at opposite ends of the parent cell.


Fig. 12.3

• The process of the formation of the two

daughter nuclei is what is specifically

what is referred to as mitosis, and is

usually followed by division of the

cytoplasm, cytokinesis.

• These processes take one cell and produce

two cells that are the genetic equivalent of

the parent.


• The mitotic (M) phase of the cell cycle alternates

with the much longer interphase.

• The M phase includes mitosis and cytokinesis.

• Interphase accounts

for 90% of the cell

cycle.

• Let’s watch

1. The mitotic phase alternates with interphase in

the cell cycle: an overview


Fig. 12.4

../Animations/Animation 12.3.1 Mitosis and.MOV

• Interphase has three subphases:

G1 phase (“first gap”) centered on growth

(protein synthesis, respiration, etc.),

• the S phase (“synthesis”) when the

chromosomes are copied,

• the G2 phase (“second gap”) where the cell

completes preparations for cell division (like

microtubule formation), and divides (M).


• By late interphase, the chromosomes have been

duplicated but are loosely packed.

• The centrosomes have been duplicated and begin

to organize microtubules into an aster (“star”).


Fig. 12.5a

• In prophase, the chromosomes are tightly coiled,

with sister chromatids joined together.

• The nucleoli disappear.

• The mitotic spindle begins

to form and appears to push

the centrosomes away

from each other toward

opposite ends (poles)

of the cell.


Fig. 12.5b

• During prometaphase (late prophase), the nuclear

envelope fragments and microtubules from the

spindle interact with the chromosomes.

• Microtubules from one

pole attach to each

chromosome at the

centromere region.


Fig. 12.5c

• The spindle fibers push the sister chromatids until

they are all arranged at the metaphase plate or

equator, an imaginary plane equidistant between

the poles. This lining up defines metaphase.


Fig. 12.5d

• At anaphase, the centromeres divide, separating

the sister chromatids.

• Each is now pulled toward the pole to which it is

attached by spindle fibers.

• By the end, the two

poles have identical

collections of

chromosomes.


Fig. 12.5e

• At telophase, two nuclei begin to form, surrounded

by the fragments of the parent’s nuclear envelope.

• Chromatin becomes

less tightly coiled.

• Cytokinesis, division

of the cytoplasm,

begins.

• Let’s watch again


Fig. 12.5f

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter11/animations.html


Fig. 12.5 left


Fig. 12.5 right

• Cytokinesis follows

mitosis.

• In animals, the first sign of

cytokinesis (cleavage)

is the appearance of a

cleavage furrow in the

cell surface near the old

metaphase plate.

• See Animation 12.3.8

3. Cytokinesis divides the cytoplasm:

a closer look


Fig. 12.8a

../Animations/Animation 12.3.8 Cytokinesis.MOV

• On the inside of the

cleavage furrow, a ring

of actin microfilaments

and the motor protein

myosin form.

• Contraction of the ring

pinches the cell in two.

• This is often referred to

as the “purse-string

method”.


Fig. 12.8a

• Cytokinesis in plants, which have cell walls,

involves a completely different mechanism.

• During telophase, vesicles

from the Golgi connect at

the metaphase plate,

forming a cell plate.

• The plate enlarges until its

membranes fuse with the

plasma membrane at the

perimeter, with the contents

of the vesicles forming new

wall material in between.


Fig. 12.8b

• The frequency of cell division varies

• Some human cells divide frequently throughout life (skin

cells), others have the ability to divide, but keep it in

reserve (liver cells), and mature nerve and muscle cells

do not appear to divide at all after maturity, although

there is current evidence that they may under certain

conditions.

• Cancer (tumour) cells seem to have lost such controls over their division, and divide repeatedly. Any cells in an organ can become cancerous.

Introduction


CHAPTER 13

MEIOSIS AND SEXUAL LIFE CYCLES


Section A: An Introduction to Heredity

1. Offspring acquire genes from parents by inheriting chromosomes

2. Like begets like, more or less: a comparison of asexual and sexual

reproduction

• Living organisms are distinguished by their ability to

reproduce their own kind.

• Offspring resemble their parents more than they do

less closely related individuals of the same species.

• The transmission of traits from one generation to the

next is called heredity or inheritance.

• However, offspring differ somewhat from parents

and siblings, demonstrating variation.

• Genetics is the study of heredity and variation.

Introduction


• Parents endow their offspring with coded

information in the form of genes, which are specific

sections of a chromosome.

• Your genome is derived from the thousands of genes that

you inherited from your mother and your father.

• Genes program specific traits that emerge as we

develop from fertilized eggs into adults.

• Your genome may include a gene for freckles, which you

inherited from your mother.

1. Offspring acquire genes from parents by

inheriting chromosomes


• In asexual reproduction, a single individual passes

along copies of all its genes to its offspring.

• Single-celled eukaryotes reproduce

asexually by mitotic cell division to

produce two identical daughter cells.

• Even some multicellular eukaryotes,

like hydra, can reproduce by budding

cells produced by mitosis.

2. Like begets like, more or less: a

comparison of asexual and sexual

reproduction


Fig. 13.1

• Sexual reproduction results in greater variation

among offspring than does asexual reproduction.

• Two parents give rise to offspring that have unique

combinations of genes inherited from the parents.

• Offspring of sexual

reproduction vary

genetically from

their siblings and

from both parents.


Fig. 13.2

• In humans, each somatic cell (all cells other than

sperm or ovum) has 46 chromosomes.

• Each chromosome can be distinguished by its size and the position of the centromere.

• A karyotype display of the 46 chromosomes shows 23 pairs of chromosomes, each pair with the same length and centromere position. Pair #1 is the longest, #2 second, etc.

• These homologous chromosome pairs carry genes that control the same inherited characters. There are a pair of blood type genes, for instance, one from that person’s mom and one from their dad.

1. Fertilization and meiosis alternate in

sexual life cycles


• Karyotypes, ordered displays of an individual’s chromosomes, are often prepared with lymphocytes, a type of white blood cell, or those from a fetus.


Fig. 13.3

• The occurrence of homologous pairs of chromosomes is a

consequence of sexual reproduction.

• We inherit one chromosome of each homologous pair

from each parent. This is a key concept!!!

• The 46 chromosomes in a somatic cell can be viewed as two sets of 23, a maternal set and a paternal set.

• Sperm cells or ova (gametes) have only one set of

chromosomes. A cell with a single chromosome set is

haploid.

• For humans, the haploid number of chromosomes is 23 (n = 23).


• IN sexual reproduction, a haploid sperm reaches

and fuses with a haploid ovum.

• The fertilized egg (zygote) now has two haploid

sets of chromosomes bearing genes from the

maternal and paternal family lines. It will divide

by mitosis to produce a new organism.

• The zygote and all cells with two sets of

chromosomes are diploid cells (2n).

• For humans, the diploid number of chromosomes is 46 (2n = 46).

• In fruit flies, the diploid number is 8 (2n = 8).


• As an organism develops from a zygote to a sexually mature adult, the zygote’s genes are passes on to all somatic cells by mitosis.

• Gametes, which develop in the gonads, are not produced by mitosis.

• If gametes were made by mitosis they would be diploid, and the fusion of gametes would produce offspring with four sets of chromosomes after one generation, eight after another, and so on.

• Instead, gametes undergo the process of meiosis, in which the chromosome number is halved. For this reason, it is also called reduction division.

• Human sperm or ova have a haploid set of 23 different chromosomes, one from each homologous pair.


• Fertilization restores the diploid condition by

combining two haploid sets of chromosomes.

• Fertilization and meiosis

alternate in sexual life

cycles.


Fig. 13.4

• Many steps of meiosis resemble steps in mitosis.

• Both are preceded by the replication of chromosomes.

• However, in meiosis, there are two consecutive cell

divisions, meiosis I and meiosis II, which results in four

daughter cells. Replication of chromosomes only

occurs before the first division, though.

• Each final daughter cell has only half as many

chromosomes as the parent cell.

3. Meiosis reduces chromosome number from

diploid to haploid: a closer look


• Meiosis reduces

chromosome number by

copying the chromosomes

once, but dividing twice.

• The first division, meiosis

I, separates homologous

chromosomes.

• The second, meiosis II,

separates sister

chromatids. Watch

• Or here, here and here.


Fig. 13.6

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html

../Animations/Animation 13.2.2 Meiosis I_ .MOV



• Division in meiosis I occurs in four phases:

prophase, metaphase, anaphase, and telophase.

• During the preceding interphase the chromosomes

are replicated to form sister chromatids.

• These are genetically identical

and joined at the centromere.

• Also, the single centrosome

is replicated.

• This is just like mitosis.


Fig. 13.7

• In prophase I, the chromosomes condense and

homologous chromosomes pair up to form tetrads.

• This does NOT happen in mitosis.

• Synapsis is the process by which homologous

chromosomes come together.

• A spindle forms from each

centrosome and spindle fibers

attach to each chromosome

and begins to

move the tetrads around.


Fig. 13.7

• At metaphase I, the tetrads are all arranged at the

metaphase plate. How is this NOT like mitosis?

• Microtubules from one pole (not from both as in

mitosis) are attached to one chromosome of each tetrad,

while those from the other pole are attached to the

other.

• In anaphase I,

the homologous

chromosomes

separate and

are pulled toward

opposite poles.


Fig. 13.7

• In telophase I, movement of homologous

chromosomes continues until there is a haploid set

(one from each pair) at each pole.

• Each chromosome still consists of two sister chromatids

(it is said to be double stranded).

• Cytokinesis by the same

mechanisms as mitosis

usually occurs simultaneously.

• In some species, nuclei

may reform, but there is

no further replication

of chromosomes.


Fig. 13.7

• Meiosis II is very similar to mitosis.

• During prophase II a spindle apparatus forms, attaches

to each sister chromatid, and moves the double stranded

chromomsomes around.

• Spindle fibers from one pole

attach to

one sister chromatid and

those of the other pole to

the other sister chromatid.


Fig. 13.7

• At metaphase II, the double stranded chromosomes

are arranged in single file at the metaphase plate.

• At anaphase II, the

centomeres of sister

chromatids separate

and the now separate

sisters travel toward

opposite poles.


Fig. 13.7

• In telophase II, separated sister

chromatids, now called single

stranded chromosomes, arrive

at opposite poles.

• Cytokinesis separates

the cytoplasm.

• At the end of meiosis,

there are four haploid

daughter cells, each with one

single stranded chromosome

from each pair.

• Let’s sing it!


Fig. 13.7

http://www.youtube.com/watch?v=hOuzYLNEuA4&feature=c4-overview&list=UUGJqRYuHwar5W6fzQO4VxxQ

• Mitosis and meiosis have several key differences.

• The chromosome number is reduced by half in meiosis,

but not in mitosis.

• Mitosis produces daughter cells that are genetically

identical to the parent cell and to each other.

• Meiosis produces cells that differ genetically from the

parent and each other.

• Mitosis produces two cells, meiosis produces 4 cells.

• What are some similarities between mitosis and

meiosis?


• Mitosis produces two identical daughter cells, but

meiosis produces 4 very different cells. Watch!


Fig. 13.8

http://www.pbs.org/wgbh/nova/body/how-cells-divide.html

Benchmark Clarification for SC.912.L.16.17

Students will:

• Differentiate the process of meiosis and meiosis

• Describe the role of mitosis in asexual reproduction, and/or the role of meiosis in sexual reproduction, including how these processes may contribute to or limit genetic variation

• Describe specific events occurring in each of the states of the cell cycle and/or phases of mitosis

• Explain how mitosis forms new cells and its role in maintaining chromosome number during asexual reproduction

• Content Limits: Items will focus on the relationship between mutations and uncontrolled cell growth, rather than a specific mutation that may result in uncontrolled growth

Of the phases shown, which is the first in

meiosis?

In many plants, a new plant can grow from a piece of the parent

plant. Strawberries reproduce this way, from runners, and

African violets can be grown from a leaf. Piece of potato tuber

can be used to grow new potato plants, as shown below. This

method of producing offspring is dependent on which

process? A. mutation during mitosis

B. mitosis during asexual reproduction

C. self-pollination during regeneration

D. meiosis during sexual reproduction

Which of these is most likely to result from the

processes of mutation and crossing over during

reproduction?

A. Offspring that are genetically identical to their

parents

B. Offspring that are genetically identical to each other

C. Decreased genetic variation among offspring

D. Increased genetic variation among offspring

Which statements (in box below) about cell

division are correct? A. statements 1 and 3

B. statements 1 and 4

C. statements 2 and 3

D. statements 2 and 4

Which statement is correct?

A. Meiosis is a way to reproduce, but mitosis is not.

B. Meiosis is a way to create diversity, but mitosis is not.

C. During mitosis, chromosomes are copied, but, during meiosis chromosomes double.

D. During mitosis, chromosome numbers double, but during meiosis, chromosome numbers remain constant.

Study the sequence below. Which cellular

process missing from the sequence

produces cells having a chromosome

number of 2n?

Which best explains how meiosis is a contributing factor to genetic variation within a species?

A. Meiosis reduces the number of mutations within an organism

B. Meiosis produces daughter cells that will contain identical chromosomes

C. Meiosis results in offspring that contain alleles from only one parent gamete

D. Meiosis allows for crossing over of chromosomes, resulting in new gene combinations

A cheetah is multicellular. A paramecium is unicellular. How

do these two organisms differ in terms of how they produce

offspring?

• A. The cheetah uses sexual reproduction, the

paramecium uses meiosis

• B. The cheetah uses binary fission, the

paramecium uses asexual reproduction

• C. The cheetah uses asexual reproduction, the

paramecium uses binary fission

• D. The cheetah uses sexual reproduction, the

paramecium uses asexual reproduction.

What would most likely result if mitosis was not

accompanied by cytoplasmic division?

A. Two cells, each with one nucleus

B. Two cells, each without a nucleus

C. One cell with two identical nuclei

D. One cell without a nucleus

When does crossing-over occur during

meiosis?

• A. when the DNA of the ciploid cell is copied

• B. when homologous chromosomes move to

opposite ends of the dividing cell

• C. when spindle fibers move the chromosomes

toward the midline of the dividing cell

• D. when homologous chromosomes pair and

portions of chromatids break off and are

exchanged

During meiosis, homologous chromosomes

line up next to each other. If one arm of a

chromatid crosses over the arm of another

chromatid, what results?

• A. an additional sex cell is created

• B. independent assortment of genetic material

• C. a possible change in the offspring cell’s

function

• D. additional variation in the DNA combination

of each sex cell formed

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