Download UNIT 2 Life Science Review 4: Cells and Cell Division Review 5

UNIT 2
Life Science
Review 4: Cells and Cell Division
Review 5: Heredity
Review 6: Adaptations to the Environment
Review 7: Interactions Among Organisms
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REVIEW 4
Cells and Cell Division
In 1665, Robert Hooke peered through his microscope at a very thin slice of bark from a cork
tree. He saw tiny little chambers arranged next to each other. These reminded him of the
small cells in which monks lived, so he called them cells. The discovery of the cell
changed the way scientists approached the study of life. Previously, it was thought that
four fluids, called humors, ran throughout the world and combined to make all matter,
including organisms. After Hooke's discovery, however, scientists eventually learned that
all organisms are composed of cells. This review looks at how these little machines of life
organize themselves to form the complex systems of living things.
Words to Know
asexual reproduction
cell
cell membrane
cell wall
chloroplost
chromosome
cytoplasm
meiosis
membrane
mitochondrion
mitosis
multicellular
nuclear membrane
nucleus
organelle
ribosome
sexual reproduction
unicellular
vacuole
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Cells
Multicellular organisms, such as plants and animals, are made of many cells. These cells, in
turn, contain many tiny structures called organelles. Each organelle has a special form and
purpose in the operation, maintenance, repair, and reproduction of the cell. The basic parts
of a plant cell are shown in the following diagram.
---see diagram
The following list briefly explains the function of each labeled component.
Cell membrane: semipermeable membrane that controls movement of molecules in and out of
the cell
Cell wall: the outer, nonliving cellulose structure that helps the plant cell keep its shape
Chloroplasts: organelles that contain chlorophyll used by plants in photosynthesis Cytoplasm:
clear, thick fluid that holds all the components of a cell
Endoplasmic reticulum: transports materials within the cell
Mitochondria: organelles that release energy to support all cell activity Nuclear membrane:
encloses and protects the nucleus
Nucleus: control center for all cell activity; contains chromosomes, which carry the genes that
help the organism reproduce
Ribosomes: organelles that contain the enzymes that help produce proteins
Vacuoles: cavities inside the cytoplasm that contain fluid and pigment (coloring)
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Animal cells have a lot in common with plant cells. Use the list of plant cell components to
label the following animal cell. (Two components of plant cells are not found in animal
cells.)
Why don't animal cells need chloroplasts?
Why is it important for plants to have cell walls? (Hint: Do plants have skeletons?)
Unicellular and multicellular organisms
Unicellular organisms have just one cell. Bacteria, yeast, and amoebas are unicellular. Their
one cell must perform all of their life functions. Humans are multicellular. Cells in
multicellular organisms, such as humans, are specialized to do different jobs. Brain cells
and liver cells, for example, have very different jobs. This specialization of cells has
allowed multicellular life to grow large and complex.
Mitosis
Both unicellular and multicellular organisms use a process of cell division called mitosis to
make new cells. In mitosis, one cell becomes two cells that are identical to each other and
to the original cell. The cell nucleus is the "master control" for each cell. It contains the
chromosomes, or DNA molecules, that contain all the
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genetic information. When a cell undergoes mitosis, each new cell must have the same
number of chromosomes as the original cell. For this reason, each chromosome of the
original cell must be duplicated before the cell divides.
What might happen to a new cell if it did not have the same number of chromosomes as
the original cell?
The following diagram shows three stages of mitosis in a cell with six chromosomes. In the
first stage, the chromosomes duplicate. In the second stage, the nuclear membrane breaks
down and the two sets of chromosomes are pulled to opposite sides of the cell. In the third
stage, the cell membrane pinches inward and splits one cell into two cells. After the cell
divides, each new cell develops a new nuclear membrane to protect the chromosomes.
---see diagram
In the example above, how many chromosomes does each new cell have?
A human cell has 46 chromosomes. If a human cell undergoes mitosis, how many
chromosomes will each new cell have?
Multicellular organisms use mitosis to grow and to repair damaged cells. Your skin cells, for
example, are constantly undergoing mitosis to replace dead or damaged skin cells.
Unicellular organisms use mitosis to reproduce, a type of reproduction called asexual
reproduction. In asexual reproduction, the offspring has one parent that multiplies
without using sex cells (egg and sperm) or fertilization. The offspring of asexual
reproduction are almost always exact genetic copies of the single adult parent.
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Name one organism (plant or animal) that reproduces asexually.
A certain organism with eight chromosomes reproduces by mitosis. Complete the following
diagram to show how the chromosomes are distributed.
---see diagram
Meiosis
The offspring of sexual reproduction have two parents. The male contributes a sperm cell; the
female contributes an egg cell. The sperm and egg cells are called gametes. Organisms that
reproduce sexually make gametes through a two-step process known as meiosis. The end
result of meiosis is four sex cells, each with half the chromosomes of the original cell. In
males, this produces four sperm cells. In females, only one of the four gametes becomes an
egg; the other three do not.
---see diagram
The illustration above shows meiosis in a cell that has four chromosomes.
Write a sentence stating how many chromosomes are in each sex cell that the organism
produces.
Most human cells have 46 chromosomes. How many chromosomes are in a human sex cell?
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Why do sex cells have only half as many chromosomes as other cells? This ensures that, when
a sperm and an egg cell unite, the new cell has the proper number of chromosomes to
produce a complete human. The process by which a sperm and an egg cell join is called
fertilization. In fertilization, half of the male's chromosomes join with half of the female's
chromosomes to make one complete set of chromosomes. The new organism now has a
complete set of genes. Because half of the genes came from one parent and half from the
other, the offspring's set of genes is different from either of the parent's set of genes.
A certain organism with eight chromosomes reproduces sexually. The diagram below shows a
sperm cell joining with an egg cell. Complete the diagram to show the process of
fertilization.
Keys to Keep
 Cells are the building blocks of life.
 Mitosis produces two cells that are copies of each other and the parent cell.
 Meiosis produces four cells, each with half the chromosomes of the parent cell.
 Some unicellular organisms use mitosis to reproduce asexually.
 Multicellular organisms use meiosis to produce sex cells.
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Explore It Yourself
In this lab, you will use pipe cleaners to simulate the stages of mitosis. Your teacher will give
you two white pipe cleaners, two green pipe cleaners, two twist ties, two pieces of yarn
each about 30 cm long, and a blank piece of paper.
Step 1: The blank piece of paper is the cell. On the paper, place one piece of yarn into a circle.
The yarn is the nucleus. Put one white pipe cleaner and one green pipe cleaner into the
nucleus. Each pipe cleaner is a chromosome. The chromosomes have not yet duplicated.
How many chromosomes does the original cell have?
Step 2: Just before mitosis, the cell duplicates its chromosomes. Show this by adding the
remaining pipe cleaners to the nucleus.
Step 3: At the start of mitosis, a structure called a centromere attaches identical chromosomes
to each other. Use the twist ties to attach the identical chromosomes to each other.
Step 4: During mitosis, the boundary of the nucleus breaks down. Show this in your model by
taking away the yarn.
Step 5: In the next step of mitosis, each pair of identical chromosomes moves to the middle of
the cell. Show this in your model.
Step 6: In the next step of mitosis, the centromeres break. The duplicated chromosomes move
to opposite ends of the cell. Show this in your model.
Step 7: In the next step of mitosis, new nuclei form around both sets of chromosomes. Show
this in your model.
Step 8: After mitosis is finished, one cell divides into two cells. Show this in your model by
drawing a line down the middle of the page.
How many chromosomes does each new cell have?
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What Does It Mean?
1. Are the two new cells identical to each other and the original cell? Or are they different than
each other and the original cell? Explain your answer.
2. Suppose that during mitosis a centromere holding together one of the pairs of chromosomes
failed to break. Suppose, also, that the cell continued through mitosis and then divided into
two cells. (It may help to use your model to enact this outcome.)
How many chromosomes would one of the new cells end up having?
How many chromosomes would the other new cell end up having?
Do you think these new cells would function normally? Explain your answer.
3. Some cells in the body need to be replaced more often than others. The cells in your skin
need to be replaced more often than the cells in your bones. How would the amount of
mitosis taking place in your skin compare to the amount of mitosis taking place in your
bones?
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AIMS Science Practice
DIRECTIONS: Use the following information to answer questions 1 through 3.
A cell with 12 chromosomes undergoes meiosis. The cell is part of a multicellular organism.
1 How many new cells does the process of meiosis produce?
A 2
B 3
C 4
D 8
2 How many chromosomes does each new cell have?
A 3
B 4
C 6
D 12
3 For what purpose is the cell undergoing meiosis?
A the growth of bones
B the repair of damaged skin
C the production of new blood cells
D the formation of sex cells
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4 The process of fertilization produces a single-celled zygote with a full set of chromosomes.
By what process does the zygote then grow and develop?
A meiosis
B mitosis
C osmosis
D plasmosis
5 Which function in the body of a multicellular organism is most likely being performed by the
process illustrated below?
---see diagram
A repair of skin after a cut
B formation of sex cells
C distribution of gamete chromosomes
D production of a new organism
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