Chromosomes

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Chapter 6 Chromosomes Cell Reproduction
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6-1 Chromosomes
About 2 trillion cells are produced By an adult
human body everyday!
This is about 2 million new cells Every second!!
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These new cells are produced When other cells
divide.
Cell division is also called Cell reproduction.
Gametes are an organisms Reproductive cells.
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Remember that all of the cells Information is
stored in the DNA.
So if/when a cell divides, the DNA must also
divide and move Into the newly created cell.
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Bacteria reproduce by a type of Cell division
called binary fission.
Binary fission is a form of asexual Reproduction
that produces Identical offspring.
In asexual reproduction, a single Parent passes
exact copies Of its DNA to its offspring.
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Binary fission occurs in 2 stages
1st the DNA is copied And 2nd the cell divides.
The bacterium divides by adding A new cell
membrane to a point On the membrane between
the Two DNA copies.
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A new cell wall forms around The new membrane.
Eventually the dividing bacterium is Pinched into
2 independent cells.
Each cell contains one of the circles Of DNA and
is a complete Functioning bacterium.
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Eukaryotic cells form chromosomes Before cell
division.
A vast amount of information encoded In DNA is
organized into units Called genes.
A gene is a segment of DNA that Codes for a
protein or RNA molecule.
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A single molecule of DNA has Thousands of genes
lined up Like train cars.
Genes play an important role in Determining how
a persons Body develops and functions.
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As a eukaryotic cell prepares to Divide, the DNA
and the proteins Associated with the DNA coil
into A structure called a chromosome.
Before the DNA coils up, the DNA Is copied.
The two exact copies of DNA that Make up each
chromosome Are called chromatids.
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The two chromatids of a chromosome Are attached
at a point called A centromere.
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Each human cell normally has 2 Copies of 23
different chromosomes, For a total of 46
chromosomes.
The 23 chromosomes differ in size, Shape, and set
of genes.
Each of the 23 pairs of chromosomes Consists of 2
homologous chromosomes, Also called homologues.
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Homologous chromosomes are Chromosomes that are
similar in Size, shape, and genetic content.
Each homologue in a pair of Homologous
chromosomes comes From one of the 2 parents.
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When a cell contains 2 sets of Chromosomes, it
is said to be Diploid.
Gametes contain only 1 set of Chromosomes.
When a cell contains 1 set of Chromosomes, it is
said to be Haploid.
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The haploid number in a human Gamete can be
written as n 23
The diploid number in a cell can be Written as 2n
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The fusion of 2 haploid gametes, A process called
fertilization, A diploid zygote is formed.
A zygote is a fertilized egg cell, The 1st cell
of a new individual.
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Of the 23 pairs of chromosomes in Human somatic
cells, 22 pairs Are called autosomes.
Autosomes are chromosomes that are Not directly
involved in determining The sex of an individual.
The sex chromosomes, one of the 23 pairs of
chromosomes in humans, Contain genes that will
determine The sex of the individual.
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In humans, and other organisms, The 2 sex
chromosomes are Referred to as the X and Y
Chromosomes.
The genes that create a male are Located on the
Y chromosome and The genes that create a
female Are located on the X chromosome.
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6-2 The Cell Cycle
Cell division in eukaryotic cells is More complex
than bacteria cells.
This is because it involves dividing Both the
cytoplasm and the Chromosomes.
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The life of a eukaryotic cell is Usually shown
as a cycle.
The cell cycle is a repeating sequence Of
cellular growth and division.
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A cell spends 90 of its time in the First 3
phases-these are collectively Called interphase.
A cell will enter the last 2 phases Only if it is
about to divide.
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The 5 phases of the cell cycle
1) First growth phase (G1)
During G1 a cell grows rapidly And carries out
its routine functions. Cells that are not
dividing remain In this phase.
Some cells never divide (muscle nerve) And thus
stay in this phase forever.
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2) Synthesis (S) phase.
A cells DNA is copied during this Phase. At the
end of this phase, Each chromosome consists of
2 Chromatids attached at the Centromere.
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3) Second Growth phase (G2)
In G2, preparations are made for the Nucleus to
divide. Hollow protein Fibers called microtubules
are Assembled. The microtubules Are used to move
the chromosomes During mitosis.
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4) Mitosis
The process during the cell division In which the
nucleus of the cell is Divided into 2 nuclei is
called Mitosis.
Each nucleus ends up with the same Number and
kinds of chromosomes As the original cell.
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5) Cytokinesis
The process during cell division in Which the
cytoplasm Divides is called cytokinesis.
Mitosis and cytokinesis produce New cells that
are identical to The original cells and allow
Organisms to grow.
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If a cell spends 90 of its time in Interphase,
how do cells know When to divide?
The cell has checkpoints at which Feedback
signals from the cell Trigger the next phase of
the cycle.
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The cell cycle in eukaryotes is Controlled by
many proteins, control Occurs at 3 principle
checkpoints
They are at the end of G1, G2, And at Mitosis.
When cells lose their control over When to
divide, cancer forms.
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6-3 Mitosis
During mitosis the nucleus divides To form 2
nuclei.
Also, the chromatids on each Chromosome are
physically moved To opposite sides of the
dividing cell.
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Spindles are cell structures made up Of both
centrioles and individual Microtubule fibers that
are Involved in moving the chromosomes To
opposite sides of the cell.
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Mitosis has 4 phases
Step 1 -gt Prophase
Chromosomes coil up and become Visible during
prophase. The nuclear Envelope dissolves and a
spindle forms.
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Step 2 -gt Metaphase
During metaphase the chromosomes Move to the
center of the cell and line Up along the equator.
Spindle fibers Link the chromatids of each
Chromosome to opposite poles.
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Step 3 -gt Anaphase
Centromeres divide during anaphase. The 2
chromatids move toward Opposite poles as the
spindle Fibers begin to shorten.
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Step 4 -gt Telophase
A nuclear envelope forms around the Chromosomes
at each pole. Chromosomes, now at opposite
poles, Uncoil and the spindle dissolves.
Mitosis is complete!
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As mitosis ends, cytokinesis begins.
During cytokinesis, the cytoplasm Of the cell is
divided in half, and The cell membrane grows to
enclose Each cell, forming 2 daughter cells.
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THE END