Genetics Part 1:
Cell reproduction
LS1I: Cell Reproduction and how the process of
genetic recombination and sexual reproduction
produces variation in offspring.
A Quick Review
Do you remember where the nucleus is found?
The nucleus is found inside all eukaryotic cells.
What is the function or purpose of the nucleus?
The nucleus is the control center of the cell.
What is stored inside the nucleus?
DNA is stored inside the nucleus.
Why is DNA important?
DNA contains the instructions on how to maintain
the function and activity of our cells. It is the
molecule that essentially controls the expression and
process of “life”.
Organelles
What is an organelle?
Definition: An organelle is a structure found within
a cell that has a specific job it
performs to keep cells
functioning properly.
What is DNA?
DNA Stands for “Deoxyribonucleic Acid”
Definition: The molecule that holds the
instructions for building an organism and how
to keep it biologically functioning.
A. Found in the nucleus of all cells
B. Stored as structures called chromosomes
C. In our cells, these chromosomes always are
found in pairs.
1. There is one exception we will talk about
later.
The shape of
DNA is described
as a
“double helix”
Chromosomes
Definition: The structure in the nucleus
that is used as the storage form of DNA
A. Each species has its own unique set
Humans have  23 pairs for a total of 46
chromosomes (23 from mom, 23 from dad).
B. Only visible during replication
C. The pairs are called “homologous pairs”.
D. Different chromosomes contain
different genes.
E. Two types of chromosomes
1.
2.
Autosomes: 22 pairs in our cells
Sex Chromosomes: 1 pair, this
determines your gender.
How Many Chromosomes?
Humans have how many singular chromosomes?
Pairs?
Can you guess how many chromosomes these
organisms have?

Goldfish = 94
Fern = 64
Mosquito = 6
Strawberry = 56
Cat = 38
Dog= 78
Karyotype
A Karyotype is a “genetic inventory” of the chromosomes present.
A female will have 2 “X” chromosomes, while a male will have
one “X” and one “Y” chromosome.
Genes
Definition: A segment of DNA that is
responsible for the control or expression
of a very specific trait.
A.
Examples of traits controlled by
genes:
1. Eye color
2. Hair texture
3. Ability to break down
lactose (milk sugar).
4. Ability for your blood
to clot if you get a cut.
Vocabulary terms
Diploid: Refers to cells that have 23 pairs of
chromosomes (all 46 in humans)
A. Abbreviated as “2n”.
B. Chromosomes are in pairs, like shoes.
Haploid: Refers to cells that only have HALF the
complete set of chromosomes
A. Abbreviated as “n”.
B. For humans, that number is 23 chromosomes.
C. There are no pairs, only singles.
Did you Realize……..
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The human body is composed of roughly 100
TRILLION cells.
Very few cells, like the neurons that compose the
nervous system, live “forever”.
The rest of our cells, have definite lifespans:

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
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Bone Cells: 25 – 30 yrs
Pancreas cells: 1 year or so
Red Blood Cells: 4 months
Skin Cells: 2 – 4 weeks
Cells lining our stomach: 2 days
So how do our cells get replaced?
Reproduction of Cells
Reproduction of cells occurs in one of 2
ways
1. Mitosis- The reproduction of regular
body (somatic) cells. (growth & repair)
2. Meiosis- The production of
reproductive cells (egg or sperm)
a. Only occurs in the testes in males, ovaries
in females.
The Cell Cycle
There are 5 phases in the
Cell Cycle
1. G-1: The growth
phase of the cell. Cell
grows rapidly and carries out
it’s normal functions. This phase occupies the major portion
of time for a cell.
2. S Phase: DNA replication occurs, all chromosomes have
been copied, so there are double the number.
3. G-2 Phase: Preparations made for cell division (organelles are
duplicated).
4. M Phase: Mitosis occurs; nucleus divides in two.
5. C-Phase: “Cytokinesis” (the cell splits into two cells).
Overview of Mitosis
1. Parent cell contains
23 pairs of chromosomes
2n
(46)
2. At the end of mitosis,
Replication of DNA
both daughter cells are
identical to each other AND
4n
the original (parent) cell.
(92)
3. This type of reproduction
occurs in all our regular
2n
body cells.
2n
(46)
(46)
Stages of Mitosis
Mitosis is made up of four stages:
1. Prophase: Chromosomes condense, become visible.
Nuclear membrane dissolves.
2. Metaphase: Chromosomes move to center of cell.
3. Anaphase: Chromosomes separate.
4. Telophase: Spindle fibers pull chromosomes to opposite
poles of the cell.
5. Following telophase is cytokinesis.
6. Then the cell enters “interphase”: the G-1 phase
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What does it look like?
Splitting Of The Chromosome
The chromosome as we
recognize it is made up of
sister chromatids held
together by a centromere.
A. The reason it looks like an
“x” is because it has
replicated.
B. Typically, the chromosome
will appear as a single strand.
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Stages of Mitosis
If you observe cells under a microscope, you can see the nuclei
In different stages (phases) of mitosis.
As you look at the cells, they are not
all in the same phases or stages of
mitosis
What Does it Look Like?
Watching Mitosis
Here is what it looks like in action:
Meiosis
Definition: the process by which haploid
cells are created from diploid parent
cells
1. Has two phases: Meiosis 1
Meiosis 2
2. Occurs only in the ovaries or testes
3. Results in 4 haploid cells from each
diploid cell
4. These cells are called “gametes”
> Eggs and Sperm
Gametes
Definition:
cells produced by organisms that reproduce
sexually. The gametes combine during
fertilization to create offspring.
A. Both cells are haploid (only have half the
full set of chromosomes)
B. Dad’s 23 in the sperm (hap) + mom’s 23
in the egg (hap)= 46 total (diploid)
Overview of Meiosis
2n
(46)
2n = 23 pairs of
Chromosomes for a
Replication of DNA
Total of 46
n= 23 single
Chromosomes,
NO pairs
2·2n
(92)
2n
(46)
Crossing 2n
(46)
over
Meiosis I
Segregation/Independent Assortment of genes
Meiosis II
n
n
n
n
(23)
(23)
(23)
(23)
Because of crossing over & segregation/I.A., these gametes are not
identical
Watching Meiosis
Parent A: Full set
Parent B: Full set
2 copies of all
23 chromosomes
2 copies of all
23 chromosomes
Meiosis
½ set
½ set
½ set
½ set
1copy
1 copy
1copy
1 copy
Sexual Reproduction
Offspring: Full set
2 copies of all 23 chromosomes
Even more Variation:
Crossover
Def: when two copies
of a chromosome
exchange sections
to produce a new
combination of the
existing genes from
the parents.
Even More Variation:
Independent Assortment
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Independent Assortment is another way in which
offspring are unique from their parents and
siblings.
When the homologous pairs of chromosomes
separate, there is no predictable pattern of which
will end up in a given egg or a given sperm cell.
mistakes in Meiosis
 Trisomy
Def: when diploid cells have three copies of a
chromosome
A. Result of uneven distribution during
anaphase
B. One gamete ends up with two copies
instead of the normal one copy
What is Down’s Syndrome?
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1. It is called a “chromosomal disorder”
because there is an extra copy or fragment of
chromsome #21
2. Lower than average cognitive ability.
3. Physical differences
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A. Small chin
B. Overly round face
C. Almond shaped eyes
D. Protruding tongue
Other Trisomy Examples
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1. Most fetuses with trisomies die within the womb early
in development.
2.Trisomy 16: Edward’s Syndrome
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A. Kidney and heart defects
B. Small head, low set ears, widely spaced eyes and heavily
drooping eyelids.
3.Trisomy 13: Patau Syndrome
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A. Mental/motor challenged
B. Extra fingers or toes
C. Low set ears, eye defects and small head
Downs
Syndrome
Edwards
Syndrome
Patau
Syndrome
 Monosomy
Def: when diploid cells have
one copy of a chromosome
1. Result of uneven
distribution during
meiosis II
2. One gamete ends up
with no copies of the
given chromosome
Turner’s Syndrome
1. All or part of an X chromosome is missing
2. Short stature, swelling, broad chest, low hairline
low set ears and webbed neck.
3. Non-working ovaries and an absence
of a menstrual cycle.
Summary
1.
2.
3.
4.
The nucleus, the control center of the cell, stores
DNA in the form of structures called
“chromosomes”.
In our body cells, chromosomes occur in pairs, in
our gametes, they occur as singles.
Cells need to reproduce to allow for growth,
repair and continuity of life. Mitosis is the process
that allows this to happen.
Our eggs and sperm (gametes) are formed by a
process called meiosis which takes place in
specialized cells in the testes or ovaries.