Download Genes, Chromosomes, and Numbers

Genes, Chromosomes, and Numbers
• Genes do not exist free in the nucleus of a cell;
they are lined up on chromosomes.
• Typically, a chromosome can contain a
thousand or more genes along its length.
Website upload 2014
1
Diploid and haploid cells
• In the body cells of animals and most plants,
chromosomes occur in pairs.
• A cell with two of each kind of chromosome is called
a diploid cell and is said to contain a diploid, or 2n,
number of chromosomes. (46 in humans)
• Organisms produce gametes that contain one of each
kind of chromosome.
• A cell containing one of each kind of chromosome is
called a haploid cell and is said to contain a haploid,
or n, number of chromosomes. (23 in humans)
Website upload 2014
2
Karyotype
• A karyotype is a photographic inventory of an
individual’s chromosomes.
• A human female karyotype is shown below.
Website upload 2014
3
diploid = 2
copies
2n
Human female karyotype
46 chromosomes
23 pairs
XX
Website upload 2014
4
diploid = 2
copies 2n
Human male karyotype
46 chromosomes
23 pairs
XY
Website upload 2014
5
Homologous chromosomes
• The two chromosomes of
each pair in a diploid cell
are called homologous
chromosomes.
• Each pair of homologous
chromosomes has genes
for the same traits.
Sister chromatids
Website upload 2014
6
Paired chromosomes
• Homologous chromosomes
– both chromosomes of a pair carry “matching” genes
• control same inherited characters
• homologous = same information
diploid
eye color eye color
2n
(brown?) (blue?)
2n = 4
homologous double stranded
homologous chromosomes
Website upload 2014
7
chromosomes
Cell division / Asexual reproduction
• Mitosis
– produce cells with same information
• identical daughter cells
– exact copies
• clones
– same number of chromosomes
• same genetic information
Website upload 2014
Aaaargh!
I’m seeing
double!
8
Asexual reproduction
• Single-celled eukaryotes
– yeast
– Paramecium
– Amoeba
• Simple multicellular
eukaryotes
– Hydra
• budding
What are the
disadvantages of
asexual reproduction?
What are the Website upload 2014
advantages?
9
How about the rest of us?
• What if a complex multicellular organism (like us)
wants to reproduce?
– joining of egg + sperm
• Do we make egg & sperm by mitosis?
No!
What if we did, then...
 
46
egg
+
46
92
sperm
zygote
Website upload 2014
Doesn’t work!
10
How do we make sperm & eggs?
• Must reduce 46 chromosomes  23
– must half the number of chromosomes
– haploid
46
23
meiosis egg
46
zygote
23
46
23
23
fertilization
sperm
Website upload 2014
gametes
11
Meiosis makes sperm & eggs
• 46 chromosomes to 23 chromosomes
– half the number of chromosomes
23
46
meiosis
46
diploid
egg
23
Website upload 2014
sperm haploid12
Why Meiosis?
• When cells divide by mitosis, the new cells have
exactly the same number and kind of chromosomes
as the original cells.
• Imagine if mitosis were the only means of cell
division.
• IF the parent organism has 14 chromosomes, it
would produce gametes that contained a complete
set of 14 chromosomes.
• The offspring would have cell nuclei with 28
chromosomes, and the next generation would have
cell nuclei with 56 chromosomes, and so on.
Website upload 2014
13
Why Meiosis?
• There must be another form of cell division that
allows offspring to have the same number of
chromosomes as their parents.
• This kind of cell division, which produces gametes
containing half the number of chromosomes as a
parent’s body cell, is called meiosis.
• Meiosis reduces the chromosome number from
diploid to haploid.
Website upload 2014
14
Why Meiosis?
• Meiosis consists of two separate divisions, known as
meiosis I and meiosis II.
• Meiosis I begins with one diploid (2n) cell.
• By the end of meiosis II, there are four haploid (n) cells.
• These haploid cells are called sex cells— gametes.
Website upload 2014
15
Why Meiosis?
• Male gametes are called sperm.
• Female gametes are called eggs.
• When a sperm fertilizes an egg, the resulting zygote
once again has the diploid number of chromosomes.
• This pattern of reproduction, involving the production
and subsequent fusion of haploid sex cells, is called
sexual reproduction
Website upload 2014
16
The Phases of Meiosis (I)
Interphase
Interphase
• The cell replicates its
chromosomes.
• After replication, each
chromosome consists of
two identical sister
chromatids, held
together by a
centromere.
Website upload 2014
17
The Phases of Meiosis (I)
Prophase I
Prophase I
• The chromosomes coil up
and a spindle forms.
• As the chromosomes coil,
homologous
chromosomes line up
with each other gene by
gene along their length,
to form a four-part
structure called a tetrad.
Website upload 2014
18
The Phases of Meiosis (I)
Prophase I
Prophase I
• The chromatids in a
tetrad pair tightly
• In fact, they pair so
tightly that non-sister
chromatids from
homologous
chromosomes can
actually break and
exchange genetic
material in a process
known as crossing over.
Website upload 2014
19
The Phases of Meiosis (I)
Prophase I
•Crossing over can occur at any location on a chromosome,
and it can occur at several locations at the same time.
•Genetic recombination results from crossing over during
prophase I of meiosis.
–This increases variation further
Website upload 2014
20
The Phases of Meiosis (I)
Metaphase I
Metaphase I
• During metaphase I, the
centromere of each
chromosome becomes
attached to a spindle
fiber.
• The spindle fibers pull the
tetrads into the middle.
Website upload 2014
21
The Phases of Meiosis (I)
Anaphase I
Anaphase I
• begins as homologous
chromosomes, each with
its two chromatids,
separate and move to
opposite ends of the cell.
• This critical step ensures
that each new cell will
receive only one
chromosome from each
homologous pair.
Website upload 2014
22
The Phases of Meiosis (I)
Telophase I
• Telophase I
• Events occur in the
reverse order from the
events of prophase I.
• The spindle is broken
down, the chromosomes
uncoil, and the cytoplasm
divides to yield two new
cells.
Website upload 2014
23
Meiosis 1 overview
• 1st division of meiosis
Copy DNA during
Divide 1
Line Up 1
interphase
prophase I
metaphase I
 4 chromosomes
 diploid
 2n
gamete
double
stranded
telophase I
Website upload 2014
 2 chromosomes
 haploid
24
 1n
The Phases of Meiosis (II)
• The phases of meiosis II
• The second division in
meiosis is simply a
mitotic division of the
products of meiosis I.
• During prophase II, a
spindle forms in each of
the two new cells and the
spindle fibers attach to
the chromosomes.
Website upload 2014
Prophase II
25
The Phases of Meiosis (II)
Metaphase II
• Metaphase II
• The chromosomes, still
made up of sister
chromatids, are pulled to
the center of the cell and
line up randomly at the
equator.
Website upload 2014
26
The Phases of Meiosis (II)
• Anaphase II
• Begins as the centromere
of each chromosome
splits, allowing the sister
chromatids to separate
and move to opposite
poles.
Website upload 2014
Anaphase II
27
The Phases of Meiosis (II)
• Telophase II
• Finally nuclei, reform, the
spindles break down, and
the cytoplasm divides.
• At the end of meiosis II,
four haploid cells have
been formed from one
diploid cell.
Telophase II
• These haploid cells will
become gametes,
transmitting the genes
they contain to offspring.
Website upload 2014
28
The Phases of Meiosis
• At the end of meiosis II, four haploid cells have been
formed from one diploid cell.
• These haploid cells will become gametes, transmitting
the genes they contain to offspring.
Website upload 2014
29
Meiosis II overview
Bye Bye 2
telophase II
telophase I
Line Up 2
• 2nd division of meiosis
– looks like mitosis
 2 chromosomes
 haploid
 1n
metaphase II
gametes
Website upload 2014
4
30
Meiosis = reduction division
• Meiosis
– special cell division in sexually reproducing
organisms
– reduce number of chromosomes
• 2n  1n
• diploid  haploid
–half
– makes gametes
• sperm, eggs
Website upload 2014
31
Meiosis & mitosis
• Meiosis to make
gametes
– sperm & egg
• Mitosis to make
copies of cells
– growth
– repair
– development
Website upload 2014
32
Comparison of Mitosis & Meiosis
• MITOSIS
MEIOSIS
Website upload 2014
33
Sexual reproduction lifecycle
 2 copies
 diploid
 2n
 1 copy
fertilization
 haploid
 1n
We’re mixing
things up here!
meiosis
 1 copy
 haploid
 1n
A good thing?
Website upload 2014
34
Putting it all together…
meiosis  fertilization  mitosis + development
gametes
46
23
meiosis egg
46
23
23
23
zygote
fertilization
sperm
46
46 46
46 4646
46
46
46
Website upload 2014
mitosis
mitosis
&
development
35
Meiosis Provides Genetic Variation
• Cells that are formed by mitosis are identical to each
other and to the parent cell.
• Crossing over during meiosis I, however, provides a
way to rearrange gene combinations.
• Thus, variability is increased.
• Random arrangement of chromosomes and the
genetic information they carry is called genetic
recombination.
Website upload 2014
36
What are the
advantages of
asexual reproduction?
What are the
DISadvantages of
asexual reproduction?
Any Questions??
What are the
advantages of
sexual reproduction?
Website upload 2014
What are the
DISadvantages of
sexual reproduction?
37