Transcript A. Chromosomes

Honors Biology
Chapter 9
JUST THE
FACTS!
Cell Reproduction = the process
by which cells divide to create
new cells = cell division
Why Most Cells Divide:
• 1. Surface Area to Volume Ratio:
- As cells grow their volume increases
faster than their surface area.
- This decreases the surface area to volume
ratio.
- So, diffusion and other forms of transport
cannot happen fast enough to keep the
cell alive.
What would the volume of a cell be if each
side measured 4 cm?
Why Most Cells Divide: (cont.)
• 2. DNA Overload:
– Genetic information cannot be processed
fast enough in a really large cell to keep
the cell functioning.
– Giant amebas have 2 nuclei with 2 sets of
chromosomes to overcome this problem!
Cell Cycle = the orderly sequence
of growth and division for a cell
• Events of the Cell Cycle:
• G1 Phase: Period of growth before DNA is
duplicated
• S Phase: DNA is replicated (duplicated)
Cell Cycle
• Events of the Cell Cycle: (Cont.)
• G2 Phase: More growth, and organelles
and materials needed for cell division are
made.
• M Phase: Cell division
– 2 Parts:
– Nuclear Division (Mitosis)
– Cytoplasm Division (Cytokinesis)
Cell Cycle
B. The Cell Cycle- Series of events that cells go
through as they grow and divide
1. Consists of 4 phases (interphase= G1, S,
and G2)
a. M phase (mitosis)division of cell nucleus and
cytokinesis
b. G1 phase (gap)-periods
of growth and activity
c. S phase (synthesis)DNA synthesized
(duplicated)
d. G2 phase (gap)- period
of growth and acitivity.
Organelles produced.
Two Types of Nuclear Division
TYPE
Mitosis
Meiosis
WHO?
(Type of Cell)
WHY?
(Purpose)
Somatic (Body)
Cells = any cell
but a sex cell
Make identical
copies of the
original cell for
growth, repair and
maintenance =
clones.
Sexual
reproductive cells
(sex cells)
To reduce the
chromosome number
by half.
To increase genetic
variation
D. Cytokinesis- division of cytoplasm and
organelles. Follows mitosis or meiosis (division of
the nucleus)
1. Animal cells- cell
membrane drawn inward until
cytoplasm is pinched into two
nearly equal parts
2. Plant cells- cell plate
forms midway between
divided nuclei. Gradually
develops into separating
membrane. Eventually cell
wall begins to appear.
Asexual Reproduction
• Single parent produces
offspring
• All offspring are genetically
identical to one another and to
parent
Chromosome = thin thread of DNA
wrapped around histone proteins.
• Structure:
•
Chromosome
A. Chromosomes- genetic information carried on
chromosomes
1. Before cell division each
chromosome is replicated
(copied)
2. Each chromosome
consists of two identical
“sister” chromatids
3. Each pair of
chromosomes attached to
area called centromere
Chromosome Number
• Chromosome number is expressed as
either:
• 1. Diploid = 2n = the number of
chromosomes found in somatic cells,
where n = the number of homologous
pairs.
Chromosome Number (cont.)
• Haploid Number = n = the number of
chromosomes in a gamete of that
organism. (Half the diploid number)
• Examples:
– Humans: n = 23
– Gorillas: n = 24
– Pea Plants: n = 7
– Why is it necessary for gametes to have half
the chromosome number of somatic cells?
CHROMOSOMES
• Homologous chromosomes = a matched
set; each chromosome has a partner
chromosome that looks just like it and
contains genes for the same things.
Homologous Chromosomes
Carry Different Alleles
• Cell has two of each chromosome
• One chromosome in each pair from mother, other from
father
• Paternal and maternal chromosomes carry different
alleles (allele = different form of a gene, but for the same
inherited trait. Ex. We have genes for eye color, but you
could have an allele for brown eyes, or an allele for blue
eyes.
Mitosis Details: Interphase
• Usually longest part of the cycle
• Cell increases in mass
• Number of cytoplasmic
components doubles
• DNA is duplicated
Mitosis Details: Prophase
Duplicated chromosomes begin to condense
Mitosis Details: Late Prophase
• New microtubules are
assembled
• One centriole pair is
moved toward
opposite pole of
spindle
• Nuclear envelope
starts to break up
Mitosis Details: Transition to
Metaphase
• Spindle forms
• Spindle
microtubules
become attached to
the two sister
chromatids of each
chromosome
Mitosis Details: Metaphase
• All chromosomes
are lined up at the
spindle equator
• Chromosomes
are maximally
condensed
Mitosis Details: Anaphase
• Sister chromatids of
each chromosome
are pulled apart
• Once separated,
each chromatid is a
chromosome
• Chromosomes
de-condense
• Two nuclear
membranes
form, one around
each set of
unduplicated
chromosomes
Telophase
• Two daughter
nuclei
• Each with same
chromosome
number as parent
cell
• Chromosomes in
unduplicated form
Results of
Mitosis
Mitosis Details - Result
• Mitosis is then followed by Cytokinesis.
• Final Result:
– 2 identical offspring cells that have the diploid
chromosome number and the same genetic
information as the original parent cell.
Mitosis Details (cont.)
• Abnormalities:
– Cancer: uncontrolled cell division
Meiosis Details
Chapter 9
Sexual Reproduction
• Involves
–Meiosis
–Gamete production
–Fertilization
• Produces genetic variation
among offspring
Sexual Reproduction
Shuffles Alleles
• Through sexual reproduction, offspring
inherit new combinations of alleles, which
leads to variations in traits
• This variation in traits is the basis for
evolutionary change
Factors Contributing to Variation
Among Offspring
• Crossing over during prophase I
• Random alignment of chromosomes
at metaphase I
• Random combination of gametes at
fertilization
Gamete Formation
• Gametes are sex cells (sperm, eggs)
• Arise from germ cells
ovaries
testes
anther
ovary
Meiosis: Phases and Events
• Two consecutive nuclear divisions
–Meiosis I
–Meiosis II
• DNA is NOT duplicated between
divisions
• Four haploid nuclei are formed
Stages of Meiosis
• Prophase I
Meiosis II
• Prophase II
• Metaphase I
• Metaphase II
• Anaphase I
• Anaphase II
• Telophase I
• Telophase II
Meiosis I
Meiosis I - Stages
Prophase I
Metaphase I Anaphase I
Telophase I
Prophase I
• Each duplicated, condensed
chromosome pairs with its
homologue
• Homologues swap segments =
Crossing Over
• Each chromosome becomes
attached to microtubules of
newly forming spindle
Crossing Over
•Each chromosome
becomes zippered to its
homologue
•All four chromatids are
closely aligned = tetrad
•Non-sister chromosomes
exchange segments
Meiosis I
Each homologue in the
cell pairs with its partner,
then the partners
separate
Effect of Crossing Over
• After crossing over, each chromosome
contains both maternal and paternal
segments
• Creates new allele combinations in
offspring
Metaphase I
• Chromosomes are pushed and
pulled into the middle of cell
• Sister chromatids of one
homologue orient toward one
pole, and those of other
homologue toward opposite pole
• The spindle is now fully formed
Anaphase I
• Homologous chromosomes
segregate(separate) from each
other
• The sister chromatids of each
chromosome remain attached
Telophase I & Cytokinesis
• The chromosomes arrive at
opposite poles
• The cytoplasm divides
• There are now two haploid cells
• This completes Meiosis I
Meiosis II - Stages
Prophase II
Metaphase II Anaphase II Telophase II
Prophase II
• Microtubules attach to the
kinetochores of the duplicated
chromosomes
• Motor proteins drive the
movement of chromosomes
toward the spindle’s equator
Metaphase II
• All of the duplicated chromosomes are
lined up at the spindle equator, midway
between the poles
Anaphase II
• Sister chromatids separate to become
independent chromosomes
• Motor proteins interact with microtubules
to move the separated chromosomes to
opposite poles
Telophase II & Cytokinesis
• The chromosomes arrive at opposite ends
of the cell
• A nuclear envelope forms around each set
of chromosomes
• The cytoplasm divides
• There are now four haploid cells
Random Alignment
• Either the maternal or paternal member of
a homologous pair can end up at either
pole
• The chromosomes in a gamete are a mix
of chromosomes from the two parents
Possible Chromosome
Combinations
As a result of random alignment, the number
of possible combinations of chromosomes
in a gamete is:
2n
(Humans = 8,388,608!!!)
(n is number of chromosome types)
1
Possible
Chromosome
Combinations
or
or
or
2
3
Results of Meiosis
• Meiosis
– Four haploid cells produced
– Differ from parent and one another
Abnormalities
• Nondisjunction: Chromosome pairs fail to
separate during Meiosis
• Causes Chromosomal abnormalities –
extra or missing chromosmes
• Ex. Down Syndrome
Spermatogenesis
secondary
spermatocytes
(haploid)
spermatogonium
(diploid male
reproductive
cell)
primary
spermatocyte
(diploid)
spermatids (haploid)
Growth
Mitosis I,
Cytoplasmic division
Meiosis II,
Cytoplasmic division
Oogenesis
first polar
body
haploid)
oogonium
(diploid
reproductive
cell)
Growth
three polar
bodies
haploid)
primary oocyte
(diploid)
secondary
oocyte
haploid)
Mitosis I,
Cytoplasmic division
ovum
(haploid)
Meiosis II,
Cytoplasmic division
Fertilization
• Male and female gametes unite and nuclei
fuse
• Fusion of two haploid nuclei produces
diploid nucleus in the zygote
• Which two gametes unite is random
– Adds to variation among offspring
Mitosis & Meiosis Compared
Mitosis
• Functions
– Asexual reproduction
– Growth, repair
• Occurs in somatic
cells
• Produces clones
Meiosis
• Function
– Sexual reproduction
• Occurs in germ cells
• Produces variable
offspring