Download The Cellular Basis of Inheritance

The Cellular Basis of
Inheritance
Chapter 8
All cells come from cells
• “Where a cell exists, there must have been
a preexisting cell…” (Rudolf Virchow
1855)
• Repair and growth
– Replacement of lost or damaged cells
– Increasing in size
All cells come from cells
• Reproduction
– Asexual reproduction= process in which a
single cell or group of cells each duplicates its
genetic material then splits into two new
genetically identical cells
• Paramecium
• Sea stars
• Geranium
All cells come from cells
• Sexual reproduction= the process in
which, the genetic material from each of
two parents combines, producing offspring
that differ genetically from either parent
– Involves the union of sex cells (egg and sperm)
The cell cycle multiplies cells
• Nucleus
– Contains almost all of an organisms genes
– Chromatin= long, thin fibers made up of DNA
and proteins, located in the nucleus of
eukaryotic organisms
– Chromosomes= condensed threads of
genetic material formed from chromatin as a
cell prepares to divide
The cell cycle multiplies cells
• Before division a cell duplicates all of its
chromosomes…each chromosome now consists
of two identical joined copies called sister
chromatids
• Centromere= the region where the two
chromatids are joined together
• Humans have 46 chromosomes
The cell cycle multiplies cells
• Eukaryotic cells that divide undergo an
organized sequence of events called the cell
cycle
• Interphase= stage during which a cell carries
out its metabolic processes and performs its
functions
–
–
–
–
G1= growth phase
S= synthesis phase…duplication of DNA
G2= cell gets ready for division
M= mitotic phase
The cell cycle multiplies cells
•
Mitotic phase (2 stages)
1. Mitosis= nucleus and duplicated
chromosomes divide and are evenly
distributed
2. Cytokinesis= cytoplasm divides
Cells divide during the mitotic
phase
• Movement of chromosomes is guided by a
football shaped framework of microtubules
called the spindle
• The spindle microtubules grow from two
centrosomes= in animal cells, area of
cytoplasmic material that contain structures
called centrioles
• Centriole
– Role unknown
– Plant cells lack them
– If a cell does not have them, the spindle will still form
normally
Mitosis
Prophase
Metaphase
Anaphase
Telophase
• Prophase
–
–
–
–
1st stage
Chromosomes appear, seen as a pair of sister chromatids
Spindle forms
Chromatids attach to microtubules in spindle
• Metaphase
– 2nd stage
– Chromosomes line up in the center of the cell
– Spindle is completely formed
• Anaphase
– Sister chromatids separate…each chromatid
is called a daughter chromosome
– Proteins in the centromere help the daughter
chromosomes travel along the microtubules
toward the poles
– At the same time the spindle fibers are
shortening, bringing the chromosomes closer
to the poles
– The spindle fibers that are not attached to the
centromeres grow longer, pushing the poles
farther apart
• Telophase
– Begins when the chromosomes reach the
poles of the spindle
– Spindle disappears
– 2 nuclear envelopes reform around each set
of daughter chromosomes
– Chromosomes uncoil and lengthen
– Nucleoli reappear
• Mitosis is now finished
• Cytokinesis completes the division
process
Cytokinesis
Animal Cells
• Indentation around
the middle of the cell
• Caused by a ring of
microfilaments in the
cytoplasm,
underneath the
plasma membrane
• Like a drawstring
Plant cells
• Disk containing cell
wall material, called a
cell plate, forms
inside the cell and
grows outward
plant cell
frog cell
What would happen if
cells did not have
mechanisms that
controlled cell division?
Licentious division - prostate cancer cells during division.
• Benign tumor= abnormal mass of
“normal” cells
• Malignant tumor= masses of cells that
result from the reproduction of cancer cells
• Cancer= a disease caused by the severe
disruption of the mechanisms that control
the cell cycle…leading to uncontrolled cell
division
• Metastasis= spread of cancer
• Cancer treament
– Radiation
– Chemotherapy
Meiosis
• Occurs in the sex organs
– Ovaries
– Testes
• Typical human body cell contains 46
chromosomes…we get 23 chromosomes from
our mother and 23 chromosomes from out father
• Karyotyping matches up our chromosomes in
pairs, according to size, location of centromere,
and staining bands
• Each pair consists of one maternal chromosome
and one paternal chromosome
• Each pair of matching chromosomes are
called homologous chromosomes
• Each homologous chromosome in a pair
carries the same sequence of genes
controlling the same inherited
characteristics
– Ex: eye color
• How do homologous chromosomes differ
from sister chromatids?
• We have 23 pairs of chromosomes
– 1 pair is called the sex chromosomes
– Females have 23 homologous pairs
• The sex chromosomes look alike (X X)
– Males have 22 homologous pairs
• The sex chromosomes are different (X Y)
• Most of the genes carried on the X chromosome
do not have counterparts on the tiny Y
chromosome and vice versa
Y chromosome
X chromosome
• Diploid cell
– Typical human body cell
– Two sets of chromosomes
– 46 chromosomes...for humans (will differ
depending on the species)
• Haploid cell
– One set of chromosomes
– (Half the number of chromosomes)
– Produced through the process of meiosis
• Gametes= sex cells (haploid)…eggs and
sperm
• Fertilization= the fusion of the nuclei and
cytoplasm of the sex cells
• Zygote= fertilized egg (diploid)
– Has two homologous sets of chromosomes
• The zygote eventually develops into a
sexually mature adult with trillions of cells
produced by mitosis
• Interphase
• Cell duplicates its DNA
• Each chromosome consists of 2 identical sister chromatids
• Meiosis I
– Prophase I
• Proteins cause the homologous chromosomes to stick
together along their length
• Paired chromosomes now consist of 4 chromatids, called a
tetrad
– Metaphase I
• Tetrads line up in middle of cell
– Anaphase I
• Homologous chromosomes separate and move to opposite
poles
– Telophase I
• Chromosomes arrive at the poles
• Nucleus forms around chromosomes
• Each pole now has a haploid daughter nucleus (it has only
one set of chromosomes even though each chromosome
consists of two sister chromatids)
– Cytokinesis
• Meiosis II
– Prophase II
• Within each haploid cell, a spindle forms
– Metaphase II
• Chromosomes line up in the middle of the cell
– Anaphase II
• Sister chromatids separate and move to opposite
poles
– Telophase II
• Chromatids are now considered individual
chromosomes
• Chromosomes arrive at the poles
– Cytokinesis
• Result is 4 daughter cells
Genetic Variation
•
•
Offspring that result from sexual reproduction
are genetically different from their parents and
from their siblings
Genetic variation depends on
1. How the homologous chromosomes line up and
separate at meiosis I
•
•
The assortment of chromosomes occurs randomly
For humans there are about 8 million possible
chromosome combinations (223)
2. Crossing over= the exchange of genetic material
between homologous chromosomes
•
Occurs during prophase I
Crossing Over
• Homologous chromosomes are stuck together along
their length
• There is a precise gene-by-gene alignment between
adjacent chromatids of the two chromosomes
• Segments of chromosomes can be exchanged at one or
more sites
• Genetic recombination= a new combination of genetic
material as a result of crossing over
• A single chromosome contains many hundred genes
• More than one crossing over event can occur in each
tetrad
• No wonder offspring can be so varied
• A karyotype is a display of chromosomes paired
according to their size, location of the
centromere, and staining patterns.
• A karyotype reveals abnormalities in
chromosome number or structure.
• Humans have 23 pairs of chromosomes; 22
pairs of autosomes and one pair of sex
chromosomes.
• Autosome= a non-sex chromosome
• Females are XX and males are XY.
Amniocentesis
• Amniocentesis uses a needle to extract
amniotic fluid from the uterus of a
pregnant woman from the 14th to 17th
week of pregnancy.
• Up to 400 chromosome and biochemical
problems can be detected by culturing
fetal cells that are in the amniotic fluid.
• There is a slight risk of spontaneous
abortion with this procedure.
Amniocentesis
Karyotyping
• Sampled fetal cells are stimulated to
divide in culture medium and another
chemical stops division during metaphase
when chromosomes are highly
condensed.
• The stained cells are photographed and
can be paired based on stained crossbands, and size and shape.
Downs Syndrome
Asexual Reproduction vs.
Sexual Reproduction
• When you think about the genetic
information:
– Is it the same for asexual reproduction?
– Is it the same for sexual reproduction?
Binary Fission
– Binary fission means “dividing in half”
– Occurs in prokaryotic cells
– Two identical cells arise from one cell
– Steps in the process
– A single circular chromosome duplicates, and the
copies begin to separate from each other
– The cell elongates, and the chromosomal copies
separate further
– The plasma membrane grows inward at the midpoint to
divide the cells
Plasma
membrane
Prokaryotic
chromosome
Cell wall
3
1
Duplication of chromosome
and separation of copies
2
Continued elongation of the
cell and movement of copies
Division into
two daughter cells
Asexual Reproduction vs.
Binary Fission
• How are they the same?
• How are they different?