Download Reproduction at the Cellular Level

REPRODUCTION AT THE CELLULAR LEVEL
CHAPTER 6
THE GENOME
 Genome: all of the DNA in a given cell
 Prokaryotic Genome
 One double-stranded DNA molecule
 Forms a loop
 Eukaryotic Genome
 Multiple double-stranded DNA molecules
 Linear
EUKARYOTIC CHROMOSOMES
 Chromosomes are a double-stranded DNA molecule
and attached proteins
 Human chromosomes would be about 2 meters long
 Winds and coils up (condenses) to pack tightly into the nucleus
 DNA molecule winds twice around histones (protein
“spools”)
 DNA + histone = nucleosome
 Looks like beads on a string under a microscope
 Coils like a telephone cord
 Have a characteristic X shape when completely
supercoiled
EUKARYOTIC CHROMOSOMES
 Chromosomes consist of one DNA
molecule during most of a cell’s life
 When a cell divides the chromosomes are
duplicated
 Each chromosome will consist of two DNA
molecules
 Each is called a sister chromatid
 They are attached at the centromere
 Constricted region where sister chromatids
are attached
centromere
EUKARYOTIC CHROMOSOMES
 Karyotyping shows all of a cell’s
chromosomes
 Images of the chromosomes can be lined up
by size, shape, length, and centromere
location
 Shows
 Types of chromosomes
 How many chromosomes
 Missing or extra chromosomes or structural
abnormalities
Fig. 8-6a, p.129
EUKARYOTIC CHROMOSOMES
 Types of Chromosomes
 Each type of chromosome has a specific length, centromere location,
shape, and genetic information
EUKARYOTIC CHROMOSOMES
 Types of Chromosomes
 Autosomes
 Any chromosome except a sex chromosome
 Both females and males have the same autosomes
 Sex chromosomes
 Differ between females and males
 XX or XY
 Determine an individual’s sex
EUKARYOTIC CHROMOSOMES
 Chromosome number
 Each species has a specific number of chromosomes
 Humans = 46
 Others
 Gorillas = 48
 Pea plants = 14
 Mosquito = 8
EUKARYOTIC CHROMOSOMES
 Chromosome number
 Haploid (n) (“n” = a single set of chromosomes)
 Only one of each type of chromosome
 Diploid (2n)
 Two of each type of chromosome
 Pairs have the same length, shape, and information about heritable traits
 Called Homologous pairs of chromosomes or Homologs
 One of the pair came from the father and one came from the mother
 Polyploid
 Multiple copies of types of chromosomes
QUESTIONS
 Chromosome
Two of each type of chromosome
 Histone
Any chromosome except the sex chromosomes
 Chromatid
DNA molecule and attached proteins
 Centromere
One of each type of chromosome
 Diploid
Protein spool that DNA wraps around
 Haploid
Image showing all of a cell’s chromosomes
 Karyotype
Constricted region where sister chromatids attach
 Autosome
One strand of a duplicated chromosome
Questions
1
2
2
4
3
CELL CYCLE
 The cell cycle is a series of events including cell growth and cell
division
 It starts when a new cell forms and ends when that cell divides
 In eukaryotes it consists of
 Interphase
 (G1, S, G2),
 Mitotic phase
 Replicated DNA and cytoplasm are separated
 The cell divides
CELL CYCLE
 Interphase
 The longest interval of the cell cycle
 Has three stages
 G1 Interval or “gap”
 Cell growth and activity before the onset of DNA replication
 Most cells remain in G1 unless they need to divide
 S Time of “synthesis” or DNA replication
 This is when the DNA is duplicated resulting in sister chromatids
 G2 Second interval or gap
 The cell prepares for division
MITOTIC PHASE
 Mitotic phase has two parts
 Mitosis which is division of the nucleus and chromosomes
 Cytokinesis which is division of the cytoplasm, membrane, and
organelles
MITOTIC PHASE
 Mitosis: mitotic spindle
 Sorts and divides chromosomes
 Also called “bipolar spindle”
 Composed of
 Centrosomes
 A pair of small barrel shaped centrioles that direct
microtubule construction
 Divided during G2 of interphase just before prophase so
there are two pair in each cell ready for mitosis
 Spindle fibers (microtubules)
Centrosome
Fibers
MITOTIC PHASE
 Mitosis: mitotic spindle
 Spindle fibers (microtubules)
 Polar fibers: run from pole to pole
 Kinetochore fibers: run from a pole to one
chromatid of a chromosome. Fiber from the other
pole connects to its sister.
 The fibers attach to a specialized area at the centromere
Polar fibers
called the kinetochore.
 Aster fibers: In animal cells the aster fibers extend
from the centrosome out towards the edge of the
cell.
 The Aster gives the bipolar spindle support.
 Plant cells do not have asters.
Kinetochore
fibers
Aster fibers
MITOTIC PHASE
 Mitosis: mitotic spindle
 When a cell commits to divide by mitosis the centrosomes migrate
toward opposite sides of the cell
 Once they reach opposite sides this creates “poles”
 Equidistant from the poles is the “equator” or metaphase plate
pole
microtubule
Chromosomes
at the equator
pole
MITOTIC PHASE
 Mitosis
 Prophase
 Chromosomes condense and become visible with a
light microscope
 During Interphase chromosomes are loosely dispersed
throughout the nucleus (chromatin)
 Condensing means that the chromosomes coil up into a
compact form
 Helps to keep the chromosomes from tangling as they
are moved and sorted out during nuclear division
 Mitotic spindle begins to form
 Nuclear envelope starts to break up
 Nucleolus disappears
MITOTIC PHASE
 Mitosis
 Prometaphase
 Chromosomes continue to condense
 Kinetochores appear at the centromeres
 Mitotic spindle microtubules attach to
kinetochores
 Centrosomes move toward opposite poles
MITOTIC PHASE
 Mitosis
 Metaphase
 Mitotic spindle is fully developed
 Centrosomes are at opposite poles
 Chromosomes are lined up at the metaphase
plate/equator
 Each sister chromatid is attached to a spindle
fiber originating from opposite poles
MITOTIC PHASE
 Mitosis
 Anaphase
 Sister chromatids are separated and pulled toward
opposite poles
 Microtubules drag the chromatids
 Polar fibers lengthen, elongating the cell
MITOTIC PHASE
 Mitosis
 Telophase
 Chromosomes arrive at opposite pole and begin to
decondense
 Nuclear envelope forms around each set of
chromosomes
 Mitotic spindle breaks down
QUESTIONS
 What are the components of the bipolar spindle?
 What organelles are located at the poles?
 What are spindle fibers made of?
 What do spindle fibers attach to?
 What is the role of the aster?
 What are the phases of Mitosis?
MATCHING QUESTION
1. Interphase
A. Kinetochores appear
2. Prophase
B. Chromosomes line up at the equator
3. Prometaphase C. Chromosomes condense
4. Metaphase
D. Chromosomes relax and new nuclear membranes form
5. Anaphase
E. The cell grows and prepares to divide
6. Telophase
F. Chromatids are pulled toward the poles
MITOTIC PHASE
 Cytokinesis
 Divides the cytoplasm and its contents between the two new daughter
cells
 Usually occurs between late anaphase and end of telophase
 Different mechanisms are used by different organisms
 Animals: Contractile Ring Formation
 Plants: Cell Plate Formation
MITOTIC PHASE
 Cytokinesis
 Animal: Contractile Ring Formation
 At the spindle equator a ring of actin filaments contracts
 Cleavage furrow is formed
 The contractile ring continues to shrink pulling the cell surface inward until the
ring pinches the cytoplasm in two
MITOTIC PHASE
 Cytokinesis
 Plant: Cell Plate Formation
 Vesicles containing cell wall materials form
 From the Golgi apparatus
 Vesicles collect at the metaphase plate and
fuse
 Deposit cell wall components
 Vesicle membranes become the plasma
membrane on either side of the new cell wall
 Starts in the center and grows toward the
cell walls
MITOTIC PHASE
 The end result of mitosis plus cytokinesis
is two identical daughter cells
G1
 Genetically identical to each other and to the
original parent cell
 Sister chromatids are separated during
anaphase ensuring that each daughter cell
receives one copy of each chromosome
 Used for growth, cell replacement, tissue
repair, asexual reproduction
S
G2
Mitosis and
Cytokinesis
REGULATION OF THE CELL CYCLE
 G0 Phase
 Cells that are not actively preparing to
divide
 The cell has exited the cell cycle
 An external signal triggers some to re-enter
the cell cycle
 Some cells remain in G0 permanently
REGULATION OF THE CELL CYCLE
 Internal Checkpoints
 To prevent cells with mistakes in the
chromosomes from continuing to divide
 Check for
 Integrity of the DNA
 Adequate reserves and cell size
CANCER AND THE CELL CYCLE
 Sometimes the checkpoints fail and cell division becomes
uncontrollable
 Tumor formation and cancer are the outcome
Basal cell
carcinoma
Squamous cell
carcinoma
Melanoma
Slow growing,
raised lump
Grows under the
surface of the skin
Spreads fastest
Dark encrusted lumps
CANCER AND THE CELL CYCLE
 Proto-oncogenes
 Normal genes that code for positive cell-cycle regulators
 When mutated become oncogenes
 Less- or non-functional in regulating the cell cycle
 causing cells to become cancerous
CANCER AND THE CELL CYCLE
 Tumor suppressor genes
 Code for negative regulator proteins that can prevent the cell from
undergoing uncontrolled division
 Mutated tumor suppressor genes
 Cell can no longer halt the cell cycle if there is a problem
PROKARYOTIC CELL DIVISION
 Binary fission (prokaryotic fission)
 Less complicated and much quicker than mitosis
 Populations can grow very rapidly
 Single circular chromosome located in the “nucleoid” region
 Origin: the starting point of replication
 Close to a site on the chromosome which binds to the plasma membrane
 Replication occurs in both directions from the origin
PROKARYOTIC CELL DIVISION
 Binary fission (prokaryotic fission)
 Bacterial DNA is attached to the plasma membrane
PROKARYOTIC CELL DIVISION
 Binary fission (prokaryotic fission)
 DNA is replicated (both directions from the origin of replication)
PROKARYOTIC CELL DIVISION
 Binary fission (prokaryotic fission)
 Both strands are attached
PROKARYOTIC CELL DIVISION
 Binary fission (prokaryotic fission)
 The cell elongates and as the membrane grows the two chromosome
copies move apart
PROKARYOTIC CELL DIVISION
 Binary fission (prokaryotic fission)
 New membrane and cell walls start (septum)
PROKARYOTIC CELL DIVISION
 Binary fission (prokaryotic fission)
 When the cell walls are finished two new cells separate
SUMMARY
 The genome
 Eukaryotic cell cycle
 Interphase
 The mitotic phase
 Mitosis
 Cytokinesis
 Control
 Cancer and the cell cycle
 Prokaryotic cell division