Download Mitosis

How cells reproduce?
• Reproduction
– Produces a generation of
individuals like parents
• Cell division
– Bridges two generations
• Each daughter cell
receives
– A required number of DNA
molecules
– Some cytoplasm
Eukaryotes and Prokaryotes
• Eukaryotic cells undergo
mitosis and/or meiosis
one
chromatid
its sister
chromatids
– Separates duplicated
chromosomes of parent cell
into two daughter nuclei
– Another mechanism
divides cytoplasm
• Prokaryotic cells divide
by a different process
one chromosome
(unduplicated)
one chromosome
(duplicated)
Mitosis and Meiosis
• Mitosis
– Basis of growth, cell replacements, and
tissue repair in multicelled species
– Basis of asexual reproduction in many
single-celled and multicelled species
• Meiosis
– Basis of sexual reproduction
– Precedes formation of gametes or sexual
spores
Chromosome structure
• Eukaryotic chromosome
– Association of DNA, histones, and other
proteins
– Proteins structurally organize the
chromosome and affect access to its genes
• Nucleosome
– Smallest unit of organization
– Double-stranded DNA looped twice around
a spool of histones
Introducing the cell cycle
• Cell cycle
– Starts when a new cell
forms
– Runs through interphase
– Ends when cell reproduces
by nuclear and
cytoplasmic division
Interphase
• Most cellular activities
occur in interphase
– G1: Cell grows in mass,
doubles number of
cytoplasmic components
– S: DNA replication
duplicates chromosomes
– G2: Cell prepares for
division
Chromosome number
• Sum of all chromosomes in
cells of a given type
• In human body cells,
chromosome number is 46
• Body cells are diploid
(have two of each kind of
chromosome)
Mitosis and chromosome number
• Mitosis maintains parental chromosome number from
one generation to the next
– Bipolar spindle divides sister chromatids
A closer look at mitosis
• Mitosis
– A nuclear division
mechanism that maintains
the chromosome number
• Mitosis proceeds in four
stages:
–
–
–
–
Prophase
Metaphase
Anaphase
Telophase
Prophase
• Duplicated chromosomes
become threadlike as they
start to condense
• Microtubules form a
bipolar spindle
• Nuclear envelope starts to
break apart
Transition to Metaphase
• Microtubules from one
spindle pole harness one
chromatid of each
chromosome
– Microtubules from the
opposite spindle pole
harness its sister chromatid
• Other microtubules extend
from both poles and grow
until they overlap at the
spindle’s midpoint
Metaphase
• All chromosomes become
aligned midway between
the two spindle poles
– Chromosomes in most
condensed forms
Anaphase
• Sister chromatids detach
from each other
– Spindles move them
toward opposite poles
• Microtubules that overlap
at spindle’s midpoint slide
past each other, push poles
farther apart
• Motor proteins drive
movements
Telophase
• Two identical clusters (one
chromosome of each type)
reach opposite spindle
poles
• Nuclear envelope forms
around each cluster
• Both new nuclei have the
parental chromosome
number
At the end!
• Interphase
– Two daughter cells
Fig. 8.7h, p.130
Cytoplasmic Division Mechanisms
• Mechanisms of
cytoplasmic division differ
in plant and animal cells
• In animal cells
– A contractile ring of
microfilaments (part of cell
cortex) contracts and pulls
the cell surface inward
until the cytoplasm is
divided
Cytoplasmic Division in Plant Cells
• In plant cells
– A band of microtubules
and microfilaments forms
around the nucleus before
mitosis starts
– Marks site where cell plate
will form
• Cell plate becomes a
cross-wall that partitions
the cytoplasm
Controls Over Cell Division
• Products of checkpoint
genes control cell cycle
– Kinases
– Growth factors
When control is lost
• Mutant checkpoint genes
can cause tumors by
disrupting normal controls
Cancers
• Altered cells grow and
divide abnormally
– Malignant cells may
metastasize (break loose
and colonize distant tissues)
benign tumor
malignant
tumor