Transcript Chapter 8: The Cell Cycle

Chapter 8:
The Cell Cycle:
the life cycle of cells
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Cells divide and cells die
• Each of a human’s trillions of cells retains
the genetic information present in the
fertilized egg
– Every cell in the body results from countless
rounds of cell division
• Cells also die in predictable ways
– Cell death is a normal part of development
– Apoptosis – programmed cell death
• Cell division and cell death must be in
balance
Chromosome duplication
• A cell’s entire genome must be duplicated
before cell division
• In prokaryotic cells, genome is single
circular DNA molecule
• In eukaryotic cells, genome divided among
multiple chromosomes housed in nucleus
– Humans have 46, chickens 78, rice 24
Why is stem cell research so controversial? See p. 158-9
How is DNA used to clone a mammal like “Dolly” the sheep? See p. 160-1
Eukaryotic
chromosomes
• Chromatin – DNA plus
packaging proteins
– Nucleosomes – stretch of DNA
wrapped around histones
(proteins)
• Chromatin barely visible when
cell is not dividing
– DNA loosely packed and
accessible for cell
• DNA condenses into
visible chromosomes
before cell division
– Chromatid – one of 2
identical copies of a
replicated
chromosome
– Sister chromatids –
pair with identical DNA
sequence
– Centromere – point of
attachment that holds
sister chromatids
together
Chromosomes in humans
• Most human cells have 46 chromosomes
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23 from each parent; 46 total (23 pairs)
Somatic or body cells
Diploid – 2 full sets of genetic information
Pair members look alike (not genetically identical)
Except sex chromosomes, X and Y
Produced by mitosis
• Sperm and egg sex cells are haploid
– 23 total chromosomes
– Produced by germ cells in testes and ovaries
– Produced by meiosis
Human Karyotype:
This is a chart called a
Karyotype and it is used to
display the chromosome
pairs that an organism
has…for humans a normal
karyotype is 22 somatic
pairs and 1 sex pair (either
XX or XY depending on
gender)
If the karyotype is abnormal
(missing or extra
chromosomes), then the
person has a “genetic
abnormality”
Ex: Turner’s Syndrome or
Down Syndrome
The Cell Cycle
• Cell cycle – events
between one cell
division and the next
• 2 major stages
– Interphase – cell not
dividing but cell very
active
– Cell division – mitosis
and cytokinesis
(cytoplasm division)
The Cell Cycle
• Interphase
– Very active time in cell cycle
– G1 – cell grows and carries out basic functions
• Signals tell cell to divide, stop for repair of DNA,
die or enter Go
– S – genetic material replicates (DNA
Replication)
– G2 – cell prepares to divide and chromosomes
begin to condense
The Cell Cycle
• Mitosis
– Overall, separates
genetic material
evenly between 2
daughter cells
– Mitotic spindle pulls
half to each new cell
– Centrosomes organize
mitotic spindle
– Kinetochores attach
chromosomes to the
spindle
Phases of Mitosis
• Prophase – chromosomes condense,
mitotic spindle begins to form
• Prometaphase – kinetochores grow on
centromeres, nuclear envelop breaks
down
• Metaphase – chromosomes line up on
mitotic spindle on equator
• Anaphase – centromeres split, one
chromatid of each pair pulled to opposite
pole
• Telophase – mitotic spindle disassembles,
chromosomes begin to unwind, nuclear
envelope reforms
• Cytokinesis (differes in plant & animal cells because of cell wall in plant cells)
– Animal cells – cleavage furrow results from
contractile ring
– Plant cells construct a new cell wall with cell
plate
Animal Cells
Plant Cells
Cell cycle regulation
• Some cells divide more or less constantly
• Signals to divide come from outside the
cell
• Growth factors – proteins stimulating cell
division
• Checkpoints – ensure cell does not enter
next stage until previous stage complete
• Telomeres
– Tips of eukaryotic chromosomes
– Lose nucleotides and become shorter at each cell
division
– Cell division stops after about 50 cell divisions
– Most cells lack telomerase, the enzyme needed
to add more DNA to extend telomeres
• Teleomerase allows cells to go beyond 50 divisions
– Present in some normal cells and cancer cells
Cancer
• Tumor – abnormal mass of tissue
– Forms when body loses control of balance
between cell division and cell death
• Benign tumor – usually slow-growing and
harmless
• Malignant tumor –invades other tissues
(metastasizes) – cancer
Cancer
• Cancer cells are not normal cells
– Look different
– Essentially immortal
– May produce their own signals to divide
– Lack contact inhibition
– May not undergo apoptosis when damaged
– Send signals to stimulate growth of blood
vessels
• Causes of Cancer
– Oncogene abnormalities –
abnormal variants of genes
that normally control cell
division
• May accelerate cell cycle and
cause cancer
– Tumor suppressor gene
abnormalities – encode
proteins that normally block
cancer development
• Promote normal cell death or
prevent cell division
• Inactivation, deletion, or
mutation can cause loss of
function
– Harmful chemicals, radiation,
and viruses can alter DNA
– Poor diet and exercise, sun
exposure, and smoking can
increase risk
• Cancer treatments
– Surgical tumor removal, chemotherapy, and
radiation
– Chemotherapy and radiation target rapidly
dividing cell – both cancerous and healthy
• Death of healthy cells cause side effects
– New more targeted drugs home in on
receptors for growth factors
– Early detection key to successful treatment
Cell death/Apoptosis
•
2 main functions
1. Eliminates excess
cells to carve out
functional
structures like
fingers and toes
2. Weeds out cells
that might harm
the organism
•
Apoptosis must
be overcome for
mitosis to occur
• Caspases –
apoptosis-specific
enzymes
• Triggered when
“death receptor”
protein receives
signal
• Cuts apart cell’s
proteins and
destroys cell
Investigating life: Cutting off a tumor’s
supply lines in the war on cancer
• Rapidly dividing cancer cells frequently
develop resistance to drugs
• Mutations allow some cancer cells to survive
treatment and proliferate
• Indirect attack of angiogenesis
• Choke off tumor supply lines
• 1997 – endostatin suppresses tumor growth
in mice, no resistance, tumors never grew
back
– Clinical trials in humans yielded mixed results
– Not unusual to find drug works well in animals but
not humans