Cell Division
Chapter 10
04/25/2016
Why do Cells Need to Divide?
• Make more cells
– Replace cells
– Grow a larger organism
– Repair damage to organism
• Reproduction of the organism
Cell Division Enables Asexual Reproduction
DNA is the instruction manual of the cell
• Codes for all proteins
• Codes directions for how proteins will be used
• Relies entirely on the sequence of nucleotide bases
• Permanent changes in this sequence are called mutations
Dealing with the DNA During Cell Division
• Human DNA measures about 6 ft in length
• In non-dividing cells DNA needs to accessible for making RNA– Termed uncondensed
• DNA need to be copied prior to cell division– New cells will need DNA
• DNA need to be divided evenly between the new cells– No tangling, messing up
sexchromosomes
Humans Have 23 Pairs of Chromosomes
1 set came from female parent1 set came from male parent
We are diploids (2 copies of each chromosome)
Often given number/letter names
chromosome 1Achromosome 1B
Chromosomes 1A and 1B are homologous chromosomes
Chromosomes with different numbers (like 1A and 4A) are non-homologous chromosomes
A
Genome
• A cell’s entire DNA, packaged as a double-stranded DNA molecule
Gene
• A segment of DNA the codes for a protein
Chromatin
• All of the DNA molecules and their associated proteins present in the nucleus
• Typically present as diffused strands throughout the nucleus
– Heterochromatin – tightly packed DNA that is inaccessible
– Euchromatin – lightly packed DNA that is enriched with genes
histone proteins
protein scaffold
DNA double helix
DNA wound around histone proteins
Folded chromosome,fully condensed in adividing cell
Coiled DNA/histone beads
Loops attached to a protein scaffold; this stage of partial condensation typically occurs in a nondividing cell
1
2
3
4
5
DNA is Tightly Wound Into Chromosomes for Cell Division
Nucleosome
• Basic unit of DNA packaging in eukaryotes
• Segment of DNA wound in sequence around 8 histone protein
• This structure is often compared to thread wrapped around a spool
Chromosome
• A double-stranded linear DNA molecule
– Human body (somatic) cells have 46 chromosomes (23 pairs)
– Human gametal cells have 23 chromosomes
replication
mitosis
centromere
2 daughter cells
1 chromosome
1 duplicated chromosome
Chromosomes Are Copied Before Cell Division
Original cell
Each has 1 chromosome
p arm
q arm
Chromatid
• All of the chromatin that is associated with a single chromosome
Ploidy
• Ploidy: number of sets of chromosomes in a cell
– Human somatic cells are diploid
– Human germ cells are special diploid cells that will divide and make gametal cells
– Human gametal cells are haploid
Definitions
Singlechromosome
DuplicatedChromosome,
“sister chromatids”
Homologous pair ofChromosomes
1A1B
1A 1A1A
1A 1B
2B2A
Homologous
Non homologous
Homologous
Sister chromatids
1A 1B
2B2A
The Cell Cycle
• An orderly set of stages that take place between the “birth” of a cell and the time it reproduces another identical cell.
Cell Division is Only Part of the Cell Cycle
Fig. 9-7
G2: cellgrowthand preparationfor celldivision; organellesare duplicated
S: synthesisof DNA;chromosomesare duplicated
G1: cell growth andpreparation for synthesis
Interphase
• The phase in a cells life when it performs growth and DNA replication
• The longest stage in a cell’s life– Some cells never complete the cell cycle and are
“permanently arrested”• e.g. nerve and muscle cells
• Stages:– G1 stage (G stands for “gap” or “growth”)
– S stage (S stands for “DNA synthesis”)
– G2 stage
Gap 1 stage
• Cell continues to produce proteins and accomplish its normal functions
• Cell’s organelles are doubled (mitochondria, ribosomes, etc.)
• Cell accumulates needed materials for DNA replication
• Cell grows
S stage
• Cell replicates DNA
• Results in two identical “sister chromatids” for each chromosome
Gap 2 stage
• Follows DNA replication (S Stage) and last to onset of Mitosis
• Cell synthesizes materials needed for cell division (mitotic stage)
M stage
• Stage when mitosis and cytokinesis occur
– Cytokinesis: Division of the cytoplasm
• Daughter chromosomes are distributed to two daughter nucleii
• When complete, two daughter cells are present
Control of the Cell Cycle
• Cells initiate M stage after receiving either external and internal protein signals– A signal is a molecule that stimulates or
inhibits a metabolic response
• Growth factors are external signals received at the plasma membrane that ask the cell to divide
• Size of the cell is an internal factor that can lead to cell division
Cell cycle checkpoints
• Points in the cell cycle when the cell ensures that all components needed to make a new cell are perfect
G1 Checkpoint
• Cell evaluates “reserves” and size
• DNA is evaluated, and if not perfect, the cell may die (apoptosis unless repairable)
G2 Checkpoint
• DNA is checked to ensure that it has successfully replicated and is not damaged
• Occurs at G2/M transition
M Checkpoint
• Mitosis stops if chromosomes are not properly aligned
• Occurs during metaphase
• AKA “the spindle checkpoint”
– Are all the sister chromatids correctly attached to the spindle microtubules?
Regulator molecules
• Molecules that control the progress of the cell cycle
Cyclins and Cdks
• Positive regulators
• Cyclins and cyclin-dependent kinases (Cdks) are responsible for the progress of the cell through the various checkpoints
Cyclins and Cdks
• Cyclins are only active when bound to Cdks and are phosphorylated
Negative regulators
• The best understood are retinoblastoma protein (Rb), p53, and p21
• All three of these regulatory proteins were discovered in cancerous cells
• Act mainly at the G1 checkpoint
Negative regulators
• p53 checks DNA integrity
– Triggers p21 and apoptosis
• Rb monitors cells size
Telomeres do not shorten
• Too much telomerase enzyme
Interphase:
Main Features:DNA not visible as chromosomesCell carries out normal cell functionsCell spends most of its time hereDNA present as chromatin
DNA is copied
Prophase:
Main features:• Start of mitosis• DNA visible as chromosomes• Duplicated chromosomes
stuck together• Nuclear envelope fragments• Nucleolus disappears• Chromosomes attach to
spindle• Spindle microtubules
form
Prophase
Metaphase:
Main Features:
• Chromosome line up in the middle of the cell
• Duplicated chromosome still stuck together
Metaphase
Anaphase:
Main Features:
• Duplicated chromosomes pulled apart
• 1 copy to each end of the cell
• Cell elongates as spindle fibers disassociate
Anaphase
Telophase:
Main features:• Chromosomes arrive at cell
ends• New nuclear membranes
form around chromatin• Chromosone uncondense• Nucleoli appear• Cleavage furrow appears• Ends with cytokinesis
(division of cytoplasm)– Final division into 2 new cells
Telophase
Cytokinesis in animal cells
• Actin filaments (contractile ring) constrict around cleavage furrow
• Filaments draw tight until two daughter cells are formed
Cytokinesis in plant cells
• No cleavage furrow
• Builds new PM and cell wall between daughter cells
Return of Interphase
Main Features:
• DNA not visible as chromosomes
• Cell carried out normal cell functions
– DNA is copied for the next round of cell division
Overview of Mitosis
Fig. 9-7
Summary: the Stages of Mitosis
• Prophase: chromosomes become visible
• Metaphase: chromosomes line up
• Anaphase: chromosomes move apart
• Telophase: two distinct cells form
G0 Phase
• Exiting the cell cycle• After G1, some cells enter a quiescent state
– Some re-emerge after a time– Some remain quiescent for the life of the
organism (heart muscle cells, cortical neurons)
Prokaryotic cells divide too!
• Peokaryotic fission
• Escherichia coli doubles in 20 minutes under ideal conditions
Big Topic in Cell Division Control:Cancer
Tumor (“-oma”)Uncontrolled cell growth• Benign – stationary• Malignant (cancer) – invasive
• Uncontrolled growth• Loss of adhesion• Loss of cell cycle control• Uncontrolled cell division
Several genes must mutate to form cancer
How does cancer begin?
• Proto-oncogenes when mutated in certain ways, become oncogenes– A proto-oncogenes is a gene that codes for a positive
cell cycle regulator
– An oncogene is any gene that, when altered, leads to an increase in the rate of cell cycle progression
• In most instances, the alteration of the DNA sequence will result in a less functional protein
• Occasionally, however, a gene mutation causes a change that increases the activity of a positive regulator
How does cancer begin?
• Tumor suppressor genes or negative cell cycle regulators can stop the progress
• A functional p53 will deem the cell unsalvageable and trigger apoptosis
What happens when the tumor suppressor gene is mutated?
• Mutated p53 genes have been identified in more than one-half of all human tumor cells
• A cell with a faulty p53 may fail to detect errors present in the genomic DNA
– Even if a partially functional p53 does identify the mutations, it may no longer be able to signal the necessary DNA repair enzymes
Big Topic in Cell Division Control: Stem Cells
Stem cells are non-differentiated cells(no specific function)
Able to divide and multiply indefinitelyrenew themselvesgive rise to differentiated cells
Two types:Embryonic Stem CellsAdult Stem Cells
Stem cell
Stem cell
NonStem cell
Embryonic Stem Cells (ESC)
• Pluripotent cell – any cell type
• Multipotent cell – multiple cell types
• Unipotent cell – one cell type
ESC are cells isolated from 5-6 day old embryosthat can become any cell in the human body
Cell Potential
ESC are pluripotent cells
Adult Stem Cells
Multipotent and unipotent cells found in small numberswithin specific tissues
bone marrow
Plants Have Stem Cells Too
http://dev.biologists.org/content/132/16/3657/F1.expansion.html
Tip of shoot and root have stem cells (meristem)Divide then give:
new stem cells (maintain meristem size)make differentiated cells (the root and shoot)
Overproduction of stem cells leads to abnormal plant growth
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