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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