Bacterial Cell Division
bacteria divide by binary fission
* no sexual life cycle
* reproduction is clonal
Replication of Bacteria
- replication begins at the orgin of replication and proceeds bidirectionally to site of termination
- new chromosomes are partitoned to opposite ends of the cell
- septum forms to divide the cell into 2 cells
Eukaryotic Chromosomes
- every species has a different # of chromosomes
- humans have 46 chromosomes (23 pairs) sex cells don’t have 46
Chromosomes
- composed of chromatin: complex of DNA (40%) and protein (60%)
- DNA of a single chromosome is one long continuous double-stranded fiber
- in the nondividing nucleus:
Heterochromatin: not expressed
Euchromatin: expressed - human chromosome 140 millon nucleotides long
Chromosome Structure
- nucleosome: complex of DNA and histone proteins (promotes and guides coiling of DNA) “basic packing”
- DNA duplex coiled around 8 histone proteins every 200 nucleotides
- histones are positively charged, strongly attracted to negative charged phosphate groups in DNA
Solenoids
solenoids are nucleosomes wrapped into higher order coils
- leads to a fiber 30 nm in diameter
- usual state nondividing (interphase) chromatin
* during mitosis, chromatin in solenoid arranged around scaffold of protein to achieve maximum compaction
- radical looping aided by condensin proteins
Karyotype
a particular array of chromosomes in an individual organism
- arranged according to size, staining properties, location of centromere
define Haploid and Diploid
Haploid (n): 1 set of chromosomes (23 in humans)
Diploid (2n): humans (2 compete sets, 46 total)
* pair of chromosomes are homologous
Replication
- prior to replication, each chromosome composed of a single DNA molecule
- after replication, each chromosome composed of 2 identical DNA molecules (held together by cohesin proteins)
- visible as 2 stands held together as chromosome become more condensed
- 1 chromosome composed of 2 sister chromatids
Eukaryotic Cell Cycle
- G1 (gap phase 1): primary growth phase, longest phase
- S (synthesis): replications of DNA
- G2 (gap phase 2:) organelles replicate, microtubules organize
- M (mitosis): subdivided into 5 phases (seperate DNA)
- C (cytokinesis): separation of 2 new cells (seperate cell)
Cell Cycle Duration
- time it takes to complete varies
- mature cells take longer to grow
- growth occurs during G1, G2, and S phase
- most variation in length of G1
(resting phase is G0: brain cells stay here forever)
Interphase
G1: cells undergo major portion of growth
S: replicates DNA
G2: chromosomes coil more tightly using motor proteins
* Centromere: point of constriction
* kinetochore: attachment site for microtubules
* each sister chromatid has a centromere
* Chromatids stay attched at centromere by cohesin
5 Phases of Mitosis
1.Prophase
2.Prometaphase
3.Metaphase
4.Anaphase
5.Telophase
Prophse
1st phase in mitosis
- stops making RNA
- chromosomes start to condense (spindle apparatus begins)
- nuclear envelope breaks down
- asters: radial array of microtubulse in animals (not plants)
Prometaphase
2nd step in Mitosis
- microtubule attachment (attaches to kinetochores)
- sister chromatids (connected to opposite poles)
chromosomes (move to center of cell-congression)
Metaphase
3rd step in Mitosis
Meta = Midde
- alignment of chromosomes along metaphase plate
- (not an actual structure)
- future axis of cell division
Anaphase
4th step in Mitosis
- begins when centromeres split
- removal of cohesin proteins from all chromosomes
- sister chromatids pulled to opposite poles
- Anaphase A: kinetochores pulled toward poles
- Anaphase B: poles move apart
Telophase
5th step in Mitosis
- spindle apparatus disassembles
- nuclear envelope forms around each set of sister chromatids (now called chromosomes)
- chromosomes begin to uncoil (RNA genes to be expressed)
- nucleolus reappears in each new nucleus
Cytokinesis
- cuts cells into 2 identical cells
- animal cells: constriction of actin filaments produce a cleavage furrow
- plant cells: cell plate forms between the nuclei
- fungi: nuclear membrane doesn’t dissolve; division of the nucleus occurs with cytokinesis
Control of the Cell Cycle
2 concepts:
1. Cell cycle has 2 irreversible points
- replicaiton of DNA (synthesis)
- separation of the sister chromatids
2. Cell cycle can be put on hold at specific points called checkpoints
- process is checked for accuracy and can be halted if there are errors
- allows the cell to respond to internal and external signs
3 Cell Cycle Checkpoints
- G1/S checkpoint
- cell “decides” to divide
- primary point of external signal influence - G2/M checkpoint
- cell makes a commitment to mitosis
- assesses success of DNA replication - Late metaphase (spindle) checkpoint
- cell ensures that all chromosomes are attached to the spindle
Cyclin-dependent kinases (Cdks)
- enzymes that phosphorylate proteins
- primary mechanism of cell cycle control
- Cdks partner with other cyclins at different points in the cell cycle
- Cdk itself is controlled by phosphorylation
Cdk-cyclin complex
- also called mitosis-promoting factor (MPF)
- Activity of Cdk is also controlled by the pattern of phosphorylation
- phosphorylation at one site inactivates Cdk
- phosphorylation at another site activates Cdk
Anaphase-promoting Complex
- aka cyclosome (APC/C)
- function of the APC/C is to trigger anaphase itself
- makes securin for destruction; no inhibition of separase; separase destroys cohesin
