Chapter 10 Flashcards
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
Control in Multicellular Eukaryotes
- multiple Cdks control the cycle as opposed to 1 Cdk in yeast (eukaryote)
- animal cells respond to more external signals
- yeast only respond to signals necessary for mating
Growth Factors
- act by triggering intracellular signaling systems
- platelet-derived growth factor (PDGF) one of the 1st growth factor identified
- growth factors can override cellular controls that otherwise inhibit cell division
Cancer
- unrestrained, uncontrolled growth of cells
- failure of cell cycle control
2 kinds of genes can distrub the cycle when they are mutated:
1. Tumor-suppressor genes
2. Proto-oncogenes
Tumor-Suppressor Genes
- P53 plays a key role in G1 checkpoint
- P53 protein monitors integrity of DNA
- if DNA is damaged, cell divison halted and repair enzymes are stimulated
- if DNA damage is irreparable, p53 directs cell to kill itself (apoptosis)
- Prevents the development of mutated cells containing mutations
- P53 is absent/damaged in many cancerous cells
- Both copies of a tumor-suppressor gene must lose function for the cancerous phenotype to develop
Proto-oncogenes
- normal cellular genes that become oncogenes when mutated (oncogenes can cause cancer)
- some encode receptors for growth factors
- if receptor is mutated in “on”, cell no longer depends on growth factors
- some encode signal transduction proteins
- Only 1 copy of the proto-oncogene needs to undergo this mutation for uncontrolled division to take place
Metastasis
cells travel to start new tumors
Cancer Cells
Angiogenesis
form new blood vessels to nourish themselves
Cancer Cells
Benign
contained with a capsule
Malignant
invasive and may spread
Cancer Cells
- don’t undergo apoptosis
- form tumors: do not respond to inhibitory signals
Cancer Treatment
- remove the tumor
- interfere with the ability of the cancer cells to reproduce
- Radiation and Chemo because cancer cells are rapidly dividing
- Hormone therapy designed to prevent cells from receiving signals for continued growth and division
BRCA
- BRCA mutation increases the risk of breast cancer
- tumor suppressor gene
