UNIT 4: CELL CYCLE (MITOSIS)

Binary Fission

asexual reproduction in single-celled organisms

Why do cells divide?

in multicellular organisms, somatic cells divide for embryonic development, growth, and repair

Chromosomes in eukaryotic cells

in eukaryotic cells, DNA is chromatin (DNA + histones) when not dividing; condenses into chromosomes only during division

How many pairs of chromosome do humans have?

humans have 23 chromosome pairs in somatic cells

Centromere

center where spindle fibers attach

Kinetochore

proteins at centromere anchoring spindle fibers

Sister Chromatids

identical copies joined at the centromere after replication

Interphase

G1, S, G2

What is interphase for?

cell growth, DNA replication, preparation for division

M Phase

mitosis/cytokinesis

What is the role of the M Phase

separation of DNA

M Phase Stages

prophase, Metaphase, Anaphase, Telophase

G1 Phase

checks that cell DNA was not damaged from last division; cell is metabolically active and growth occurs

What happens to cytoplasmic organelles?

cytoplasmic organelles are duplicated as well as proteins; centrioles replicates (46 chromosomes and chromatids)

G0 Phase

resting phase; metabolically active but no division

S (Synthesis)

DNA replication occurs (46 Chromosomes & 92 Chromatids)

G2 Phase

centriole replication finishes; DNA error check and repair

Prophase

chromatin condenses into chromosomes, nucleolus disappears; mitotic spindle fibers form, centrosomes move apart

Prometaphase (Late Prophase)

nuclear envelope breaks down, and spindle fibers attach to kinetochores

Metaphase

chromosomes align at metaphase plate and spindle fibers attach to opposite sides of sister chromatids

What is the importance of spindle fibers?

to help ensure that sister chromatids are pulled apart and distributed evenly to the daughter cells

Anaphase

sister chromatids separate and chromosomes move to opposite poles as the cell elongates (92 Chromosomes & Chromatids)

Telophase

nuclear envelopes reform; nucleoli reappear and chromosomes revert to chromatin; mitosis completes (46 Chromosomes & Chromatids)

Cytokinesis

Division of cytoplasm begins in anaphase, and ends in telophase

What do animals and plant cells form during cytokinesis

animal cells form a cleavage furrow, while plant cells form a cell plate that becomes cell wall

Growth Factors for Regulation of the Cell Cycle

signaling proteins that stimulate cell division and growth

Anchorage Dependence

cells need attachment to a surface to divide

Density-Dependent Inhibition

cells stop dividing when space is full

G1 Checkpoint

checks size, nutrients, growth factors, and DNA damage

What happens if cells get a go-ahead signal during G1 Checkpoint?

If a cell gets a go-ahead at checkpoint, it completes S, G2, and M, and divides

What happens if there’s no G1 Checkpoint?

Without the signal, it exits the cycle and enters G0

G2 Checkpoint

checks DNA for errors after replication; apoptosis occurs if irreparable

M Checkpoint

ensures proper spindle attachment before chromatids separate

Why do apoptosis happen?

occurs when cells are damaged, old, or infected

What happens during apoptosis?

DNA and organelles break down, and the cell shrinks; parts are packaged in vesicles and digested by scavenger cells

Ras

controls G1 checkpoint; moves the cell to S phase when activated

P53

tumor suppressor; inspects copied DNA for damage after S phase

What does P53 trigger?

triggers apoptosis if damage is irreparable (early G1 + G2 checkpoint)

MAD 1

ensures correct spindle attachment to divide chromosomes equally in metaphase

What do cancer cells do?

cancer cells ignore checkpoints and divide continuously

What do cancer cells develop

develops in an organism only after one or more of the following changes, resulting when there are mutations in genes

Proto-Oncogenes

normally promote division; mutations turn them into oncogenes

Example of Proto-Oncogenes

mutation Ras creates a mutated RAS protein

Tumor Suppressor Genes

stop cells with errors from continuing through the cell cycle

What happens if tumor suppressor genes are turned off

if turned off, cell ignores checkpoints and results in cancer (p53 mutation)

Benign

remain in place

Malignant

spread and metastasize