Cell Division and Interphase

Cell Division

• Cells divide for growth and repair
• Cells also divide to get food, water, and remove waste from the body quicker
• The cell cycle is a repeating series of events that take place between one cell division to the next
• The cell cycle is constantly repeated

Interphase

• Interphase is a period of growth and development
• It is the longest phase of the cell cycle
• Some cells stay in interphase, such as nerve and muscle cells

Interphase: Stages

• Interphase is divided into three parts: G1, S, and G2
• G1 checkpoint—decides whether a cell can divide
  • Checks environmental conditions
  • Looks at the health and size
  • If conditions are favorable, the cell proceeds to the synthesis phase; if not, the cell enters a resting period.
• Synthesis phase (S)—DNA is copied
  • Each chromosome consists of two chromatids attached at the centromere
• G2 checkpoint—DNA replicates
  • Checks for errors in DNA replication

Interphase: Three Things Happen

• Cell grows in size, making new additional structures
• The cell makes a copy of its heredity material (DNA)
• Produces structures needed for division (spindle fibers, centrioles)

Phases of Interphase

• First Growth Phase (G1):
• Cell grows rapidly
• Carries out everyday functions
• Synthesis Phase (S):
• Cell’s DNA is copied
• Second Growth Phase (G2):
• Assembles hollow microtubules
• Microtubules move chromosomes during mitosis

Chromosomes (Basic Structure)

• Chromosomes contain DNA
• A chromosome is made up of two chromatids connected by a centromere
• Each end of a chromatid is called a telomere

Chromosomes (Information)

• Humans have 46 chromosomes
• Carrot has 18
• Cats have 32
• Dogs have 78
• Earthworms have 36
• Fruit fly has 8
• Chimpanzees have 48
• Adders' tongue fern has 12

Control of Cell Cycle: Checkpoints

• There are three checkpoints
• G1 Checkpoint—decides if a cell can divide
  • Based on the environmental conditions, health and size
  • If favorable, conditions for S phase occur, else cell enters a resting phase
• G2 Checkpoint—checks for DNA replication errors
  • If passed, mitosis occurs
• Mitosis Checkpoint—signals the end of mitosis, and G1 phase starts again

Control of Cell Cycle: When Checkpoints Fail

• Cancer occurs—uncontrolled cell growth
• Mutation missed by checkpoint—causes overproduction of growth hormone
• Damage to a cell—causes cells to constantly repair

Mitosis

• Mitosis is the division of the nucleus
• Results in two identical nuclei

Four Stages of Mitosis

• Prophase
• Metaphase
• Anaphase
• Telophase

Prophase

• Chromatin condenses to form chromosomes
• Centrioles move to opposite ends of the cell
• Spindle fibers form a bridge between the ends of the cell
• Nuclear envelope breaks down

Metaphase

• Chromosomes line up in the middle of the cell
• Chromosomes attach to spindle fibers by a centromere

Anaphase

• Centromeres split
• Chromatids separate to become chromosomes
• Chromosomes move to opposite cell ends
• Cell begins to stretch

Telophase

• Chromosomes stretch out
• New nuclear envelope forms around each region of chromosomes

Cytokinesis

• The cell membrane pinches in around the middle of the cell
• The cell splits into two daughter cells
• Each daughter cell has an identical set of chromosomes and half the organelles.
• Usually starts around the same time as telophase

Cytokinesis in Animal Cells

• Cell membrane pinches together around the middle to form two new cells
• Each daughter cell gets about half the organelles

Cytokinesis in Plant Cells

• Cell plate forms down the center of the cell
• Cell plate develops into a cell membrane
• New cell walls form around the cell membrane

DNA

• Deoxyribonucleic acid

• Heredity material passed on from parents

• Found in the nucleus as chromatin