Cell Growth and Division Overview

Cell Growth and Division

• All cells originate from pre-existing cells.
• The cell cycle produces new cells.
• Skin cells reproduce to replace dead cells.
• Skin consists of epidermis (outer layer) and dermis (inner layer).
• The epidermis has two layers: dead cells on top and living cells at the bottom.
• Living skin cells reproduce constantly; the new cells move outwards.
• Skin cell reproduction occurs every 24 hours.
• Renewed cell production helps wound healing.
• Mitosis is the type of cell reproduction involved in growth & repair.
• Cell reproduction also results in growth; increasing size from a baby to an adult.
• All the trillions of cells in the body originate from a single fertilized egg via cell reproduction.

Reproduction

• Cell division results in growth and repair within organisms.
• Cell division plays a significant role in creating organisms.
• Some less complex organisms reproduce via simple cell division.
• In this method, a single cell or group of cells duplicates genetic material and splits into two genetically identical cells.
• This type of reproduction is called asexual reproduction.
• Asexual reproduction involves producing offspring from one parent.
• Offspring are genetically identical to one another and their parent
• The creation of offspring from two parents is sexual reproduction.
• Sexual reproduction involves the combination of sex cells (eg: egg and sperm).
• Most multicellular organisms reproduce sexually; some also reproduce asexually.
• All multicellular organisms depend on cell division (mitosis) for growth.

Chromosomes and Cell Division

• Most eukaryotic cell genes reside in the nucleus as long, thin fibers called chromatin.
• Chromatin condenses into visible structures called chromosomes as a cell prepares to divide.
• Chromosomes are composed of DNA and protein.
• The number of chromosomes varies by species.
• Human body cells have 46 chromosomes.

Mitosis

• Before cell division can begin, a cell must first duplicate each of its chromosomes.
• Each chromosome now is called two identical sister chromatids.
• The cell cycle, in which the cell carries out metabolic processes and other functions in the body, often takes up 90% of the cycle.
• The cell cycle has three phases: G1, S and G2
• G1: cell growth; S: Duplication of genetic material; G2: cell prepares to divide.
• The mitotic phase is when the cell actively divides and involves two processes: Mitosis and cytokinesis.
• During mitosis, the nucleus and duplicated chromosomes divide equally to form two identical daughter nuclei.
• Cytokinesis begins before Mitosis completions, and involves the division of cytoplasm to form two daughter cells.
• Mitosis is divided into 4 stages: Prophase, Metaphase, Anaphase, Telophase.

The Cell Cycle

• How often a cell divides depends on its type.
• Eukaryotic cells that divide undergo a sequence called the cell cycle.
• The cell cycle is a continuous process that includes actual cell division (mitosis) and the time the cell needs to prepare for division (interphase).

Interphase

• The cell may spend as much as 90 percent of the cell cycle in interphase.
• Interphase is broken into three phases: G1, S, and G2.
• During interphase, the cell performs its metabolic processes and functions in the body.
• The cell makes duplicates of many of its organelles and grows in size during interphase.

Mitosis Phases

• Prophase: First stage of mitosis; chromatin fibers condense into chromosomes. The nucleolus disappears and the cell stops making ribosomes. The nuclear envelope breaks down, and a spindle forms.
• Metaphase: Chromosomes align in a plane across the middle of the cell. The mitotic spindle is fully formed, and spindle fibers are attached to the centromeres of each chromosome.
• Anaphase: Sister chromatids separate and move towards opposite poles of the cell. Proteins at the centromeres help move the chromosomes along the spindle fibers.
• Telophase: Chromosomes reach the poles of the spindle, and the spindle disappears. Two nuclear envelopes form around each set of chromosomes. The chromosomes uncoil and lengthen, and the nucleoli reappear.

Cytokinesis in Animals and Plants

• Cytokinesis is the actual division of the cytoplasm into two separate cells.
• In animals, an indentation (cleavage furrow) forms around the middle of the cell, and microfilaments contract, pinching the cell in two.
• In plants, a cell plate forms inside the cell, grows outward, and divides the cell into two.