Chapter 10 – Cell Growth and Division

10.1 Cell Growth, Division, and Reproduction As cells grow, a few problems arise. These problems limit a cell’s ability to grow and grow and grow. The 2 Major Problems: 1. As the cell grows larger, the demands on the DNA become too large. 2. A larger cell is not as able to remove wastes and take in nutrients through its cell membrane.

So how does a cell solve this problem? By Dividing!

Types of Reproduction Reproduction is the formation of new individuals. For a single-celled (unicellular) organism, cell division is considered a form of reproduction called… Asexual Reproduction – production of genetically identical offspring from a single parent. Examples: Binary Fission

Types of Reproduction And then there is… Sexual Reproduction – where offspring is produced by two parents, inheriting some genetic info from each parent. This mixture of genetic info creates offspring that are different looking from the parents and their siblings.

10.2 – The Process of Cell Division

Eukaryotic Chromosomes Recall that Eukaryotes keep their DNA enclosed in a Nucleus. DNA molecules are linear (think long strands of thread) and for 99% of a cell’s life they are so thin, they’re basically invisible. Key Terms to get straight: Chromatin – uncoiled DNA molecules. (99%) Chromosomes – coiled DNA molecules. (1%) When cells prepare for Cell Division, they coil up their DNA for the process. At that point they become visible.

Eukaryotic Chromosomes Sister chromatids – two identical chromosomes. Produced during S phase’s DNA replication process. Centromere – constricted region where the two chromatids are joined. Weird Side note: the entire structure is referred to as a chromosome. The process of Mitosis separates the sister chromatids. At that point, each separate sister will now be referred to as a chromosome.

The Cell Cycle A cell’s life exists in a cycle, with no definite beginning or end. It’s basically just… Growth  Preparation  Division  Repeat ***It could be said that the cell’s life*** is lived between divisions. For humans, the cycle is the same for any type of cell, it’s the timing that varies. For skin cells, a cycle could last one week. Nerve cell cycles could last a lifetime.

The Cell Cycle There are 4 distinct phases of the cell cycle. Even though this is a cycle, let’s consider this point as the start

G 1 Phase: Cell Growth In G 1 phase - cells grow in size and make new proteins/organelles.

S Phase: DNA Replication In S (or synthesis) phase - new DNA is synthesized when the chromosomes are replicated (copied).

G 2 Phase: Preparing for Cell Division In the G 2 phase, many of the organelles and molecules required for cell division are produced.

M Phase: Cell Division In eukaryotes, cell division occurs in two stages: mitosis and cytokinesis. Mitosis is the division/splitting of the cell nucleus. Cytokinesis is the division of the cytoplasm.

Important Cell Structures Involved in Mitosis –Centrioles – tiny structures located in the cytoplasm of animal cells that help organize the spindle –Spindle – a fanlike microtubule structure that helps separate the chromatids

4 Phases of Mitosis P - prophase M - metaphase A - anaphase T - telophase

Prophase 1) The chromatin coils and becomes chromosomes.

Prophase 1) The chromatin coils and becomes chromosomes. 2) The centrioles move to opposite sides of nucleus and organize the spindles.

Prophase 1) The chromatin coils and becomes chromosomes. 2) The centrioles move to opposite sides of nucleus and organize the spindles. 3) The spindle forms and attaches to the centromeres.

Prophase 1) The chromatin coils and becomes chromosomes. 2) The centrioles move to opposite sides of nucleus and organize the spindles. 3) The spindle forms and attaches to the centromeres. 4) The nuclear membrane breaks down.

Metaphase The chromosomes line up across the middle of the cell.

Anaphase The chromatids are pulled apart and separate to become individual chromosomes.

Telophase The chromosomes unravel back into chromatin.

Telophase 1) The chromosomes unravel back into chromatin. 2) A nuclear membrane re-forms around each cluster of chromosomes.

Cell Timeline, from division to division.

Cytokinesis is the division of the cytoplasm - it splits the cell into two daughter cells. The process of cytokinesis is different in animal and plant cells. Cytokinesis

Cytokinesis in Animal Cells –The cell membrane is squeezed in until the cytoplasm is pinched into two equal parts. –Each part contains its own nucleus and organelles.

Cytokinesis in Plant Cells –In plants, a cell plate forms between the two nuclei that develops into cell membranes. –A cell wall then forms in between the two new membranes.

The Stages of Cell Division