The Cell Cycle

Section 5.1

Objectives

• SWBAT describe the stages of the cell cycle.• SWBAT compare rates of division in different

types of cells.• SWBAT identify factors that limit cell size.

Starter

• Why do you always have to cut your hair, your fingernails, and the lawn?

• The answer is, of course, that new cells are being made – and will be made all the way up to the time of your death.

• So, in this section and the following section, we will learn how new cells are created.

Crash Course

• Crash Course (SciShow) video on Mitosis– https://www.youtube.com/watch?v=L0k-

enzoeOM

Vocabulary – Section 5.1

• Cell cycle• Mitosis (called M Phase)• Cytokinesis (also part of M Phase) • Interphase• Gap 1 (G1)• Gap 2 (G2)• Synthesis (S Phase)

Metaphase of Mitosis

Onion Root Tip Seen Through Electron Microscope

Varying phases of mitosis. Note, not all cells are dividing at the same time.

Cell cycle of a species of asparagus

Cell Cycle Diagram

This is a cycle of growth, DNA synthesis (replication), and cell division.

Controlled cell growth is essential to allow you (or any organism) to grow and heal.

Out of control cell growth is abnormal and is known as cancer.

Cell Cycle

Cell cycle definition: the regular pattern of growth, DNA duplication, and cell division that occurs in eukaryotic cells.

It consists of four stages: three of the stages (Gap 1, Synthesis, and Gap 2) are collectively known as Interphase.

Cell Cycle – Stage 1• Gap 1 (G1) – cells spend most of their time in this

stage.• During G1, a cell carries out its normal functions.

– For example: if it is a skeletal muscle cell, it contracts to move joints; if it is an intestinal cell, it absorbs nutrients; etc.

• In this stage, the cell needs enough nutrition, maturity (size), and undamaged DNA before it can move into the next stages.

Cell Cycle – Stage 2

• DNA Synthesis (S) – when your cell copies its nuclear DNA– Remember, in eukaryotes DNA is kept in the

nucleus– By the end of S stage, the cell nucleus contains 2

complete sets of DNA.• It is hard to see the DNA in this phase because it is long

and stringy. It hasn’t formed condensed, easily seen, chromosomes yet.

Cell Cycle – Stage 3

• Gap 2 (G2) – cells continue to carry out normal functions and growth continues. – Like in G1, there are critical factors like adequate

nutrition, cell size, and undamaged DNA. – Duplicated DNA is present.

Cell Cycle – Stage 4

• Mitosis – the division of the cell nucleus and its contents.

• Cytokinesis – divides the cell’s cytoplasm (remember, the new cells need cytoplasm and organelles to function).

• The result of mitosis and cytokinesis is two genetically identical daughter cells (they are clones of the parent cell).

Mitosis• During mitosis:

– the nuclear membrane dissolves;– the duplicated DNA condenses into easily seen chromosomes (more in

next section);– and then separates to form two new nuclei.

Your cells and the cells of all other eukaryotes are controlled by the same proteins.

This has been demonstrated – for instance, yeast regulator proteins can work in human cells and vice versa. Why is this?

Cells divide at different rates – depending on type of cell.

Some cells, like neurons, appear not to divide ever. They are referred to as G0

because they never divide.

Cells that receive a lot of wear and tear divide frequently, others hardly at all (like internal organs)

The size of cells

• Cells have upper and lower size limits.– Too small and cannot contain all the organelles

and molecules they need.– Too big and the cell would surpass the minimum

surface area, cell membrane, to volume ratio it needs to be able to move all of the oxygen, nutrients, wastes, etc. into and out of the cell as quickly as is necessary.

• Volume increases much faster than surface area.

Cell size is limitedVolume increases faster than surface area

Cell size is limited• Surface area must allow for adequate exchange of

materials.– Cell growth is coordinated with

division.– Cells that must be large have

unique shapes.