Cell Growth &

Reproduction

Section 8.2 Chapter 8

Pages 203 - 210

All cells come from preexisting cells.

Cell division is the way that new cells are produced from one cell.

Cell Reproduction

Cell division gives you two cells that are identical to the original or parent cell.

Cell Reproduction

Cells are constantly getting old, dying and being replaced.

New cells are needed for growth.

Cell Reproduction

Early biologists observed that stringy structures appeared in the nucleus just before cell division.

They also noticed that these structures seemed to vanish soon after cell division.

These stringy structures are chromosomes.

The Discovery of Chromosomes

Chromosomes contain DNA.

The Discovery of Chromosomes

Chromosomes carry the genetic information that is copied from one generation to the next.

It is important that this information is accurately passed along during cell division.

The Discovery of Chromosomes

When most of you think of chromosomes, this is probably the image you have in your head.

Electron scanning microscope’s photo of chromosomes

Humans have 23 pairs of chromosomes

This is a male… the sex chromosomes are an XY pair

Karotype

For most of a cells lifetime, chromosomes exist as chromatin.

Chromatin are long strands of DNA wrapped around proteins.

The structure of eukaryotic chromosomes

Under an electron microscope, chromatin looks like a string of beads.

The structure of eukaryotic chromosomes

Before a cell can divide, the long strand of chromatin needs to be organized… kind of like when we wrap up a cord to store it.

In this wrapped up coil, the chromatin is very tightly packed.

The structure of eukaryotic chromosomes

The chromosomes of eukaryotic cells change shape depending on which phases of the cell cycle they are in.

Chromosome Structure

A metaphase chromosome is a compact arrangement of DNA.

Chromosomes during interphase are long and tangled.

Chromosome Structure

What kind of cycles can you think of?

Seasons, tides, day and night…

The Cell Cycle

Cells have cycles too.

The cell cycle is the sequence of growth & division.

One period is for growth,

the other is for division.

There are two periods in the cell cycle.

Interphase is the growth period.

Most of the cells life is spent in interphase.

Interphase

During interphase a cell:

•grows in size

•carries on metabolic functions

•duplicates chromosomes for division

Interphase

Mitosis is the period of division.

During mitosis, two daughter cells are formed from one parent cell.

Each daughter cell has a complete set of chromosomes.

After mitosis, the cytoplasm divides to separate the two daughter cells.

Mitosis

The Cell Cycle

G1 phase

G2 phase

M phase

S phase

Interphase is the busiest time of the cell cycle.

G1 The cell grows and protein production is high.

S The cell copies its chromosomes. This is the only time during the cell cycle that DNA synthesis occurs.

G2 The cell then enters another growth period where mitochondria and

other organelles are manufactured and other parts needed for cell division are produced.

Interphase is divided into three parts:

Interphase now ends and mitosis

begins!

Cells undergo mitosis when they reach maximum cell size.

Remember, a cell can only grow so large until the surface area of the cell membrane becomes too small in proportion to its volume.

The cell membrane transports nutrients and waste into and out of the cell.

The Phases of Mitosis

Although cell division is a continuous process, it can be broken up into four phases.

The 4 phases of mitosis are:

prophase

metaphase

anaphase

telophase

How long does the cell cycle take?• Each turn of the cell cycle is a generation.

• Generation time varies greatly depending on the species and type of cell.

• The minimum time needed is about 10 minutes.

• It takes about 2 hours for sea urchin cells to divide.

• Most growing plant and animal cells need 8 – 10 hours to divide.

How long does the cell cycle take?

• The generation time for a bean seed is 19 hours

• The G1 phase lasts about 5 hours

• The S phase lasts about 7 hours

• The G2 phase lasts about 5 hours

• The M phase lasts about 2 hours

How long does the cell cycle take?

• The generation time for some mouse cells is about 22 hours

• The G1 phase lasts about 9 hours

• The S phase lasts about 10 hours

• The G2 phase lasts about 2 hours

• The M phase lasts about 1 hour

How long does the cell cycle take?• Many mature

cells like nerve cells and red blood cells, never divide.

• They are said to be in the G0 phase which is a lot like the G1 phase.

Cell Division occurs in a series of stages, or phases.

Mitosis Notes

1st phase: Interphase• (Not mitosis… but part of the cell cycle!)

• Chromosomes are copied (# doubles)

• Chromosomes appears as threadlike coils (chromatin) at the start, but each chromosome and its copy (sister chromosome) change to sister chromatids at the end of this phase

Interphase2

2nd phase: Prophase

• Mitosis begins (cell begins to divide)

• Centrioles (or poles appear and begin to move to opposite ends of cell

• Spindle fibers form between the poles

Prophase4

Spindle fibersDisappearing nuclear envelope

Doubled chromosome

Prophase

•longest phase of mitosis.

•The long stringy chromatin coils up into visible chromosomes.

•Each chromosome is made of two halves.

Prophase: the first phase of mitosis

The two halves of the doubled structure are called sister chromatids.

Sister chromosomes are an exact copy of each other.

Sister chromosomes are made when DNA is copied during interphase.

Sister chromatids are held together with a structure called a centromere.

Prophase: the first phase of mitosis

2 sister chromatids held together with a centromere

2 sister chromatids

centromere

telomeres

Centromeres play a role in chromosome movement during mitosis.

The position of the centromere also helps scientists identify the chromosomes.

As prophase continues, the nucleus disappears as the nuclear envelop and the nucleolus disintegrate.

In animal cells, pairs of structures called centrioles migrate to opposite ends of the cell.

Centrioles are small, dark, cylindrical structures that are made of small tubes.

Centrioles play a role in chromatid separation.

As the pairs of centrioles move to the opposite ends, another structure called spindles, form between them.

The spindle fibers make a cage like structure made of microtubules.

Plant cells form spindles without centrioles.

The spindle fibers separate sister chromatids during mitosis.

3rd phase : Metaphase

• Chromatids (or pairs of chromosomes) attach to the spindle fibers

Metaphase7

Centromere

Sister chromatids

Metaphase

Metaphase is a short stage.

The sister chromosomes become attached to spindle fibers by their centromeres.

The spindle fibers pull on the chromatids until they line up in the center of the cell.

Metaphase: The second stage of mitosis

Each sister chromatid is attached to its own spindle fiber.

This helps to ensure that each new cell receives an identical and complete set of chromosomes.

Metaphase: The second stage of mitosis

4th phase : Anaphase

• Chromatids (or pairs of chromosomes) separate and begin to move to opposite ends of the cell

Anaphase10

Anaphase

The separation of sister chromatids is the beginning of anaphases.

During anaphase, the centromeres split apart and chromatid pairs from each chromosome separate.

The chromatids are pulled apart by the shortening of the microtubules on the spindle fibers.

Anaphase: The third phases of mitosis

5th phase : Telophase

• Two new nuclei form• Chromosomes appear as

chromatin (threads rather than rods)

• Mitosis ends

Telophase13

Telophase

Nuclear envelope reappears

Telophase begins as the chromatids reach the opposite sides of the cell.

During telophase, many of the changes that occurred during prophase are reversed.

Telophase: The fourth phase of mitosis

The chromosomes which were tightly coiled since the end of prophase are now unwinding.

The spindles breakdown, the nucleolus reappears, and a new nuclear envelope forms around each set of chromosomes.

Finally a new double membrane begins to form between the two nuclei.

Telophase: The fourth phase of mitosis

6th phase : Cytokinesis

• Cell membrane moves inward to create two daughter cells – each with its own nucleus with identical chromosomes

Cytokinesis16

After telophase, the cell’s cytoplasm divides in a process known as cytokinesis.

Cytokinesis is different in plant and animal cells.

Cytokinesis – After Mitosis

With animal cells, towards the end of telophase, the cell membrane pinches in around its equator and the two new cells are separated.

This is the furrow created when the cell membrane of a frog cell pinched in two.

Plants have a rigid cell wall so the plasma membrane does not pinch in.

Instead a structure known as a cell plate is laid down across the cell’s equator.

Then cell membrane forms around each cell and a new cell wall forms on each side of the cell plate until separation is complete.

Results of Mitosis

Mitosis results in two new cells with chromosome sets that are identical to the parent cell.

The new cells will carry out the same cellular processes and functions as the parent cell.

The new cells will grow and divide just like the parent did.

Results of Mitosis

When mitosis is complete, unicellular organisms are still unicellular – the organism simply multiplied!

In multicellular organisms, cell growth and reproduction result in groups of cells that work together as tissue.

Tissues work together to form organs.

Multiple organs work together to form organ systems.

Organ systems work together for the survival of the organism.

Levels of organization

Cells of complex multicellular organisms

are organized into tissues, organs, and organs systems.

The different systems have developed through cell specialization.

No matter how complex, the cell is still the most basic unit of life.