The Cell Cycles –Mitosis and Meiosis

Essential question:

How do cells reproduce and why?

Objectives – Section 10.1

Explain why cells divide in terms of growth and cell size

Review - Types of Cell Division

• 2 Types of cell division

– Asexual

• Several purposes

– Sexual

• One purpose

Review - Types of Cell Growth

• Individual Cell Size Increase

• Number of Cells Increases

10.1 Why Divide?

10.1 Cell Growth

A set amount of DNA is unable to meet the needs of an ever-growing cell.

Limits to Cell Size:

1. DNA “Overload”

What is the job of the DNA of a cell?

2. Exchange of Materials

How do substances get in and out of a cell?

As a cell grows larger, both the surface area (the cell membrane) and the volume (the cytoplasm and organelles) grow larger, but at different rates!

10.1 Cell Growth

Calculating Surface Area to Volume Ratios

Which cat has the greatest surface area to volume ratio?

10.1 Cell Growth

How do you increase surface area without decreasing overall size of the organism?

Has the volume changed?

2cm

2cm

2cm8cm

8cm

8cm

Surface area = 2cm x 2cm x 6sides =

24 cm2

Volume = 2cm x 2cm x 2cm =

8cm3

SA:V = 24:8 24/8 = 3:1

Surface area = 8x8x6 =

384 cm2

Volume = 8x8x8 =

512cm3

SA:V = 384:512 384/512 = 0.75:1

Which cell has the biggest surface area to volume ratio?

Calculating Surface Area to Volume Ratios

Let’s Practice Calculating Cell Size

1cm 5 cm 10 cm 20 cm 40 cm 80 cm

So as organisms get bigger their surface area/volume ratio gets smaller. It becomes more difficult for them to exchange materials with their surroundings.

If the volume of a cell becomes too big for the surface area to support, substances such as food, O2, and wastes cannot diffuse in and out fast enough.

Objectives – Section 10.1

Explain why cells divide in terms of growth and cell size

10.1 Cell Growth Review

1. Give 2 reasons why cells divide that are not related to growth.

The larger a cell becomes, the more demands the cell places on its DNA and the more trouble the cell has in moving enough nutrients and wastes across the cell membrane.

2. As a cell increases in size, which increases more rapidly, its surface area or its volume?

Its volume.6:1

3:1

3. Calculate the surface area, the volume, and the ratio of surfacearea to volume of an imaginary cubic cell measuring 4 cm on each side.

3 cm

3 cm

3 cm

Surface area: l x w # of sides:

Volume: l x w x h:

SA:V ratio:

3 x 3 x 6 = 54 cm2

3 x 3 x 3 = 27 cm3

54:27 = 6:3 (or 2:1)

How does surface area to volume affect the physiology of organisms?

(Let’s look at a related concept!)

Is it better to be a large animal or a small one in cold climates?

Organisms also exchange heat with their surroundings. Large animals have an advantage in having a small surface

area/volume ratio: they lose less heat than small animals.

Why do penguins huddle?

Which animal has the greatest surface area to volume ratio?

The MOUSE of course!

How might the small surface area to volume ratio help the hippo in its natural environment?

Chapter 10.2

The Cell Cycle and Mitosis

Objectives

• Describe the main events of the cell cycle

• Explain each of the four phases of mitosis

Some Cell Division Terminology

Don’t panic…

Just sharpen your pencil!

• Each of the time periods of a cell’s life are referred to as phases

• This is similar to the way you can breakdown your life into phases

• Certain events are associated with each phase and can be used for identification of the phase

Phases

Chromosomes• Chromosomes are made up of DNA wound tightly

around proteins called histones

• Histones + DNA = chromatin

• When a cell prepares for division it duplicates its DNA and condenses the chromatin into neat packages called chromosomes

Chromosomes

• Exist in homologous pairs– One from mom

– One from dad

• Humans have 23 pairs– 22 autosome pairs

– 1 sex pair (these determine the sex of the organism)

Sister Chromatids

• During replication (S phase) a pair of sister chromatids are made for each homolog of a chromosome pair

Chromosomes

• Are generally represented in their duplicated form

• A single duplicated chromosome has two parts, each called a chromatid

• Each chromatid is referred to as a “sister” and is attached at a centromere

Cell Cycle

• During the cell cycle, a cell grows, prepares for division, and divides to form 2 daughter cells, each of which then begins the cycle again.

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_cell_cycle_works.html

Cell Cycle• It includes two major stages

1) Interphase• G1, S, G2

2) M phase (nuclear and cellular division)• Mitosis (division of the

nucleus)

• Cytokinesis (division the cell)

Stage 1 - Interphase – “In Between”

• Interphase

G1= cell grows

S =chromosomes duplicate

G2= organelles duplicate, DNA checked

Stage 2 – Mitosis – Nuclear Division

• First part of M Phase• The division of the

nucleus of somatic (body) cells during the cell cycle.

• Its divided into 4 phases.• Remember Interphase

and Cytokinesis are NOT part of Mitosis.

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.html

KEY CONCEPT

• The phases of the M Phase of the cell cycle do not occur in isolation

• The are a continuum starting with prophase and proceeding sequentially through metaphase, anaphase and finally telophase

• Sometimes it can be difficult to determine the phase a cell is in because it will appear to be transitioning from one phase to another

Prophase

• First stage of Mitosis

• Nuclear membrane breaks down

• Chromatin condenses into chromosomes

• Centrioles (animal cells only) separate and a spindle apparatus begins to form a cage like structure across the cell

Prophase

Metaphase

• Second Phase of Mitosis

• Each chromosome is connected to a spindle fiber at an area called the kinetochore on the centromere.

• the chromosomes line up in the middle of the spindle (called the metaphase plate)

Metaphase

Anaphase

• Third Phase of Mitosis

• The sister chromatids separate at the centromere into individual chromosomes and move apart to opposite ends of the cell.

• the spindle fibers shorten and pull the chromosomes to each end (aster)

Anaphase

Telophase

• Fourth and Final Stage of Mitosis

• Nuclear envelopes reform around each complete set of chromosomes

• Chromosomes unwind back into chromatin losing their distinct shape

Telophase

Final Step - Cytokinesis• The last step of M phase

• The equatorial belt pinches cytoplasm in half

• Each daughter cell has an identical set of chromosomes

The Whole Cell Cycle – Putting it all together!

So what is the END RESULT?

One Parent Cell

DIVIDES INTO

Two identical Daughter Cells

Regulating the Cycle

• Proteins called cyclin help regulate the cell cycle in eukaryotic cells.

• They respond to internal and external events. These proteins are regulators.

When Things Go Wrong

• Some cells begin to grow without regulation• They produce a mass of cells called a tumor

– Benign tumors do not cause damage to surrounding tissues

– Malignant tumors DO cause damage to tissues and may become cancerous

• Cancer cells do not respond to the signals that regulate the growth of most cells.

• As a result, they form masses of cells called tumors that can damage surrounding tissues.

• Brief animation!

11-4 Meiosis

Cell Division for Sexual Reproduction

Homologous Chromosomes

• All sexually reproducing organisms inherit one set of

chromosomes from each parent

• Each parental set contains exactly the same number of

chromosomes

• The corresponding chromosome from each parent is called

a homolog

• 2 homologs make a homologous pair

Chromosome Pair 1

Chromosome Pair 2

Chromosome Pair 3

Chromosome Number

• Diploid (2 N)

– Cells with a complete set of homologous pairs

• Haploid (N)

– Cells with one half the homologous chromosomes in a diploid cell

Let’s do a quick activity to investigate this.

What is Meiosis?

• A cell division cycle which reduces the number

of chromosomes in new cells by half

• Diploid (2N) somatic (body) cells are reduced

to haploid (N) gamete (sex) cells

• Chromosome reduction occurs by separation

of homologous chromosomes

Meiotic Cell Division – the Basics

• Meiosis involves 2 distinct phases

– Meiosis I

• Homologous pairs are separated

• 2 haploid cells are formed

– Meiosis II

• Sister chromatids are separated

• 4 haploid cells are formed

• End result is 4 genetically distinct haploid cells

Meiotic Cell Division – the Basics

Start with Interphase

• Meiosis I– Prophase I

– Metaphase I

– Anaphase I

– Telophase I

– Cytokinesis

• 2 daughter cells result

• Meiosis II– Prophase II

– Metaphase II

– Anaphase II

– Telophase II

– Cytokinesis

• Each daughter cells divides into 2 new daughter cells

• 4 daughter cells in total

A Closer Look at Meiosis I

• Meiosis I involves a special step that leads to genetic changes in duplicate sister chromatids

• During PROPHASE I– Homologous

chromosomes join together in a structure called a TETRAD

Tetrads and Crossing Over

• TETRAD formation (synapsis),

– the arms of one chromatid of each

homolog CROSSOVER one another

– a gene segment from one parent

chromosome is exchanged with an

equal gene segment from the

other parent chromosome

Cytoplasm distribution in human gamete formation

Males Females

• Cytoplasm distribution is even

• Meiosis results in – Four equal sperm cells

– Occurs in the testes

• Cytoplasm distribution is uneven

• Meiosis results in – One large egg

– Three smaller polar bodies

– Occurs in the ovaries

Zygote Formation

• Fertilization

– The joining of a haploid egg and a haploid sperm cell

– Results in the formation of a ZYGOTE (a fertilized egg)

• What is the chromosome number of a zygote?

– Haploid or Diploid?