Introduction to the Cell Cycle

Why do cells need to divide?

As cells grow larger, they start to face several difficulties in functioning:

• Growth & Repair– In order for multi-cellular organisms to grow, cells must divide. If cells are

lost due to injury or damage to DNA, new cells must replace old cells.

• Information Overload– The larger a cell becomes, the more demands the cell places on its DNA.– There needs to be enough DNA to support the protein needs of the cell.

• Exchanging Materials– The rate of diffusion is slower in larger cells and faster in smaller cells/ – A larger cell is less efficient in moving nutrients and waste materials across the cell membrane.

Surface Area to Volume Ratio• The volume of the cell increases faster than the surface area of

the cell membrane!!

Volume = content inside the cellSurface Area = Amount of Cell Membrane Small cells have a LARGER surface area : volume ratio!

More cell membrane = faster exchanging of materials

Cell Division• The process by which a cell divides into two

new “daughter” cells.– DNA replication takes place before the cell divides• Creates two identical copies of DNA (genetic information)

– Each daughter cell gets one complete copy of genetic information

– Results in the an increase in the surface area:volume ratio for each daughter cell

Asexual Reproduction• The production of genetically identical offspring

from a single parent.• Primary form of reproduction in single-celled

organisms such as Bacteria & Protists.

• Unicellular Organisms– Bacteria using cell division to reproduce is called Binary

Fission.

• Multicellular Organisms– Budding - a new organism develops from an outgrowth or

bud on another one due to cell division at one particular site. The new organism remains attached as it grows, separating from the parent organism only when it is mature.

Asexual Reproduction

Advantages• Do not need a partner to

reproduce• Can reproduce quickly and

easily

Disadvantages• Produce genetically

identical offspring – Could result in death of a

population if there is a change in the environment that is not favorable to the characteristics of that organism.

Sexual Reproduction

• Offspring produced by sexual reproduction inherit some of their genetic information from each parent.

• Offspring are produced by the fusion of reproductive cells form by each of the two parents.

• Most animals and plants produce sexually, as well as some single-celled organisms.

Sexual Reproduction

Advantages• Creates genetic diversity in

a population– If the environment changes,

some offspring may have the right combination of characteristics to survive

Disadvantages• Invest more time, energy,

and resources– Need to find a mate– Growth, development, and

caring for offspring

What is a chromosome?• DNA is packaged into chromosomes.• Chromosomes are threadlike structures of

DNA wrapped around proteins.

CHROMATIN – DNA tightly coiled around histone proteins

CHROMOSOME

Eukaryotic Chromosome Structure

• Chromosomes are only visible when a cell is dividing so we usually see them in their double-stranded form.

Eukaryotic Chromosome Structure

• The halves of a double-stranded chromosome ( sister chromatids) are identical to each other because the DNA (original chromosome) replicated before cell division.

(part of chromosome where the two chromatids attach)

Why are chromosomes important in cell division?

• Chromosomes make it possible to separate DNA precisely.

• Each new cell will get the exact same DNA as the cell it came from. – DNA Replication takes place before cell division!

Prokaryotic Chromosome Structure

• Where is the nucleus?– THERE IS NO NUCLEUS! IT’S A PROKARYOTIC CELL!

ONLY EUKARYOTIC CELLS HAVE A NUCLEUS!

Types of Cells• There are two types of human cells: somatic

cells and gametes.– Somatic Cells = body cells (brain, muscle, heart,

liver, skin, etc)• Contain 46 chromosomes in humans• Diploid (2n)= cell that contains two sets of homologous

chromosomes

– Gametes = sex cells (sperm or egg)• Contain 23 chromosomes in humans• Haploid (n)= cell that contains one set of chromosomes

FertilizationWhen sperm meets egg a zygote is formed. A zygote is the first cell of the embryo.

Which of the cells above are haploid (n)? Diploid (2n)?

Homologous Chromosomes

• Chromosomes containing the same type of genetic information

• one comes from male parent, one comes from female parent

DAD MOM

Homologous Chromosomes

What is the diploid number of chromosomes in humans?

46

How many pairs of homologous chromosomes do humans have?

23

The chromosomes diagrammed below are arranged in a karyotype, the 46 chromosomes have been arranged in homologous pairs.

The first 22 pairs of homologous chromosomes are called autosomes or autosomal chromosomes.

The 23rd pair of chromosomes determines the sex of the individual and are called sex chromosomes.

The sex chromosomes of a female are XX.The sex chromosomes of a male are XY.

What’s the difference?

Which of the following is an example of a haploid cell?

1 2 3 4

0%

10%

86%

5%

1. Stomach cell2. Brain cell3. Sperm Cell4. Skin cell

How many chromosomes are in your body cells?

1 2 3 4

13%

88%

0%0%

1. 232. 323. 644. 46

Your body cells have how many pairs of homologous chromosomes?

1 2 3 4

4%9%

4%

83%

1. 46 pairs2. 23 pairs3. 32 pairs4. 2 pairs

This is a picture of a

1 2 3 4

25%

0%

25%

50%

1. Single chromosome2. Chromatin3. Duplicated

chromosome4. Cell

_________ are located on chromosomes.

1 2 3 4

6%0%

69%

25%

1. Cells2. Karyotypes3. Genes4. Eggs

Human somatic cells contain _____ pair(s) of autosomes.

A. 22B. 1C. 23D. 46

22 1 23 46

52%

5%10%

33%

Somatic cells contain _____ pair(s) of autosomes.

0% 0%0%0%

1. 222. 13. 234. 46

Countdown

10

Gametes are _________, meaning they contain one set of chromosomes.

A. DiploidB. HaploidC. TriploidD. Monoploid

Diploid

Haploid

Triploid

Monoploid

14%

32%

0%

55%

If an organism contains 14 chromosomes in each somatic cell, then its gametes each

contain ____ chromosomes.A. 7B. 14C. 21D. 28

7 14 21 28

64%

8%8%

20%