CELL DIVISION

Why do Cells Divide?

• Unicellular organisms do it to make new individuals

• Multicellular organisms do it to grow, repair, and maintain SA:V ratios-– there are about 10,000,000 mitotic cell divisions/

sec in the human body!

How do they do it?

• PROKARYOTES use binary fission:– DNA attaches directly to membrane

• DNA replicates and attaches to different points on membrane

• Plasma membrane grows between these two points

• A segment of cell wall begins to form between the DNA

Eukaryotic Cells

• Use Mitosis and Meiosis– Nuclear contents (DNA) are copied

& divided– Cell splits into two by cytokinesis

– Complexity of Eukaryotes:

• they are big and contain many organelles

• they contain 700x the DNA of Proks• they contain many chromosomes

(not just one)

The Cell Cycle• Represents recurring events

that take place from the beginning of one cell division to the beginning of the next.

• Includes:– cell division– growth – DNA replication– preparation for the next

division

A few Questions?• Why is the cell cycle called a “cycle”?

– It represents recurring events• Why do you think that it is important for a cell

to grow in size during its cell cycle?– If a cell did not grow in size, each cell

division would produce progressively smaller cells

• What might happen to a cell if all events leading up to cell division took place as they should, but the cell did not divide?– The cell would grow increasingly larger--- to

a point at which the cell could no longer exchange materials with the environment efficiently enough to live.

– Animation

THREE PARTS TO THE CELL CYCLE

• Interphase- – majority of life cycle,

preparation for cell reproduction

• M Phase– Nuclear division

• Cytokinesis-– The cytoplasm divides And the cell splits in 2

Interphase• G0- Gap0 Phase- where cells spend most of

their time, just acting like cells, until signals are sent to their nucleus that it’s time to divide.– some cells stay here

ex. muscle & nerve cells• G1- Gap 1 Phase-

– cell grows and carries out normal metabolism – organelles duplicate

• S (synthesis) Phase– all DNA in chromosomes replicates

• G2 – Gap2 Phase– Cell grows and preps for mitosis– Animation

• Draw a labeled picture of the Cell Cycle

C & C P. 216

1. How does the importance of cell division differ in unicellular and multicellular eukaryotes?

Cell division in unicellular organisms results in the production of new individuals; in multicellular organisms, it results in new tissue

2. What is the major event in M phase? S phase? G0 phase?

M, mitosis; S, DNA synthesis (replication); G0 stopping point, metabolism

3. What determines whether a cell stays in interphase or divides?

Signals received in G1 or G24. What are some factors that might influence the cell cycle?

A trigger protein may help control the cell cycle. This protein may itself be influenced by hormones, cell size, and position in a tissue

5. Explain why DNA replication is important in cell division

Each offspring/daughter cell needs a complete set of genetic material.

DNA: The genetic material

A few questions to ponder today:• What is DNA?• How did we figure out the structure of

this important biomolecule?• How is it replicated for the cell cycle?

• Steps to the discovery of DNA:– 1869- Freidrich Miesher

• purified DNA from pus cells obtained from discarded surgical bandages

– 1923- Robert Feulgen • developed a procedure that stained this material in the nucleus• This enabled him to measure the amount of “stuff”in the

nucleus• He concluded that virtually every cell nucleus in any plant or

animal has the same amount of this “stuff”

The Unknown…Was it DNA or a protein that carried the hereditary instructions

from generation to generation?Early scientists saw the structure of the DNA molecule as too simple to possibly contain the complex genetic information of an organism

• 1828- Fred Griffith was trying to prepare a vaccine against a pneumonia-causing bacterium.

• He discovered that harmless bacteria could turn virulent when mixed with bacteria that caused disease.

• In other words, he discovered what is now called transformation

a change in phenotype caused when bacteria cells take up foreign genetic material. animation

• 1944- Oswald Avery discovered that DNA is responsible for the transformation that occurred in Griffith’s mice.

• 1952- Alfred Hershey and Martha Chase settled the controversy. – DNA is the molecule that store genetic info in living cells

• 1951- Roseland Franklin fired x-rays through DNA crystals, trying to learn how the atoms of the DNA molecule were arranged

• 1953- James Watson and Francis Crick were shown Franklin’s photo and immediately determined the double helical structure of DNA.

DNA StructureNucleotides are the subunits:

– a phosphate group– a 5-carbon sugar molecule

• Deoxyribose• 5’ end- where the Phosphate group is• 3’ end –OH (hydroxyl) end

– a nitrogen –containing base• Purines- made up of 2 rings of carbon

and nitrogen atoms– Adenine (A)– Guanine (G)

• Pyrimadines- made up of a single ring of carbon and nitrogen atoms

– Thymine (T)– Cytosine (C)

Chargaff’s Rule- (1947)- Erwin Chargaff, a chemist who worked at Columbia

found that:– for each organism studied, the amount of adenine

always equaled the amount of thymine – and cytosine always equaled guanine. – This became known as the Base-Pairing Rules

Why does this complementarity exist?□ The two chains of nucleotides are held together

by 2 or 3 weak hydrogen bonds that form between the nitrogenous bases:– Cytosine and guanine have three hydrogen

bonds– Thymine and adenine have two hydrogen

bonds□ Each rung of the DNA ladder needs to have a

total of three rings • Also, One of the strands of DNA runs in the

3’ to 5’ direction and the other runs in the 5’ to 3’ direction.

• This is called _______________

antiparallel

What does 5’ and 3’ mean???

The symbols 5' and 3' (pronounced "five prime" and "three prime") refer to the two ends of a strand of DNA.

The carbons on the deoxyribose sugar are numbered 1-5. One end of a DNA strand begins with a phosphate group; this is attached to the 5' carbon (carbon #5) of a deoxyribose.

A new nucleotide is attached by combination of its phosphate to the 3' carbon of the sugar. The two strands of a DNA molecule are antiparallel, so the 5' end of one strand is paired with the 3' end of its complementary strand.

http://wiki.answers.com/Q/What_do_5'_and_3'_mean_when_talking_about_a_cell_reproducing_itself

DNA cont.• DNA is the molecular

“blueprint”—– This “instruction manual”

makes up an organisms genes. – Encoded in the precise order of

its nucleotides – The genetic alphabet only

contains 4 letters (ACTG). – Human DNA contains more

than 3 billion base pairs!

COMPARING PROKARYOTIC AND EUKARYOTIC DNA:

PROKARYOTES EUKARYOTES

Configuration of DNA Circular linear

Length 1000 micrometers 1.8 meters

DNA housed in nucleus?No Yes

Associated proteins Transient proteins Histones

# of Chromosomes per Cell

1 2- 1,262

HOW DO ALL THESE BASE PAIRS FIT IN THE NUCLEUS?

– a NUCLEOSOME consists of 2 loops of DNA wrapped around a central cluster of 8 histone molecules.

– This shortens the length of DNA by 1/6. – This is shortened even further by its coiling into

thicker fibers

DNA REPLICATION• We know we need to copy a cells DNA before a cell

can divide, but how is DNA copied?• There were 3 possible models for DNA copies to be

made while obeying the Base Pairing or Chargaff’s Rules :– Conservative replication

• leaves the original DNA molecule intact and generates a completely new molecule

• Sort of like a photo copier– Dispersive replication

• produces two DNA molecules with sections of both old and new DNA interspersed along each strand.

– Semi-conservative replication • produces molecules with both old and new DNA,

but each molecule would be composed of one old strand and one new one.

• Journey Into DNA• Investigating DNA Replication

The Process of DNA Replication• Simplified DNA Replication Activity

1. The double helix unwinds with the help of enzymes called DNA HELICASES. • The enzymes break the hydrogen bonds between nitrogen bases• Strands are held apart and prevented from twisting back into their

double-helical shape by a SINGLE STRAND BINDING PROTEIN.• The two areas on either end of the DNA where the DNA helix

separates are called REPLICATION FORKS.2. RNA PRIMASE- inserts as starter of RNA nucleotides at the initiation

point.

Animation

– Recall, one of the strands of DNA runs in the 3’ to 5’ direction and the other runs in the 5’-3’ direction

– DNA Polymerase can only move/build in one direction- new DNA nucelotides can only be added to the 3’ end (replication proceeds 5’-3’ on each strand)

– Because DNA is antiparallel, DNA Polymerase molecules move in opposite directions along the two strands.

– The two newly synthesized strands grow in opposite directions- one towards the fork, the other away from it.

– The strand that is assembled continuously and towards the replication fork is called the LEADING STRAND,

– The strand that is assembled discontinuously and away from the replication fork is called the LAGGING STRAND. These short fragments are called OKAZAKI FRAGMENTS.

3. DNA POLYMERASE III moves along each of the DNA strands reading the nucleotide on the template and joins the complementary nucleotide onto the end of the new strand by following the Base-Pairing Rules (as many as 1000 base pairs can be added each second)

4. DNA POLYMERASE I- strips away the RNA PRIMERS and inserts DNA nucleotides

5. LIGASE- seals the gaps in the sugar phosphate backbone where Okazaki fragments come together.

Checking for Errors

• Errors sometimes occur. • An important feature of DNA

replication is DNA polymerases ability to “proofread”. – This enzyme is capable of

backtracking, removing incorrect nucleotides and replacing it with the correct one.

– Still one error per one billion nucleotides typically occurs.

– This type of error is called a MUTATION.

Multiple Forks

• Replication does not begin at one end of the DNA molecule and end at the other.

• Each chromosome contains a single long strand of DNA.

• The replication of a typical human chromosome with one pair of replication forks would take 33 days!

• For this reason, each human chromosome is replicated in about 100 sections that are 100,000 nucleotides long (this takes only 8 hours).

C & C p. 2221. What are 3 different roles of DNA polymerase?

DNA polymerase synthesizes DNA from the four nucleotides, proofreads its own work, and corrects single base insertion errors

2. Explain how DNA synthesis can proceed in both directions form a replication origin, even though DNA polymerase can synthesize DNA only in one direction.

Back synthesis on the lagging strand products short fragments that are joined by enzymes.

3. How is the number of replication errors in cells kept to a minimum?

Enzymes detect and repair the errors; DNA polymerase has proofreading and error-correcting functions.

4. How does the cell repair damaged DNA?

By enzyme-mediated mechanisms, including excision repair

5. How are histones involved in gene expression?

Histones bind to DNA and form nucleosomes. This tightly packed structure turns off DNA by excluding the enzymes involved in gene expression.