DNA replication

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DNA is Here to Stay

By Dr. Fran Balkwill and

Mic Rolph

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The Cell Cycle and DNA Replication

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The Cell Cycle

• To understand how organisms grow and develop, we will study how cells make copies of themselves.

• We will study the Cell Cycle and two of its components:– DNA replication (copying genetic information

so that both new cells have exactly the same DNA)

– Mitosis (dividing one cell into two)

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• The lifespan of a eukaryotic is best described as a circle (as opposed to a line)

• As a cell grows, it reaches a point where it gets big enough to divide. To do this, all the DNA and other cellular components must be copied and shared equally.

• Thus one cell becomes two identical daughter cells in a process called mitosis.

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Phases of the Cell Cycle

• Cell Cycle can be divided into 2 main parts: 1. Interphase - the

period between cell divisions

• Chromosomes are not visible in the nucleus

2. Mitosis – cell division• Chromosomes visible

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InterphaseInterphase has 4 sub-phases

1. G1 phase (Gap 1 – growth, normal cell activities, preparation for DNA replication)

During G1, cells can enter

2. G0 phase (Gap 0 or nondividing cells)

a. Cells still metabolically active, just not making new cells.

b. Examples- Nerve cells do not re-enter G1

- Muscle, liver cells can re-enter G1 if damaged

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Interphase

3. S phase (DNA synthesis – DNA is copied in preparation for mitosis)

4. G2 phase (Gap 2 – cell continues to grow and produces proteins needed for mitosis)

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Mitosis

Mitosis – replicated DNA distributed to daughter cells

Cytokinesis – cytoplasm divided between daughter cells. - Happens at the very end of

mitosis

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• http://www.cellsalive.com/cell_cycle.htm

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Mitosis Vocabulary

• Chromosomes– Molecules of DNA complexed

with specific proteins responsible in eukaryotes for storage and transmission of genetic information.

• Chromatin – Protein/DNA complex making

the chromosome

• Chromatid – Each of a pair of identical DNA

molecules after DNA replication, joined at the centromere.

• Histones – Proteins forming complexes

with Eukaryotic DNA.

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Mitosis Vocabulary

• Centromere– The point at which two

chromatids of a chromosome join and at which the spindle fibers attach during cell division

• Spindle fibers– a network of microtubule

fibers along which chromosomes are drawn apart during mitosis

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Mitosis

• Mitosis further divided into 4 phases:1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

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Mitosis

• Animation: http://www.mhhe.com/biosci/esp/2001_gbio/default.htm

• ABC # 16

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Mitosis• Prophase

– Nuclear envelope breaks down

– Chromatin condenses into chromosomes (each with two sister chromatids) and becomes visible

– Microtubules begin to assemble into the mitotic spindle

• Spindle attaches to chromosomes at the centromere

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Mitosis

• Metaphase– Chromosomes align at the center of the cell

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Mitosis

• Anaphase - chromatids (daughter chromosomes) are pulled apart by the spindle fibers and begin moving to the cell poles

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Mitosis

• Telophase – Daughter chromosomes

arrive at the poles, 2 new nuclei form

– Spindle fibers disappear – Cytokinesis divides

cytoplasm between 2 daughter cells

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Mitosis

• Animation: Repeat http://www.mhhe.com/biosci/esp/2001_gbio/default.htm

• http://www.cellsalive.com/mitosis.htm

• Also consult http://www.mhhe.com/biosci/esp/2001_gbio/default.htm for general Biology help

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Mitosis

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Mitosis

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DNA Replication

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DNA

Review…– Polymer of nucleotides

(monomers)

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DNA

Nucleotides made of– 5 carbon sugar

• Deoxyribose (DNA) – missing an –OH group

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DNA– Phosphate group – link sugars together

into sugar-phosphate backbone

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DNA

– Nitrogenous base a. Purines (large double ring

organic base)

1. Adenine (A)

2. Guanine (G)

b. Pyrimidines (single ringed organic base)

1. Cytosine (C)

2. Thymine (T)

3. Uracil (U) – found only in RNA

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DNA

DNA polymer made by combining monomers (nucleotides) in specific order.

- Catalyzed by enzyme DNA polymerase

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DNA

– DNA is a double helix – 2 strands held

together by hydrogen bonds between complementary base pairs:

• Purine - pyrimidine• A-T (2 H bonds)• G-C (3 H bonds)

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• The two DNA strands are anti-parallel (run in opposite directions)– Both strands go in

the 5’ to 3’ direction

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DNA REPLICATION

• Replication based on base-pairing rules– Involves over 20

enzymes and other proteins

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• Animation http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ge/m4/s2/index.htm

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• Helicase unzips the DNA double helix by breaking the H-bonds between bases, forming a replication bubble– There are multiple bubbles on each eukaryotic chromosome

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• DNA polymerase adds nucleotides to the new DNA strand according to base-pairing rules – The entire length of the DNA molecule is replicated as

the bubbles meet.

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• DNA polymerase can only work from 5’ to the 3’ direction. – This means one strand

is synthesized continuously – the leading strand

– The other – the lagging strand - forms a series of short Okazaki fragments, which are all joined together to complete replication

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• DNA replication is semi-conservative– When DNA is copied, one half of the

old strand is always kept in the new strand.

– This helps reduce the number of copy errors.

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DNA REPLICATION

• Results in two exact copies of DNA.

• Highly accurate—only one error for every 2x109 (billion) nucleotides.

• Any error (mutation) can be harrmful, even deadly, but a few can cause beneficial change.

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• Practice replicating…

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Histones

• What are histones?– Proteins that form complexes with eukaryotic

DNA • DNA wraps around histones to form nucleosomes

– Consist of many basic amino acids, so they have a strong positive charge

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Histones

• What do histones do?– Allow negatively-charged DNA (nucleic acid)

to be packed into small space of the nucleus– Help control which DNA can be used to make

proteins (more on this later…)

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Lab 8B

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Control of Cell Cycle

• Review cell cycle and mitosis

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• What would happen if cells divided whenever they “felt” like it?

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• God is a God of order, not chaos.– This is true of our world and of our bodies!

Every activity in our bodies is remarkably and precisely controlled and coordinated through chemical signals (hormones, neurotransmitters)

– Breakdown in control, coordination is called disease

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• Our Creator has placed controls called checkpoints throughout the cell cycle to keep it under control.

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What controls the cell cycle?The cell receives chemical signals that stimulate or restrict

it from continuing the cycle - CHECKPOINTS

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Checkpoint #1

1. The cell growth checkpoint (G1)

Is the cell large/mature enough?

Is the DNA intact, ready to be copied?

If “yes”, proteins called cyclins, acting as chemical signals, cause the cell to enter the S phase.

If “no”, the cell cycle stops and the cell enters the G0 phase. (nerve cells)

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Checkpoint #2

2. DNA synthesis checkpoint (G2)

Did DNA replication occur without error? (Repair enzymes check the DNA)

If “yes”, different cyclins trigger mitosis

If “no”, the DNA repair enzymes try to correct the problem so the cell can continue the cell cycle.

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Checkpoint #3

3. Mitosis/Spindle Assembly CheckpointCan mitosis proceed without errors?

If “yes”, mitosis can be completed and the G1 phase can begin. The cell can return to Interphase.

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• Sometimes the checkpoint controls are damaged by…– Mutation - any change in the nucleotide

sequence of DNA• A single base or a larger piece of DNA can be

accidentally added, deleted or changed • Change in DNA of a gene can change which

amino acid goes in the protein it produces. This can change the structure/function of the protein.

Cancer

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Cancer• Cells cannot stop dividing and go through

uncontrolled cell division, becoming cancerous cells – Cancer refers to any abnormal growth of cells.– Tumor is a mass of cancerous cells

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Cancer

• Types of tumors– Benign tumors do not spread beyond their

original area of growth• Can be harmful if they grow large enough to

interfere with normal body function– Examples – lipoma, tumors on acoustic (hearing) nerve,

meningiomas

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Cancer

– Malignant tumors can metastasize - spread to other parts of the body and establish new colonies

• These are the most familiar “bad guys” – lung, colon, melanoma, breast, pancreatic and other cancers

– What are some things that you know of that cause cancer?

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Cancer• Carcinogens/mutagens are agents known to cause

cancer/mutations– Radiation

• X-rays• UV light from the sun and tanning lamps

– Carcinogens• Nicotine, tar (mainly from tobacco products)• Asbestos• PVC’s• Alcohol

– Viruses• Hepatitis B• Herpes • Papilloma virus (HPV)

– Many cancers also have a genetic component (run in families) like breast, colon, lung and some leukemias

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Cancer and the Cell CycleAt a very basic level, cancer is usually due to one

of the following factors:

1. Oncogenes - mutations cause normal “go” genes to become cancer-causing genes

• Normally, proto-oncogenes act with a growth factor to stimulate cell division.

• Mutation causes proto-oncogene to become an oncogene that “tells” the cell to divide without the presence of a growth factor (by-passes checkpoints)

• Cells start dividing and keep dividing with no control mechanism to stop them.

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Cells also contain “brake” genes that keep dividing cells from becoming tumors. These genes are called…

2. Tumor-Suppressor genes - mutations to tumor suppressor genes ‘inactivate’ them. This cell cycle ‘brake’ is removed.

– Checkpoint proteins are inactivated – With no inhibitors to stop it, the cell begins

rapid and out of control division. – Many cancer cells result from mutations in

tumor-suppressor genes.

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Tumor Suppressor Genes

BRCA1 and BRCA 2 are genes that (with the help of other proteins) are directly involved in the repair of damaged DNA.

Variations/mutations in this gene lead to an increased risk for breast cancer.

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Tumor Suppressor Genesp53 is a protein that blocks the cell cycle at

the G2 checkpoint if the DNA is damaged.

– Normal p53 function gives the cell time to repair its DNA

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Tumor Suppressor GenesCells with mutant p53 can’t inhibit the growth

of abnormal cells.

A p53 mutation is the most frequent mutation found in cancer cells. p53 is mutant in about 50% of all cancers.

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FIGHTING CANCER

• Surgery – many solid tumors can be removed.

• Radiation – Kills cells by damaging DNA – Most effective when aimed at a particular

region of the body.

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FIGHTING CANCER• Chemotherapy – using chemicals (drugs)

to attack rapidly dividing cells– Drugs typically interfere with DNA replication

or cell division• Antimetabolites look like normal nutrients, but

once incorporated into cell cause it to die– Methotrexate

• Helicase inhibitors – prevent DNA from unzipping, so it can’t replicate

• Alkyloids – disrupt the mitotic spindle so chromatids don’t separate

– Vinchristine (from vinca plants)

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FIGHTING CANCER

• Also harms normal healthy cells which divide frequently– White blood cells, lining of digestive tract, hair

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GENE EXPRESSION

• A single segment of DNA can have thousands of protein-coding regions.

• How does a cell know where to start transcription?

• How does a cell know which is the coding strand?

• How does a cell know when to stop the protein synthesis?

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GENE EXPRESSION

• Genes have four regions:• The promoter region• Protein coding region• Termination sequence • Regulatory region(s)

• The promoter is a specific sequence (recognized by RNA polymerase) found upstream of the protein coding sequence of a DNA.

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CODON

• Here are examples of codons:

» AUG codes for a start

» UAA, UAG, and UGA code for a stop

» The other 61 remaining codons code for amino acids.

• The human genome has 46 chromosomes and about 5,000,000,000 base pairs.

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GENE EXPRESSION

• RNA polymerase finishes making an RNA when it encounters the termination site.

• The mRNA is then used to translate to direct protein synthesis.

• A gene is expressed when its product, usually a protein, is produced for use by the cell.

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GENE EXPRESSION

• Regulatory regions are important as to when and how genes are expressed.

• Gene regulation works by using proteins that bind to the regulatory regions.

• These proteins are called transcription factors and are only present at certain times and make the promoter region more noticeable.

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SPLICING

• Unlike in prokayotic cells, in eukaryoatic cells, transcription of the protein-coding regions are not continuous.

• Transcription starts at the promoter and finishes at the termination site.

• The regions that are not needed (introns) are spliced (removed).

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GENETIC ENGINEERING AND BIOTECHNOLOGY

• Knowledge of the biology of DNA has resulted in major advances in:

» Medicine» Agriculture » Industry» Environmental sciences

• We now have organisms with genetic changes in their DNA known as genetically modified (GM) or transgenic organisms.

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DNA and CHROMOSOMES

• When the chromatin fiber is condensed it is referred to as a chromosome.

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• During replication of the DNA molecule, each strand serves as a template (model) for the assembly of a new strand. The DNA strands separate, and free, unattached nucleotides associate with their complementary bases through hydrogen bonding. DNA polymerase is the enzyme that adds nucleotides to the newly-forming strand. Ultimately two DNA complexes are formed, each containing one old strand and one newly formed strand.

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• www.pbs.org/faithandreason/media/chrom-body.html

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Mitosis

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DNA

• All cell’s activities are controlled by its DNA– DNA found in the

nucleus– Chromosomes are

made of DNA – Genes are specific

sections of DNA that code for proteins

• Not all DNA codes for proteins

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DNA REPLICATION

• Copying DNA when a cell is preparing to divide into 2 identical cells (daughter cells)– Each daughter cell keeps the same amount of DNA.

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DNA REPLICATION

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MUTATION

• Types of mutations:– 1 base is substituted for another, and the amino acid

changes• THQ ONE BIG FLY HAD ONE RED EYE

– Frameshift – 1 base is added or deleted• THE ONE QBI GFL YHA DON ERE DEY E (insertion)• TEO NEB IGF LYH ADO NER EDE YE (deletion)

– Nonsense – base change codes for a stop codon, so the protein is incomplete.

• THE ONE BIG FLY

• THE ONE BIG FLY HAD ONE RED EYE

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Sometimes the checkpoint controls are damaged…

…a cell repeatedly divides (ignoring checkpoint commands)…

What is this condition called?

CANCER

Cells that repeatedly divide may form a mass of cells called a…

TUMOR

So, what is cancer?

It is a cell division disorder. It occurs when the cell cycle malfunctions.