Unit 4 B Asexual Reproduction and Mitotic Cell

Unit 4 B Asexual Reproduction and Mitotic Cell Division

The Cell Theory states: 1) cells are the basic unit of life 2) all living things are made up of at least 1 cell, and 3) all cells come from pre-existing cells (cell reproduction/division)

What is the purpose of cell reproduction? • In Unicellular Organisms: to form a new organism-asexual reproduction • In Multicellular Organisms: a. Growth- addition of new cells b. Repair- replacement of lost or injured cells c. Reproduction- formation of reproductive cells (egg + sperm) in sexual reproduction

Cell Reproduction • As you grow from a baby to a teenager, do your cells get bigger or do you gain more cells?

Why do cells divide into more cells rather than continue to grow? 2 Reasons: 1. Larger size puts larger demands on the DNA (information shortage) 2. Larger cells are less efficient at moving materials in and out of the cell (exchanging materials)

1. Information Shortage • DNA contains instructions for making proteins for the cell • As the cell gets bigger, the DNA stays the same • If a cell continues to get larger, eventually the DNA would not be able to make enough proteins quickly enough for the cell

2. Exchanging Materials • Cells pass material in and out through the cell membrane – Surface area (SA) of the membrane determines the rate of exchange of materials • Cells use and break down materials within the cell – Volume (V) of the cell determines how much material is needed (larger cells need more nutrients, O 2, etc. )

Ratio of Surface Area to Volume • A ratio is a comparison – Ex. Surface Area: Volume (Surface Area/Volume) – The amount of surface area for each unit of volume • Volume = l x w x h • Surface Area = l x w (x 6 sides)

Ratio of Surface Area to Volume The greater the SA / V ratio, the easier it is for the cell to exchange materials. 1. 2. 3. Cube 1 SA (mm 2) V (mm 3) SA/V Cube 2 Cube 3

Determine the SA/V Ratio for cubes 2 & 3 and fill in the table Cube 1 Cube 2 Cube 3 SA (mm 2) V (mm 3) SA/V • Which cell has the greatest SA / V ratio? • So, which cell has the easiest time moving material in and out of the membrane and getting nutrients, O 2, etc. to all places in the cell? 1. 2. 3.

Determine the SA/V Ratio for cubes 2 & 3 and fill in the table Cube 1 Cube 2 Cube 3 SA (mm 2) V (mm 3) SA/V • From one cube to the next, the SA increased by a factor of ______. • The V increased by a factor of _______. • Which is increasing faster as the cell gets bigger? (circle) SA or V

Ratio of Surface Area to Volume • As a cell grows, its volume increases much faster than its surface area • If a cell becomes too large, not enough nutrients can come through the membrane to feed the whole cell. • Similarly, wastes from the huge cell would not be released fast enough. • Therefore, smaller size allows for efficient movement of materials in, out, and throughout the cell.

Thus, instead of growing too large, cells divide in half! • This increases the SA / V ratio • 1 cell divides into 2 “daughter” cells. • This process is called cell division How a high SA/V ratio aids in diffusion into a cell

Before a Cell Divides… 1. The cell gets the signal to divide 2. It replicates (copies) all of its DNA 3. Then, the cell divides and each daughter cell gets a complete copy of the DNA 1 chromosome Copy of that chromosome Replicated Chromosomes (DNA and associated proteins)

Cell Division Solves the Growth Problems 1. Each daughter cell gets its own DNA so there is no information shortage 2. Volume is reduced in division, so efficient exchange of material can occur through the cell membrane

Practice Two factors important in the transport of materials into or out of cells: • The surface area of the cell membrane should be large / small • The volume of cell matter that materials have to travel through to get to all parts of the cell should be as small / large as possible.

Practice • The best combination of surface area and volume factors is one in which • the surface area is small / large • and the volume is small / large • the surface area to volume ratio: SA /V is small / large • Cells that conduct transport most efficiently will be (larger / smaller) cells.

Structure Leads to Function • Different cell types in the body have different functions – Ex: muscles cells and nerve cells have different functions Muscle Cells • All cell types are structured to maximize their surface area to volume ratio! Nerve Cell • This allows for efficient How do the structures of movement of materials. these cells maximize SA/V ratio?

The Process of Cell Division

Chromosome Recap • Packages of DNA and histones highly condensed. Prokaryotic Chromosomes • Prokaryotes have no nucleus • Usually prokaryotes have 1 circular chromosome in the cytoplasm. • Not all prokaryotes have histones

Eukaryotic Chromosomes • DNA binds to proteins called histones • DNA and histones condense to form chromatin

Eukaryotic Chromosomes (cont. ) • Chromatin is condensed completely to form chromosomes after the DNA has replicated and the cell is ready for division. • Chromosomes make it possible to separate DNA precisely during cell division.

Chromosomes can be seen in two forms: 1. Single-Arm: are composed of a single chromatid 2. Double- Arm (Replicated Form, Duplicated form): are made up of paired, genetically identical chromatids, called sister chromatids. • Sister chromatids are joined at the centromere. • Because the sister chromatids are formed during replication of DNA, they are identical right down to the nucleotide sequences!

The Cell Cycle • The cell cycle is the series of events that cells go through as they grow and divide • The events in the cell cycle are: – Growth – Preparation for division – Division into two daughter cells

Prokaryotic Cell Cycle • Also called binary fission (a form of asexual reproduction) • Cell grows, doubles its DNA, and splits in two, dividing the DNA and cytoplasm between the daughters.

Eukaryotic Cell Cycle- Cell division in body (somatic) cells • Consists of 4 phases: – – G 1 (Growth) S (DNA Replication) G 2 (Preparation for Cell Division) M (Cell Division)

Eukaryotic Cell Cycle • Only during M phase does division actually occur. • The other 3 phases are part of Interphase- the “in -between” period of growth

G 1 Phase: Cell Growth • Cells increase in size • Synthesize new proteins • Make new organelles G 1

S Phase: DNA Replication • New DNA is synthesized (chromatin replicated) • At the end of S Phase the cell will have twice the amount of DNA S

G 2 Phase: Preparing for Cell Division • Shortest of the phases in Interphase • Cell produces organelles and molecules necessary for division • Now the cell is finally ready to divide G 2

M Phase: Cell Division • The making of 2 daughter cells • Very quick compared to the lengthy interphase. • Consists of Mitosis (division of the nucleus) and Cytokinesis (division of the cytoplasm). M

Mitosis • Consists of 4 phases: – Prophase – Metaphase – Anaphase – Telophase – Remember PMAT

Prophase • Longest phase of mitosis • Duplicated chromatin condense to form the double armed chromosomes seen in the picture. • Both copies attach to each other at the centromere

Prophase (cont. ) • The spindle begins to form – System of microtubules that will help separate the duplicated chromosomes. – Spindle fibers extend from the centrosome regions where centrioles are located (no centrioles in plant cells) – Centrioles move to opposite ends (poles) of the cell

Prophase (cont. ) • At the end of prophase… – Chromosomes coil more tightly – The nucleolus disappears – The nuclear envelope breaks down

Metaphase • Shortest part of Mitosis • Centromeres (and thus chromosomes) line up across the center/middle of the cell • Spindle fibers connect each centromere to both poles of the spindle

Anaphase • Sister chromatids separate into single armed chromosomes and begin to move apart • Separated chromosomes move along spindle fibers to either pole of the cell • When the movement stops, anaphase is over

Telophase • The condensed chromosomes begin to spread out (back into chromatin) • A nuclear envelope reforms around both clusters of chromosomes • Spindle breaks apart • Nucleolus reforms in each daughter nucleus • Mitosis is now complete

Cytokinesis • The second portion of M phase • Splits the cytoplasm in half, and splits the cell in two • Usually occurs at the same time as telophase

Cytokinesis Differs in Animal and Plant Cells • Animal Cells- the cell membrane is drawn inward by microfilaments creating a cleavage furrow – The cytoplasm is pinched into 2 parts until the daughter cells separate • Plant Cells- a cell plate forms in between the daughter cells. – Cell membrane and cell wall form at the cell plate from vesicles containing membrane and cell wall materials

Cytokinesis Differs in Animal and Plant Cells Stages of Mitosis Animal Cell Plant Cell

After cytokinesis is complete… • The cell may enter G 1 again and continue the cycle • OR the cell may enter a phase called G 0 or resting phase. – During this time a cell is not preparing for division but rather is making protein and doing normal cell functions. – Different cells stay in G 0 for different amounts of time. (Nerve? ____ Skin? _______)

Compare plant and animal cell division: • Differences: – Centrioles in animal cells only – Cleavage furrow for animal cell cytokinesis and cell plate for plant cell cytokinesis • Similarities: – The rest!

Regulating the Cell Cycle

Cell Growth and Cell Division are Controlled Carefully in Multicellular Organisms • These controls can be turned on and off • Example: if you get a cut, new cells fill in the space only until the cut is healed • How do cells know when to divide?

Cyclins and Regulatory Proteins • Cyclins- A family of proteins that regulate the timing of the cell cycle in eukaryotes. – Act like policemen at different points in the cycle to prevent it from going forward if mistakes have occurred. – Work with a family of proteins called CDK

Other regulatory proteins: • Proteins inside and outside the cell that help regulate the cell cycle

Internal Regulatory Proteins • Proteins that respond to events inside the cell, letting the cell know whether or not to proceed with the cell cycle. • Examples: 1. An internal regularity protein keeps the cell from entering mitosis until the chromosomes have replicated. 2. A different protein prevents the cell from entering anaphase until the spindle fibers have attached to all chromosomes.

External Regulatory Proteins • • • Proteins that respond to events outside the cell Direct the cell to speed up or slow down the cell cycle Examples: 1. Growth Factors- tell the cell to grow and divide 2. Inhibitory signals- proteins on the surface of other cells; tell the cell to slow down or stop the cell cycle - This prevents excessive replication

Cells die and are replaced all the time • Two ways that cells end their life cycle 1. Accidental death by damage or injury 2. Programmed cell death called apoptosis. - helps shape structures during development - kills infected cells - kills cells with DNA damage or cancer - etc.

Cancer: Uncontrolled Cell Division • Cancer- a disorder in which body cells lose the ability to control division - Cancer cells do not respond to signals that regulate cell growth and thus, divide uncontrollably. Cancer is a disease of the cell cycle!

Tumors • A mass of cells • Cancerous tumors are called malignant- invade and destroy surrounding tissue • Not all tumors are cancerous – Some are benign – do not spread to surrounding healthy tissue or around the body

What Causes Cancer? • Defects/mutations in genes that regulate the cell cycle – Example: the protein from the gene p 53 normally halts the cell cycle until all chromosomes have replicated (an internal regulatory protein) – If this gene is defective, apoptosis will not occur, and the cell cycle will proceed continuously Normal Problem with replication occurs Cell division proceeds normally

What Causes Cancer? (cont. ) • The defective/mutated gene is not the same for all cancers – Some cancer cells do not respond to internal regulation, others do not respond to external regulation, etc.

Possible Reasons for Defective Cell Cycle Genes • • • Innate mutation (born with it) Smoking or chewing tobacco Radiation exposure Other defective genes Viral infections Etc.

Will a mutation in a somatic cell mean the organism’s offspring will also have the mutation? Or if you get skin cancer during your lifetime, will your children be born with the mutations for skin cancer?

Treatments for Cancer • Some tumors can be removed by surgery • Radiation is used to kill cancerous tumors • Chemotherapy- using chemicals (drugs) to kill cancer cells – These drugs target rapidly dividing cells – Will also kill normal, healthy cells that are dividing (Like? ) – Scientists seek to find a drug that kills cancer cells but not healthy cells