Chapter 4 Cell Reproduction• Which of these pictures shows an organism with cell division taking place?
I. Introduction - Cell Division: • Occurs more often in young, growing organisms or when damaged tissue is repaired. • Some tissues like skin & intestinal cell grow throughout life while some- like nerve cells, do not divide again after full size achieved. • Genetic information must be divided between cells. • How does this happen?
Cell Division & Reproduction -allows for growth & reproduction of individuals -passes on genetic information to future generations.
In this chapter:• Chromosome Structure – Tightly coiled DNA molecules• Cell Division- – Mitosis (nuclear division) – Cytokinesis (division of cytoplasm)• Cell Reproduction- – Meiosis- Process reduces # of chromosomes to half of original cell to produce gametes.
II. Chromosome Structure• Chromosomes are rod-shaped structures made of DNA and protein.
Chromosome Structure• When a cell is NOT When eukaryotic cells dividing, the DNA are ready to divide, is loosely coiled in the DNA coils tightly the cell NUCLEUS around proteins called & is called HISTONES & forms CHROMATIN. CHROMOSOMES.
Chromosomes• Consist of 2 identical halves• Each half is called a CHROMATID.• Are attached to each other at the CENTROMERE. http://www.micro.utexas.edu/courses/levin/bio304/genetics/chromosome.gif
How many Chromosomes in an Organism?• Look at this table- Does the # of chromosomes show how complex the organism is?..........NO!
B. Chromosome Numbers In various species:• Adder’s Tongue Fern 1,262• Carrot 18• Cat 32• Chimpanzee 48• Human 46• Dog 78• Potato 48• Earthworm 36• Fruit fly 8• Garden pea 20• Lettuce 18• Can different organisms can have same # chromosomes? …….. YES.• Can any conclusion about relatedness be made because of this? NO.
Sex chromosomes or Autosomes.• Sex Chromosomes (X, Y) - determine sex & carry genes for other characteristics.• Autosomes- all of the other chromosomes in an organism.• Example: Humans have 2 sex chromosomes & remaining 44 are autosomes. Humans have 46 chromosomes!
Sex chromosomes:• Females are XX• Males are XY• Males determine the sex of the baby- the sperm may contain either an X or a Y; eggs can only have Xs.
Homologous Chromosomes• Most organisms have 2 copies of each chromosome.• These pairs are called homologous chromosomes or “homologues”. geneticsmodules.duhs.duke.edu/Design/page.asp...
2 copies of each gene • On each homologue- there are a genes for the same traits• For Example – if one of the homologous chromosomes has a gene for eye color, so will the other. http://course1.winona.edu/sberg/ILLUST/homolog1.gif
Homologous chromosomes•The 2 chromatids•are exact copies of eachother- they’re identical.• attached by a centromere.•TheHomologouschromosomes•ARE NOT exact copies.(One set is from mom, one from dad) www.emunix.emich.edu•They have genes for the same The large red & blue chromosomes are homologous.traits The small purple & green chromosomes are homologous.
Homologous chromosomes in humans www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=g...
Karyotype• A picture (photomicrograph) of chromosomes.• Notice that humans have 46 chromosomes – 22 homologous pairs of autosomes – Plus 2 sex chromosomes. (Is this person male or female?) Karyotype Copied from: http://homepages.uel.ac.uk/V.K.Sieber/human.htm
Diploid & Haploid cells• Diploid- cells having 2 sets of chromosomes.• Haploid- have only 1 set of chromosomes These are Sperm cells & Egg cells.**What is the number of chromosomes in a human haploid cell? (23)
• Diploid is usually abbreviated 2n( where “n” is the number of chromosomes in that organism)(in humans, the diploid- or 2n number is 46• Haploid is 1n.• When an egg cell (1n) combines with a sperm cell (1n), the new cell will be diploid (2n)**What would happen if haploid cells were not used in sexual reproduction?
III. Cell DivisionA.Prokaryotic Cell Division• Prokaryotes are bacteria that lack nuclei & membrane bound organelles.• Have a single circular stand of DNA – molecule is not coiled & do not form chromosomes.• Division is much simpler than in eukaryotic cells• Binary fission - how prokaryotic cells divide.
Binary fission:- Is the division of a prokaryotic cell into 2 identical offspring cells. (creates clones)
Cell Reproduction: Mitosis & Meiosis B. 2 kinds of cell division in eukaryotic cells: • Mitosis occurs in cells for growth, development, repair or asexual reproduction. • Meiosis occurs during formation of gametes for sexual reproductionwww.ccs.k12.in.us/.../Humanembryology.htm
2. The Cell Cycle• The cell cycle is the repeating set of events in the life of a cell. 2 main parts; further divided:• INTERPHASE – Time between cell divisions – G1 phase – S phase – G2 phase• CELL DIVISION- When chromosomes & cytoplasm are equally divided. – Mitosis - nuclear division – Cytokinesis - cytoplasm division
3 Stages of Interphase• G1 phase- cell grows to mature size. – “G1” stands for the time gap – after cell division & before DNA replication.• S phase- cell’s DNA is copied (synthesized). – (“S” stands for synthesize, meaning to “make”)• G2 phase- the cell prepares for cell division. – “G2” is the time gap between DNA synthesis & cell division.Some cells Exit the cell cycle:• Go phase- state where cells do not copy DNA or prepare for cell division. (example- CNS cells)
C. Mitosis- is division of the nucleus- results in: - two offspring cells - Diploid # chromosomes - genetically identical (clones) of the original cell.
Stages of MitosisDivided into 4 phases:1. PROPHASE2. METAPHASE3. ANAPHASE4. TELOPHASE• Memory aid: Pro met Anna on the telophone
What happens during each phase of mitosis?1. PROPHASE- of DNA into chromosomes. - coiling -The nucleus starts to disappear. -Centrosomes appear (In animal cells -centrioles appear also) & move to opposite poles of the cell. -Spindle fibers, made of microtubules, radiate from the centrosomes & forms the mitotic spindle & will be used to equally divide the chromosomes in the next phase.
1. Prophase• Can you list some things that happen in prophase?• Find the structures in the drawing: – Nuclear membrane – Homologous Chromsomes – Chromatids – Centromere – Centrosomes – Spindle fibers; (look in Drawings from: www.tqnyc.org book for 2 types of spindle fibers.)
2. METAPHASE• chromosomes are moved to the center of the cell• Note how they line up.• Chromosomes are easiest to ID under a microscope during metaphase -most karyotypes are made from cells in metaphase.
3. ANAPHASE- the chromatids separate at the centromere & move to opposite poles. - after they separate, each chromatid is now considered to be a separate chromosome.(Tricky question- what “n” is a cell in anaphase? 4 )
4. TELOPHASEAfter the chromosomes reach the opposite ends of the cell:• the spindle fibers disassemble,• the chromosomes return to a less tightly coiled state,• the nuclear envelope reforms in each newly forming cell.
D. Cytokinesis• Division of the cytoplasm:• In animal cells- a cleavage furrow forms as the area of the cell membrane pinches the cell into 2.• In plant cells- a cell plate forms a new cell wall as vesicles from the golgi apparatus join together at the midline of the 2 dividing cells.
E. Control of Cell Division• A cell spends most of its time in interphase.• What triggers a cell to leave interphase & divide?• feedback from Checkpoints: – 1. Cell growth checkpoint (G1)- if the cell is healthy & grows to mature size, protiens will intiate DNA synthesis. – 2. DNA synthesis checkpoint (G2) proteins check results of DNA replication , signal OK – 3. Mitosis checkpoint, if ok- growth cycle starts.
Cell Division Errors• If mutations occur• proteins may not function properly• growth may not be controlledWhen control is lost = cancer.
IV. Meiosis• How it is different than mitosis: – reduces the number of chromosomes in new cells to half the number in the original cell.• Produces four haploid cells (gametes)• Different than two diploid cells as in mitosis.
Beginning meiosis:• Cells begin meiosis just like they begin mitosis, after interphase.• Cells must divide twice to reduce the number of chromosomes & so we have Meiosis I & Meiosis II.• Remember- the whole point of meiosis is to make gametes (which are 1N)
A. Meiosis- 8 phases• MEIOSIS I 1. PROPHASE I 2. METAPHASE I 3. ANAPHASE I 4. TELOPHASE I• MEIOSIS II 5. PROPHASE II 6. METAPHASE II 7. ANAPHASE II 8. TELOPHASE II
MEIOSIS I -what happens in each phase?1. PROPHASE I -Spindle fibers appear & the nuclear membrane disappears as in mitosis.NOTE IN MEIOSIS I -HOMOLOGUES LINE UP NEXT TO EACH OTHER. -This pairing of homologous chromosomes, which does NOT occur in mitosis, is a synapsis. -Each pair of homologous chromosomes is a tetrad.
Genetic RecombinationDuring synapsis, chromatids within a pair of homologues may twist around each other.Parts of the chromatids may break off & attach on the homologous chromosome.This is called crossing-over.Crossing over permits exchange of genetic materials= genetic recombination. •http://regentsprep.org/Regents/biology/units/evolution/crossovr.gif
Important to understand:• Crossing-over, – which is when portions of homologous chromosomes line up to form tetrads in synapsis. – exchange genetic material, – occurs during prophase I – results in genetic recombination.
2. METAPHASE I• Pairs of homologous chromosomes (tetrads) line up on metaphase plate.3. ANAPHASE I• Tetrads split- reducing the cells to haploid• The chromosomes have randomly separated- this is called Independent Assortment.4. TELOPHASE I• 2 new cells begin to split off. Cytokinesis occurs.
MEIOSIS IIMeiosis II occurs in both cells made in meiosis I5. PROPHASE II- Chromosomes condense, nuclear membrane breaks down, spindle fibers form. NOT preceded by replication or recombination.6. METAPHASE II- Chromosomes line up on metaphase plate7. ANAPHASE II- The chromosomes divide at the centromeres8. TELOPHASE II- Chromosomes begin to uncoil, nuclear envelope reforms, followed by cytokinesis.
The biggest differences between mitosis & meiosis occur at:*prophase I (Meiosis I has synapsis, tetrads & crossing-over. Mitosis does not.)*metaphase I (Meiosis I has tetrads lining up on metaphase plate.)*anaphase I (Meiosis –the centromeres do not separate & haploid number chromosomes results.
B. What are Gametes?Gametes are 1N cells for sexual reproductionMeiosis -occurs to form haploid gametes for sexual reproduction.- Spermatogenesis (production of sperm) -occurs in humans in testes -forms 4 haploid spermatids (develops 4 sperm cells)- Oogenesis (produces 1 egg cell, 3 polar bodies) - All of the cytoplasm goes to 1 egg cell - 3 useless polar bodies are formed from the other nuclei & they eventually disintegrate.
Sexual reproduction• is the formation of offspring through meiosis and the union of a sperm and an egg.• Offspring produced by sexual reproduction are genetically different from the parents
Questions1. Which of the following statements about prokaryotic chromosomes is true? A. Prokaryotes have at least two chromosomes. B. Prokaryotic chromosomes consist of a circular DNA molecule. C. Prokaryotic chromosomes include histone and nonhistone proteins. D. Prokaryotic chromosomes are made of DNA wrapped tightly around histone proteins.
2. Crossing-over occurs during which process? A. mitosis B. meiosis I C. meiosis II D. interphase
3. Which type of cell division is shown in the diagram? A. mitosis B. meiosis C. binary fission D. sexual reproduction
• 4. What are the differences between mitosis & meiosis?• 5. Explains the difference between cytokinesis in plant & animal cells.
Your body contains over 200 different cell types!• Cells are specialized into different types.• The adult human body is made up of about 60-90 trillion cells. Thats a lot of cells!!!• If you lined up all the cells in a human body end-to-end, you could actually circle the earth 41/2 times!
Stem cells• Tissue precursor cells that have the ability to self-renew and differentiate into more specific cell types.• They are important because they can replace dying, old or damaged cells.• These cells are found in human embryos, fetuses, children and adults, i.e. at all stages of development and in most tissues but it is the embryonic cells which have raised the most controversy.
• Early human embryos (5 - 6 day old blastocysts) have an outer cell layer from which the placenta develops, and an inner cell mass, in the region of 200 cells, which gives rise to the fetus. This inner cell mass is the source of embryonic stem cells.
Growing stem cells is making clones of those cells.• Mixed reactions to the prospect of cloning for biomedical research.• Supported by some for its medical promise• Opposed by others who view it as intentional exploitation and destruction of human life created specifically for research purposes.• Bioethical problems-
A mere speck nestled in the eye of a needle, a five-day-old embryo(photographed using an electron microscope) contains controversialstem cells. Photograph by Yorgos Nikas, M.D. Stem cell research has been living up to its reputation for being fast-paced. In the few weeks since the July National Geographic cover story went to press,several important advances were reported —along with some significant political milestones. >> See Story from National Geographic
Stem cell research: the US politics• 2006- The Senate opens debate on bill that matches H.R. 810, which would ease Bushs restrictions. Matching bill passed the House May 24, 2005. Senate also opens debate on two other stem cell bills. One would encourage research into creating stem cell lines without destroying human embryos and the other would ban the creation of a fetus solely for the purpose of destroying it and harvesting its body parts. President Bush says despite divided GOP, he will not ease policy and if passed will likely veto. Post Story• 2006- In the first veto of his presidency, Bush vetoes the stem cell bill at a White House ceremony where he was joined by children produced by what he called "adopted" embryos. Post Story
Embryonic stem cell advance Updated: 7:34 p.m. ET June 6, 2007 http://www.msnbc.msn.com/id/19067616/• Reprogrammed mouse cells avoid controversial destruction of embryos• NEW YORK - In a leap forward for stem cell research, three independent teams of scientists reported Wednesday that they have produced the equivalent of embryonic stem cells in mice using skin cells without the controversial destruction of embryos.• If the same could be done with human skin cells — a big if — the procedure could lead to breakthrough medical treatments without the contentious ethical and political debates surrounding the use of embryos.• Experts were impressed by the achievement.
Stay Tuned-• Remember- science is a DYNANIC field- it keeps changing daily.