• One of the important life functions of livingthings is their ability to reproduce.• Reproduction depends on the cell.• Cells reproduce in order to make identicalcopies of themselves.• In order to understand humanreproduction, we must first look at how cellsreproduce
How Body Cells ReproduceCells reproduce through a continuous sequenceof growth and division called the cell cycleThere are two main phases:– Growth Phase– Division PhaseThe growth stage, also called interphase, is astage in which the cell produces newmolecules
Interphase• Three Parts of Interphase:1. G1 (gap 1)Phase2. S Phase• DNA made and copied3. G2 (gap 2)Phase
Division Phase• Two Processes involved in cell division1. Mitosis (pronouced “my-toe-sis”)• Division of the nucleus (including DNA)2. Cytokinesis• Division of the cytoplasm• Overall length of the cell cycle variesdepending on the species and where it isliving
Function of Mitosis• Cells which go through the process of mitosisdivide to produce two new cells• This allows an organism to:– Grow– Regenerate (repair) damaged tissues or body parts– Replace malfunctioning cells– Replace dead cells
Mitosis…• Occurs in an organism’s somatic (body)cells, not sex cells• New cells are identical copies of the parentcell• Maintains the number of chromosomes fromcell to cell (ie. 46 in humans)• Each parent cell divides producing two newdaughter cells that are genetically identical tothe parent cell
Prophase• In this phase, chromatin in the cell’snucleus forms thick, condensedstructures called chromosomes.• A chromosome is made up of two sisterchromatids which are held together by astructure called a centromere• Each sister chromatid contains anidentical copy of the genetic informationor DNA.• Other events which occur during thisstage are:1. The nuclear membrane and thenucleolus disappear.2. The centrioles migrate to oppositepoles of the cell.3. Spindle fibers start to form from eachcentriole.• The cell is now ready to enter the secondstage of mitosis called metaphase.
Metaphase• The spindle fibers attachto the centromere of thechromosomes and pullthe chromosomes to thecenter or equator of thecell.• Spindle fibers from eachcell pole then attach tothe sister chromatids.• The cell is now ready toenter the third stage ofmitosis called anaphase.
Anaphase• In this stage thecentromere splits apartand the sister chromatidsare pulled to oppositepoles of the cell by thespindle fibers.• The cell is now ready toenter the fourth stage ofmitosis called telophase.
Telophase• This is the last phase of mitosis.• The chromatids reach the oppositepoles of the cell.• The chromatids, now calledchromosomes, begin to unwind andform into chromatin again.• The spindle fibers break down anddisappear.• The nucleolus reappears.• A nuclear membrane forms aroundthe chromatin.• The cell is now ready to divide intotwo new cells, a process calledcytokinesis
Mutations Affecting Cell Division• A mutation is a permanentchange to the DNA which makesup a gene. This causes the geneto either function improperly ornot at all.• There are a number of causes ofmutation:– Chemical compounds– Radiation– Viruses– Mistakes during DNA replication• Once a mutation occurs in a cellany division of this cell will passthe mutation on to the newlyformed daughter cells.Most mutations in somatic cells arenot serious because the mutatedcell can easily be replaced by a newnormal cell.If a mutation occurs in a gene whichcontrols cell division the cell maydivide rapidly and uncontrollably.This will produce a mass of cellswhich create a tumor or cancer.Genes which cause cancer whenthey become mutated are calledoncogenes.
How Reproductive cells are ProducedThe Function of Meiosis (“my-oh-sis”)• Meiosis is a special type of cell division which occurs inreproductive organs.• Meiosis produces the sex cells which are called gametes.– In males the gametes are sperm cells– In females the gametes are the egg cells.• Gamete cells are called haploid cells.– This means that they contain only half the number ofchromosomes as a number body cell.– Haploid cells are represented by the letter n .• Somatic cells are called diploid cells.– This means they contain the normal number of chromosomes.– Diploid cells are represented by 2n.
• A normal body cell in humans contains 46chromosomes. A sperm or an egg will containonly 23 chromosomes.• Meiosis is referred to as reduction - division.– The first part of meiosis reduces the chromosomenumber from diploid to haploid so that thegametes which are produced will only contain 23chromosomes.– Of these 23, 22 are autosomes (bodychromosomes) and one is a sex chromosome (X orY). A female has two X chromosomes (XX) and amale has an X and a Y chromosome (XY).
Phases of Meiosis• The phases of meiosis are very similar to thephases of mitosis. However, meiosis involvestwo sequences of phases.• The sequence of phases for meiosis– Interphase– Prophase I, Metaphase I, Anaphase I, Telophase I– Prophase II, Metaphase II, Anaphase II, TelophaseII
Interphase• The chromosomesreplicate during thisphase.• Each chromosome ismade up of two sisterchromatids joined atthe center by acentromere.
Prophase I• Pairs of chromosomes grouptogether.– The pairs are calledhomologous chromosomes.• Two pairs group together andwe call this a tetrad.– A tetrad is a group of fourchromosomes.• Crossing over mayoccur, where chromosomesexchange genetic information
Metaphase I• The tetrads line upacross the equator ofthe cell.• Spindle fibers attachto the centromere ofeach pair ofchromosomes
Anaphase I• Pairs of homologouschromosomes arepulled to oppositepoles of the cell bythe spindle fibers.
Telophase I• Telophase I does not occur inall cells.• If telophase I does notoccur, the cell moves into thesecond part of meiosis whichwe call meiosis II.• If telophase does occur thefollowing events take place:1. Chromosomes uncoil to formchromatin.2. Spindle fibers disappear.3. The cytoplasm divides.4. The nuclear membrane formsaround each groupofchromosomes and two cellsare formed.
Meiosis II• The stages of meiosis IIare identical to mitosis• At the end of meiosisII, four cells are produced.These four daughter cellswill develop into gametesin animals and eithergametes or spores inplants.
Meiosis vs. Mitosis• The final productof mitosis is twodaughter cellswhich aregeneticallyidentical to theparent cell.• The final productof meiosis is fourdaughter cellswhich aregenetically uniqueor different fromthe parent cell.
Gamete formation• The purpose of meiosis is to produce gametesor sex cells• The production of gametes by meiosis is calledgametogenesis– gameto = sex cell– genesis = creation
• In males, the gamete sperm is produced and this iscalled spermatogenesis– spermato = sperm– genesis = creation• In females, the gamete egg is produced and this iscalled oogenesis– oo = eggs (or oocyte)– genesis = cr
Spermatogenesis vs. Oogenesis• In males, meiosis occurs in the male reproductive organ called thetestes.• The process starts with a diploid cell called the spermatogonim.• The spermatogonium goes through the stages of meiosis I andmeiosis II and the final product is four haploid sperm cells.• Each sperm contains two parts:– a head and tail.• Some species produce sperm continually (humans) while otherspecies only produce sperm during breeding season (migratorybirds).
• In females, meiosis occurs in the female reproductiveorgans called the ovaries.• The process starts with a diploid cell called theoogonium.• The oogonium goes through the stages of meiosis I andmeiosis II and the final product is four haploid cells, but only one cell survives. We call this cell the ovumor egg.• The production of eggs or ova in human femalescontinues from the start of puberty until menopausewhich will occur between the ages of 40 and 50.• There are a number of differences between the spermand eggs which are produced by meiosis.
GametogenesisGametogenesis is abiological process bywhich diploid or haploidprecursor cells undergocell division anddifferentiation to formmature haploid gametes.Depending on thebiological life cycle of theorganism, gametogenesisoccurs by meioticdivision of diploidgametocytes into variousgametes, or by mitoticdivision of haploidgametogenous cells.
Binary fission is a form of asexual reproductionwhich is used by all prokaryotic organisms, andsome eukaryotic organisms like fungi as well. Inaddition to being used to duplicate wholeorganisms, binary fission is also utilized withinthe cells of eukaryotic organisms by some of theorganelles. In this process, two daughter cells areproduced by a single parent cell which effectivelyclones itself. The Escherichia coli bacterium hasbeen used extensively by researchers studyingbinary fission, as it provides a classic example ofthis asexual reproduction method in action, andit is very abundant. In binary fission, the cellstarts by duplicating its DNA to create twocomplete sets, and then growing to a size muchlarger than it usually is. As the cell grows, thesets of DNA move to opposite ends of the cell.Once the cell has achieved the right size, it splitsin two, creating two daughter cells with identicalDNA. Binary fission is classically used when anorganism is living in a stable environment.