Chapter 8 Cell Reproduction

8-1 Chromosomes Recall: DNA: long, thin molecule that stores genetic information

Chromosomes DNA during cell division DNA coiled into very compact rod-like structures with proteins Can be seen if you stain a cell as it is undergoing cell division

Histones Chromosomes = DNA + proteins Eukaryotic DNA wrapped tightly around histones – Help maintain shape of chromosome – Aid in tight packaging

Nonhistones Do not help in packing DNA Involved in controlling activity of specific regions of DNA

Chromatids Chromosome: two identical halves Each half is a chromatid Form as DNA makes copy of itself before division When cell divides each of two new cells will receive one chromatid from each chromosome

Centromere Construction area of each chromatid Holds the two chromatids together until they separate in cell division

When the cell is not dividing… Chromatin:- less tightly coiled DNA DNA regions are uncoiled so that they can be read Why read? – Information used to direct cells activities

Prokaryotes Much simpler Usually just one chromosome attached to inside of cell membrane Chromosome = One Circular DNA molecule + proteins

Chromosome Number Each species has characteristic chromosome number Some species have same number of chromosomes Ex: – Potatoes, plums, and chimpanzees all have 48 chromosomes

Sex Chromosomes Chromosome that determines sex of an organism May carry genes for other characteristics X or Y – Female: XX – Male: XY

Autosomes All other chromosomes Every cell of an organism produced from sexual reproduction has two copies of each autosomes in each cell – One from mother – One from father

Homologous Chromosome aka Homologues: Two copies of autosomes Same size and shape Carry genes for same traits – Two genes for eye color; Two genes for hair texture – Two for every trait that makes you, YOU!

Karyotype Photomicrograph of chromosomes in a dividing cell Shows all chromosomes 46 chromosomes of humans look like 22 homologous pairs PLUS two sex chromosomes

Diploid Cells having two sets of chromosomes Have both chromosomes of each homologous pair ALSO have two sex chromosomes ALL human cells EXCEPT sex cells (sperm, egg) 2n Human 2n=46

Haploid Cells that contain only one set of chromosomes Half number of chromosomes as diploid cells 1n Ex: sperm, egg

Let’s talk numbers… Haploid + Haploid = Diploid 1n + 1n = 2n Egg + Sperm = New organism If reproductive cells were diploid, new cell would have too many chromosomes and not be functional

8-2: Cell Division

Quick Look: “All cells come from preexisting cells” Cell Division: process by which cells produce offspring cells Different in prokaryotes and eukaryotes

Prokaryotes Binary fission: division of prokaryotic cell into two offspring cells Three stages

Stages 1.Chromosome (attached in inside of cell membrane) copies itself 2.Cell continues to grow until it is 2x the normal size 3.Cell wall forms between two chromosomes and cell splits RESULT: TWO cells with identical chromosomes

Eukaryotes Both cytoplasm and nucleus divide More complicated Types: – Mitosis – Meiosis

Mitosis Division of nucleus New cells with genetic material identical to original cell Occurs in reproduction of unicellular organisms and in addition of cell or tissue in multicellular organism

Meiosis Reduces chromosome number by half New cells join together later in organism’s life cycle to produce cells with complete set of chromosomes

The Cell Cycle Repeating set of events that make up life of a cell 1.Cell division: 1.M Phase or Mitosis: nucleus divides 2.Cytokinesis: cytoplasm divides 2.Interphase: time between cell divisions

Interphase Most of cells life here! After cell division, offspring cells about ½ usual cell size Phases – G 1 – S – G 2 – G 0 *ONLY when cell is exiting cycle*

G 1 Phase Offspring grows to mature size “G 1 ” stands for time gap following cell division and before DNA replication

S Phase After cell has reached mature size DNA is copied or synthesized

G 2 Phase Time when cell prepares for cell division “G2” stands for time gap following DNA synthesis and before cell division

G 0 Phase ONLY when cell is exiting cycle Usually from G 1 Cells do not copy DNA or prepare for cell division Many cell in human body are in G 0 phase Ex: fully developed cells in central nervous system

Mitosis Stages Continuous process Can see stages under light microscope REMEMBER: PMAT 1.Prophase 2.Metaphase 3.Anaphase 4.Telophase

Prophase Begins with shortening and tight coiling of DNA – DNA  Chromosomes Nucleolus and nuclear membrane break down and disappear Centrosomes: Two pairs of dark spots appear next to nucleus

More prophase… Centrioles: cylindrical bodies that exist as pairs in centrosomes Centrosomes move towards opposite poles of cell Spindle fibers made of microtubules stretch from centrosomes to prepare for mitosis

Spindle Fibers Mitotic spindle: array of spindle fibers FUNCTION: equally divide chromatids between two offspring cells during mitosis Types: – Kinetochore fibers: extend from kinetochore (disk shaped protein of centromere) from each chromatid to centrosome – Polar fibers: extend across dividing cell from centrosome to centrosome

Metaphase Kinetochore fibers move chromosomes to center of dividing cell Once in center, each chromosome held in place by kinetochore fiber Chromosomes seen most easily NOW! Karyotypes usually made from cells during metaphase

Anaphase Chromatids of each chromosome separate at centromere Then move to opposite poles of cell When chromatids separate they are considered to be individual chromosomes

Telophase After chromosomes reach opposite sides of dividing cell, spindle fibers disassemble Chromosomes  Chromatin Nuclear envelope surround each set of chromosomes Nucleolus forms in new cells

Cytokinesis Splitting of cytoplasm during telophase Different in plants and animals

Animal cytokinesis Area of cell membrane pinches and separates cell into two cells Cleavage furrow: area that pinches cell into two cells through action of microfilaments

Plant cytokinesis Vesicles from Golgi body fuse at midline of dividing cell forming – cell plate: membrane bound cell wall Then, cell plate separates cell into two cells

8-3: Meiosis

Quick Look: “Process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell” 2n  1n Produces gametes

Gametes Haploid reproductive cells Human: egg and sperm cells Fusion of gametes  zygote Human: = 46

Meiosis Prepare: G 1, S, G 2 Stages of interphase SO! DNA is replicated SO! Cells must divide TWICE to become haploid 2n  4n  2n  1n First stages: Meiosis I Second Stages: Meiosis II

Prophase I DNA coils into chromosomes Spindle fibers appear Nucleus and nucleolus dissemble Chromosomes line up next to homologues Synapsis: pairing of homologous chromosomes

Tetrads Each pair of homologous chromosomes Chromosomes aligned lengthwise so genes on one chromosome are adjacent to genes on corresponding chromosome

Crossing-over During synapsis, chromosomes twist around one another Portions may break off and attach to adjacent chromosome Permits exchange of genetic material between maternal and paternal chromosomes Result: genetic recombination- new mixture of genetic material produced

Metaphase I Tetrads line up randomly along midline of dividing cell Spindle fiber from each pole attach to one homologous chromosome each

Anaphase I Each homologous chromosome moves to an opposite pole of dividing cell Independent assortment: random separation of homologous chromosomes RESULT: maternal and paternal chromosomes separated; Genetic recombination

Telophase I Final phase of meiosis I Chromosomes reach opposite ends and cytokinesis begins New cells will contain diploid numbers 1 cell  2 cells One 4n  Two 2n

Meiosis II VERY similar to meiosis I EXCEPT: NO DNA synthesis before Some species start meiosis II after nuclear membrane re-forms Other species begin meiosis II directly after meiosis I

Prophase II Spindle fibers form and begin to move chromosomes toward midline of dividing cell

Metaphase II Chromosomes move to midline of dividing cell facing opposite poles of dividing cell

Anaphase II Chromatids separate and move toward opposite poles of cell

Telophase II Nuclear membrane forms around chromosomes in EACH of FOUR new cells

Cytokinesis II Occurs during telophase II FOUR new cells Each of FOUR new cells contain half of the original cell’s number of chromosomes

Formation of Gametes In animals: produces gametes Meiosis occurs only within reproductive organs Humans: testes and ovaries

Male Gamete Formation Spermatozoa: sperm cells Diploid reproductive cell divides meiotically to form four haploid cells called spermatids Spermatid develops into mature sperm cells Spermatogenesis: production of sperm cells

Oogenesis Production of mature egg cells Diploid reproductive cell divides meiotically to produce ONE ovum (mature egg cell) During cytokinesis I and cytokinesis II of oogensis cytoplasm of cell is unequally divided RESULT: One ovum and THREE polar bodies

Asexual Reproduction Production of offspring from one parent NO meiosis or union of gametes Usually binary fission or mitosis RESULT: offspring genetically identical to parent

Sexual Reproduction Production of offspring through meiosis and union of sperm and egg RESULT: offspring genetically different from parents All offspring contain unique combination of parental genes

Sexual Reproduction and Evolution Sexual reproduction enables species to rapidly adapt to new conditions Lots of genetic combinations = Lots of different kinds of organisms Lots of different kinds of organisms = lots of possible survivors to specific evolutionary change in environment

No genetic variation = ALL the same If all the same is not good for evolution, all die