2 Introduction: DNA, chromatin, chromosomes Prokaryotes: circular DNA (no histones)(Fig: 9-4, book)Eukaryotes: linear DNA + histones = chromatinChromosome is the condensed form of chromatinChromosomes are visible during cell division („transport” form of the genetic material)Chromosomes are not visible in non-dividing cells (but present !!!!, in extended form: chromatin)Role in chromosome packaging histon proteins and scaffolding proteins (help maintain chromosome structure)
5 Chromosomes Number of chromosomes is different among species In humans 46 chromosomes (23 paternal, 23 maternal)This is the human KARYOTYPE (chromosomal constitution)Structure of a duplicated chromosome (after replication)2 sister chromatids !with the same genetic material !associated at the regions of centromeres !kinetochores are attached to each centromere (they are binding sites for microtubules)before replication only one chromatid !!!!
6 The cell cyclefrom beginning of one division to beginning of next divisioninterphase and M phase (division)most of the cell’s life is spent in interphase (no division)IN INTERPHASE:G1 phase (first gap phase) normal metabolism, growthS phase (synthesis) DNA replicationG2 phase (second gap phase) preparation for divisionIN M PHASE:Mitosis nuclear division (two nuclei produced)Cytokinesis division of the cell cytoplasm (two daughter cells are produced)
7 Regulation of the cell cycle Is it possible to leave the cycle ? Yes, G0 phase: restingIs it controlled ? Yes, very strictly3 important checkpointscheckpoint 1First one in the G1 phase Is the DNA intact (OK) ? If not, first to correct it !G0 phaseSecond checkpoint in G2 phase Is all DNA replicated ?Third checkpoint in M phase Are all chromosomes attached to the spindle ?(Molecules called cyclins and cyclin dependent kinases are important regulators of the cell cycle)checkpoint 3checkpoint 2
8 Interphase: S phase In synthesis (S) phase DNA replication takes place Chromosomes are not visible in interphase (present in form of extended chromatin)centrioles are also duplicated !!centrioles (9 x 3) form the centrosome, and they are the MTOCs (microtubule organizing centers)
9 Mitosis: Introduction is the division of body cells, 2 genetically identical daughter cells are produced (with same number of chromosomes !!!)46in humans:4646before replication chromosomes composed of one chromatid !!!!after replication chromosomes composed of two chromatids !!!!1 chromatid = 1 DNA moleculeG1SG2MNumber of chromosomes46464646Number of chromatids46929292Number of DNA molecules46929292Number of centromeres46464646
10 Mitosis: Prophase prophase is the first stage chromosome condensation starts (packaging), condensin, ATP is neededchromosomes become visible (in microscope)nucleolus and nuclear envelope disappearcentrioles move to the poles of the cellmitotic spindle formed between centriolesMicrotubules (spindle fibers)kinetochores begin attaching to microtubulesKinetochores (proteins)
11 Mitosis: Metaphase metaphase is the second stage chromosomes line up along the cell’s midplanechromosomes attached to the spindle fibersThis is the best stage for chromosome analysis, most visible, organized,microtubules don’t touch the centrioles, they end in the pericentriolar materialMicrotubules (spindle fibers)CentriolesKinetochores (proteins)Pericentriolar materialDetails on the next slide
13 Mitosis: Metaphasechromosomes lined up along the cell’s midplane (or metaphase plate)centrioles: 9x3 structure of microtubulespericentriolar material: protein fiberspericentriolar material + centrioles form the centrosomemitotic spindle: centrosomes and further microtubulesasters (astral microtubules): positioning of the cell polekinetochore microtubules (or spindle fibers): separation of chromatidspolar microtubules: push the poles apartKinetochores (proteins)Kinetochore microtubulesCentriolesPericentriolar materialPolar microtubulesAstersmidplane
14 Mitosis: Anaphase anaphase is the third stage sister chromatids are separated at centromereschromosomes move toward the opposite polesthey use spindle fibers as trackspolar microtubules push the poles apartanaphase ends when chromosomes have arrived to the poles
16 Mitosis: Telophase telophase is the fourth stage chromosomes arrived to the polestwo new nuclei are formednew nuclear envelopes, nucleoli are reformedchromosomes start to decondense (uncoil)the spindle disappearsCYTOKINESIS usually starts during the telophase and produces 2 daughter cells this is the actual division of the cytoplasm two genetically identical daughter cells organelles are distributed more or less equally between the two daughter cells (and mitochondria ?)Cleavage furrow
19 Sexual and asexual reproduction There are so called somatic (body) cells in our organism, they contain chromosomes (23 pairs)They are diploid (2n)They contain 2 sets of chromosomes (1 set = 23 chromosomes (in humans)) (maternal and paternal)There are some cells in our reproductive organs, called gametes, they contain 23 chromosomesThey are haploid (n)They contain 1 set of chromosomes (either maternal and or paternal)Gametes (cells needed for sexual reproduction) are sperms and egg cells„n” refers to haploid chromosome number, n=23 in humansgametes fuse to form a zygote >>> This is the basis of sexual reproductionIn asexual reproduction >>> there are no gametes, basis is mitosisSomatic cells are generated by mitosis (2n>2n)Gametes are generated by meiosis (2n>n)
20 Homologous pairs Each chromosome has its pair !! Maternal chromosomesPaternal chromosomesEach chromosome has its pair !!There are 23 homologous pairs (similar size, shape and genetic material)
21 MeiosisDuring meiosis, gametes (haploid cells) are producedMeiosis reduces the chromosome number by half („make smaller”)A single diploid cell (2n) produces four haploid cells (n)23232346232323During meiosis two cell divisions takes place, but the DNA is duplicated only once, that is why is the chromosome number reduced after meiosisPhases of meiosis:Meiosis I. (prophase I, metaphase I, anaphase I, telophase I.)Meiosis II. (prophase II, metaphase II, anaphase II, telophase II.)(between meiosis I and meiosis II, there is a short phase called interkinesis (similar to the interphase), but there in no DNA duplication !!!!!!!
22 Meiosis I: Prophase I Meiosis I (or first meiotic division) Prophase I is a very specific phase of the whole meiotic division >>>> homologous chromosomes are physically attached (synapsis)The process is called synapsis, the structure is the synaptonemal complexSister chromatidsSister chromatidsThe place where the crossing over occurs is called chiasma (ta)In prophase I of meiosis there is an exchange of genetic material between homologous chromosomes, so new combinations of parental genes are created !!!! This is the process of genetic recombinationHomologous pair (tetrade, bivalent)
23 Meiosis I: Metaphase IThe homologous pairs are aligned along the midplaneIn mitosis there is no connection between the members of the homologous pairs
24 Meiosis I: Anaphase IThe homologous pairs are separated (not sister chromatids !!!!!) and are distributed to the two polesIn mitosis the sister chromatids were separatedIn TELOPHASE I two haploid cells with duplicated chromosomes are produced
25 Meiosis II: The same phases like in meiosis I (P, M, A, T) In meiosis II the sister chromatids are separated (like in mitosis)Diploid cell (2n), duplicated chromosomes DNA content: 4C (after replication)Meiosis I.Haploid cells (n), duplicated chromosomes DNA content: 2CMeiosis II.Haploid cells (n), chromosomes with one chromatid DNA content: CFrom 1 diploid cell, 4 genetically different haploid daughter cells are produced !!
26 MEIOSIS versus MITOSIS 2n 4C2n 4CMeiosis I.2n 2Cn 2CMitosisMeiosis II.n C
27 MEIOSIS versus MITOSIS Two genetically identical daughter cells, diploid (2n)one division, one replication > chromosome number the samechromosomes arranged in a line along the midplane of the cellno crossing over in prophasesister chromatids separate during anaphasedivision of somatic (body) cellsMEIOSIS4 genetically different daughter cells, haploid (n)2 consecutive divisions, only 1 replication > chromosome number reducedhomologous pairs attached to each othercrossing over occurs in prophase I.members of the homologous pairs separate in anaphase Iduring meiosis, gametes are createdsister chromatids separate in anaphase II.