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CHAPTER 13 Notes DNA, CHROMOSOMES, AND GENES (REVIEW ) As YOU KNOW…genetic material in eukaryotic cells are chrosomoes Chrosomes are linear strands of DNA in unrepilcated state in G1 each chrosome is a single molecule of DNA, after S phase in g2 and M phase, each chrosome have 2 moelcules of identical dna called sister chromatids…. Each CHROMSOME HAS 2 identical sister chromatids If we look at the entire collection of DNA of the cell, it’s called the cell’s genome…. A cell that is in G2 has a genome that is twice as big as a cell that is in G1 because the chromosomes in G2 are in their replicated state,( 2 molecules of DAN per chromsome ) On each chrosome there are regions that are going to be used to make an RNA… Functional UNITS on chrosomes are called genes. Genes-contain information to make a protein and in some cases make functional RNA (rna that doesn’t go on and contribute to production of protein) EUKARYOTIC CELLS HAVE MUTLIPLE Within the genome of an individual, there are 2 versions of each chrosome type…. There are two diff versions of chrosome 1 , chrosome 2, etc…. You got one version of each chromosome from each of your parents… The two different versions of the same chrosomeome type are called homologous chrosomes or Homologs… Look AT THE TOTAL number of chrosomomes in human cells, (46) there are 23 PAIRS/homologs pairs of chrosomes in human cells. Or there are 23 different types of chorsomes in human cells, and most of the tpe there’s two copies/two versions of each chrosome so the total number is 46. When counting homologs, we don’t care whether the chrosomes are in their replicated or unreplicated state, it has nothing to do with homologus pairs. Each chrosomome contains two molecules of DNA, two sister chromatids, we still consider it one homologus pair.. Ploidy(n) descrives the number of chrosomes of a particular chromosome type (how many copies/versions of each chrosome type ar ein a cell) Most of our cells are 2n…a cell that is 2n is dipoid Gametes are haploids, they can only contain one chromosome of each chromosome type so that when two come together you restore diplid state

HOMOLOGOUS CHROMOSOMES Homologous pair, in their unreplicated state, what you see is they are the same size, shape, and same gene in the same order…but are not identical… -if you compare the same gene on each homolog, you find they aren’t the same version of that gene, they are slightly different it’s called ALLELE Allelee- different version of a particular type of gene HAPLOID NUMBER/GRAPH If YOU isolate cells in G2 or early M, and break them open, and look at their chromosomes, it’s called the karyotype Karyotpe-number and types of chrosomes If you do this you analyse karyotype you can see how many homolous pairs there are, and because you do it in early M, each chromsostme will be in the replicated state, you do it in M phase because its when they are condensed and easier to see. YoU SEE that there are two versions of each chromosome so they are diploid 2n, then you count the total number of chromosome in each cell, (12) so you say that the karyotype of this cell is 2n=12 It’s in diplod state, and in the diploid state there’s a total of 12 choromsomes.

TABLE 13.1 Sometimes when compare the karyoptypes of individuals of different sezes, you find that theyr karyptoes don’t match They have the same number of chorsmomes (diploid number) but if you look at the chromsomes they are different. It shows that many specisies there is one homologs pair of choromsomes that varies depending on the sex….EX: MAMALS X AND Y chrosmome. Those are homologous pair…they are more dissimilar then they are similar WHY MEIOSIS? Mitosis makes cells that are genetically identical to the original cell If you take a diploid cell and divide it by mitosis you’ll get another diploid cell… It’ll have too many chromosomes 2 from mom 2 from dad…not compatible… Polyploid-more than two copies of each chromosome type Generally don’t make it far its too much DNA To maintain a concstant chromosome number in generations, each parent must give half of their total chromosomes that’s what meiosis does

MIEOSIS MITOSIS PICTURE… So in Mitosis-you start with diplod cell, and end up with 2 diplod daughter cells In Meiosis-two rounds of cell division back to back without DNA replication in between, Because of that you go from single diplod to 4 haploid cells.CUT GENEITC INFO IN HALF Because meiosis involves two rounds of cell divison without DNA replication in between That way you have cells with half geneic content that can be gametes and pair up during firtilizaiotn DOG PICTURE That WAY you have cells with half genetic content that can be gamemtes and paur up during firtilizano to make a new dipoid cell, called ziygote Zygote is what develops into the individual Important to spend time on the step up… You need to know the why of meiosis MEIOSIS FOLLOWS S AND G2, like mitosis These cells go through cell cycle G1-S-G2-MEIOSIS The difference is inteaded of mitosis after g2, its meiosis! The chromsomes prior in meiosis are in their replicated state, there are two sister chormatids per chrosmosme. (in example, it’s simple, 2n=2, sister chromatids are attached at centremer) TABLE 13.1 Terminology that you should know! Homologus pair where chrosmomes are in their replicated state (two sistet cchromatids) if you are comparing a chromatid on one member of homolous pair, to a chromatid on the other member o the same homologous pair, then you call that non sister chromatids… Sister chromatids two identical DNA molecules in a replicated choromotme Non sister are the DNA molecuels between the two members of homologus pair, but not two members of the same chromosome Cell that under go meio sis, one of the first things they do is members of homolous pair, line up and physically attach to each other to create bivalent

MEIOSIS OVERVIEW Chrosmomes are in replicated state, now whats ogin to happe n is two rounds of cell division 1ST ROUND-meiosis 1. Members of homolous pair get separated to opposite cells, but each chromosome remains in its replicated state, you don’t mess with sister chormtids you just separate. END UP WITH two daughter cells, each containting onle 1 versoin of each chormosme type. Cells are already haploid 2ND CROUND- Meiosis 2- going to separate the sister chromatids of each chromosome. You end up with 4 total cells that are haploid, each chromosome in these cells in the end of meiosis 2 are in their unreplicated state MEIOSIS 1: SEPARATING HOMOLOGUS CHROMSOMES Cells IN G2 begin to process duplicating the centrosome need to make a spindle Then the cells go to early prophase 1, it shows that its in meiosis 1, you know when you’re in early phrospe because the chromsomes are beinging to condense and the homologs are beginning to line up next to each other to make bivalents. Late prophase 1, the nuculear envelope breask down the choromsome finish condensing and the microbtubles form the two halfs of the spindle beging to attach to the chromsomes Late prhosae 1, the nonsister chromatids make physical points of connection with each other, they overlap one another and then break and reassemble. At the end of late prophase 1 some of the nonsister chromatids will have swaped genetic material (process called crossing over) After late prohpshe 1, the homolous pairs line up on the midpoint in the cell, they line up so that one member of each homolog is facing one pole of the cell or the other, then in Anaphase 1, the homologs get pulled apart, Then telophase wehre the spindle breaks down and the chorsmome begin to decondense a little bit and then you have cytokinesis where you divide the cellin half These cells are haploids Red and blue show maternal homoloug versus paternal homologys Each chromosome is still in the replicated state

MEIOSIS II: Each of these haploid cells go independtly through MEISIS 2, Prophse – wehre the spindle reforms, Metaphase- where the sister chromatids on each chromosome attach to one half of the spindle Anaphse – where the connection between sister chormatids break and get pulled apart, Telophase- nucluear envelope reforms and chrosmomes decondense Cytokinesis OVERALL; go from one 2 end cell, to 2 one end cells, then the 2 one end cells divide again to give us 4 one end cells and at the end you have 4 haploid cells each cell has 1 copy of each chorosme type and one copy of each corosme type and they are in their unreplicated state N is re KEEPING TRACK OF CHROSMOME CHANGES IN MEIOSIS: n doesn’t tell us anything about wehitehr chormoomse are in their replicated or unreplicated state n is referring to how many different versoins of each chrosmome type are there in the cell, and that is independent wheterh the chormsome is in their replicated or unreplicated state C-number of chromatids of a particular chromosome type N number of types of chrosmomes Ex: cell is diploid, so its 2n, because the chromsomes are in their replicated state, theres’ a total of 2 chromids 2 DNA cmolcules within this homologs pair , so its 2C, When this cell goes through S phase and ends up in G2, and early m, it’s still 2n, still only has two versons of each chromosome, but because they are in their replicated state it’s 4C, it has 4 total chromtads for each chrosmome type. Not making more chrosmoes just doubling DNA content Diploid in G1, still Diploid in G2 and M phase but you go from each crhosmome having single DNA molecue to having 2 sister chromatids so total number withing homolous pair is 4C BE CONFORTABLE WITH THIS MITOSIS Use this system to track the changes in Chromsomes and track total DNA amount, compare between mitosis meiosis So mitiosis- particular cell is in prometaphase (nuclear envelop is broken down and spindle is starting to attach to chrosmomes) We have 4 total chrosmoes in this cell, two versions of blue and two versions of red, diploid 2n, dipold number would be 4 (4 total chrosmomes)

Within a homolous pair, there is 4 total chormotids (4c) counting the number of chromatids of a particular chromosome type… Each cell now has 4 total chromsomes, (two large two small) still diploid but with chromosome type there is only 2 total crhomatids(, two for the same, two for the large,) End of mitosis there’s 2C, in mitosis you start 2n, 2c, you go through DNA replication, 2n, 4c, then you to through Mitosis and youre back at 2n, 2C Meiosis 1: Take a cell that is in 2n, 2c, you go through replication and now it’s 2n, 4c, In meiosis 1: separating homologs form each other , so at the end of meiosis 1, each cell only has one versoin of each chrosmome type, the end of miesis 1 the cell is 1n, but each chrosmome has a chromatid so it’s 2c Meisos 11: so the 1n, 2c cells go to meisiis 11, separate sister chromids and you end up wth cells that are 1n (1 version of each chromosome type), and that chrosome only have 1 chormatid so it’s 1c. COMPARING THE TWO ‘MS’ Mitosis: start with cell that is 2n, 2c End up with two daughter cells that wre 2n, 2c Meisos: You start with cells that are 2n, 2c End up with 4 daughter cells that are 1n, and 1c Graph comapritn gtow m’s Right away in meiosis you end up with daughter cells that are differently genetically than parent cells they have half total of DNA

MEIOSIS 1 ALLOWS No two cells from the same individual going through meiosis are genetically identical Reason: when cells are in meisis 1, they have two processes that shuffle the DNA , The shuffling is random So one cell going through meiosis and another shuffle differently Two resons for this: 1) Independnet assortment. Means no bias in how the matural and patonal homologs of the different chromosome types segregate in mieis 1 its entirely random a. Ex; CELL that has two chrosmome tyeps (in diplod state 2n=4) when chrosmomes line up. Anotehr cell might line up differently as shown in the example. How many cobinations are there in Independent assortment? -2n 2) Crossing over. the other event in meiosis 1 that contributes to schuffling of genetic material. The NON sister chromatids within a homolous pair, physical overlap each other (points of overlap looks like little X) called chiasma. Points of contact a. After they overlap they break, when they reheal you get exchanges of material between the sister chromatids (looks like they’re shuffled) b. Process of crossing over, is happening during Prophase 1, so by the time the chrosomes align at metaphase 1, the crossing over has already occurred c. Shuffling material between non sister chromatids then randomly segregating homologs, maternal and fraternal into daughter cells, d. Crossing over events occur in different cells going through meiosis Indpendent assortemtn is in metaphase 1 anaphase 1, because of how the homologs align, Crossing ovaer occurs in prophase 1, shuffling materials in non sister in random way

INDEPENDENT ASSORTMENT Look AT THIS in another way -A cell that’s haploid number is 5( 5 diff chrosome types) so it 11-11 is homologs pair for chromosome one, and there is 2 chromatids per chorosme (1 chrostid for mom, sister chromatid, 1 chromotid for dad and its sister chrosmome) a. all the different ways that these different homologus pairs could line up, if you did all the exapme it’s 25 or 32 different possible types of gametes just through independent assortment 3) Independe assortment Happens during metaphase 1,

CROSSING OVER CAUSE GENETIC RECOMBINOIN Indpendent assortemtn is in metaphase 1 anaphase 1, because of how the homologs align, Crossing ovaer occurs in prophase 1, shuffling materials in non sister in random way These two events make it statistically almost impossible that any two gametes are going to be exactly the same… Closer look at the interactions between homolougus chrosomes leading to chrossing over Start with condensed chrosome in homolous pair, they line up to make the bivalent, (there’s a complex of proteisn that helps stick these homologous chrsoomes together) Non sister chortaids over lap (ppoints of connetion are called chiasma), there’s two breaks and exchagend of pieces occur at the break, Piece of non sister gets switched… After crossing over the exchange of material is called genetic recombinaition. Recombingn the genetic material between non sister chromatids. NONDISJUNCTION Meisos doesn’t happen perfectly all the time -both members of homologs pairs stick together in meiosis 1 -or sister chromatids stick together in meisos 2 when that happens you end up with daughter cells with too many or too few chromosomes Process is called nondisjunction- errors in chromosome separation during meisis (EX: non disjunction in meiosis 1. Samller chrosome, the homologus pair stayed attached so they both moved to one daughter that has an extra copy and one daughter that is missing a copy) More common in meiosis 1 a cell that has 1 too few or one too many chrosomes is called aneuploid A CELL will too few or too many Nondisunction resutsl in anueplody THE RATE OF NONDIS Downsyndrom is a nondisjunction error in the oocytes in women Specifically in chrosome 21 2 homologs in chrosome 1 stick together in meisos 1 and more together to one daughter cell which causes one that has too many, and one that has too less…. Risk increases with the age of the mother UHH GO BACK lol

ASEXUAL AND SEXUAL REPRODUCTION To understand why meosis is importat and how it plays a role, need to look at different types of reproduction In asexual reproduction in eukaryotes there is no fusion of gametes, only mitotic divisions that give rise to daughter cells that are genetically identical to the parent cell. In sexual reproduction there must be a fusion of gametes and these gametes must arise from meiosis. -Selfing is when a single organism produces both gametes (egg and sperm). These hermaphrodites still undergo meiosis to produce the gametes. One common example used in research is C. elegans a small worm – -Outcrossing improves the amount of genetic diversity because gametes from two different individual fuse to form a zygote and offspring. This allows for the offspring to be genetically distinct from both parents – can have different ALLELE combinations. SELF-FERTILIZATOIN AND Shows selfing can produce gametes that can fuse to form a new organism - Both crossing over (prophase 1) and independent assortment (metaphase 1) still occur and can give rise to genetic diversity - Limited to alletes found in the parent When outcrossing occurs we get normal genetic variation from meiosis (crossing over and independent assortment) when each parent produces gametes. We get additional genetic variation in the offspring as new combinatons of alleles are possible when different parents contribute gametes.

ASEXUAL REPRODUCTOIN Meiosis can generate gametes for sexual reproduction is a costly process It doesn’t make sense to sexually reprosuce. ASEXUAL reproduction is capable of producing lots more offspring and could outcompete Drawbacck to sexual reproduction is MALES- these are offspring that cannot produce any offspring on their own -Diagramm form asexual, assume four offspring are possible for each After 1 generation four offspring capable of reproduction. Each of these products produces four more, so 16 at the end of two generations

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For sexual, a female can reproduce four offspring (half males and half females), but ONLY females can produce offspring. Therefore, each of the two daughters can have childram and of these children only half will be females

In long run, asexual reproduction should be favored by a lot-more efficient and no males. Simple model like this would say that esexual reproduction is unlikely to persist in the world WHY DOES SEXUAL REPRODUCTION EXIST? Sexucal reproduction allows for allele combinations to occur that would not normally, and this improvse the genetic diversity This overall diversity can be very important as a population adapts to a different environment The following experiment shows that this is indeed possible as it looks at the ability of different populations of C. elegans to respond to a pathogen in its environment. Why does SEXUAL REPRODUCTOIN EXIST? In enviromesnt where evolving pathogens are preseeng, sexual reproduction is favored…...