Learning Guide 6: Cell Division PDF

Learning Guide 6: Cell Division Ms. Lapointe 1 What is Heredity? - What we used to think - Life can arise from nonliving things - What we know now - Life arises only from the division of living cells - Thus: - The passing of traits...

Learning Guide 6: Cell Division Ms. Lapointe 1 What is Heredity? - What we used to think - Life can arise from nonliving things - What we know now - Life arises only from the division of living cells - Thus: - The passing of traits from one generation to the next - Through the passing of combinations of DNA from parents to their offspring 2 How do we get to that point? Every day our cells are being replaced - From the moment of fertilization, cells are continuously growing, dying, and being replaced. - Daughter cells must be identical and contain the same genetic information as the parent cell - We do this through DNA replication and cell division 3 DNA - Genetic information found in the nucleus of every cell in the body (~3m) Chromosomes: structures of tightly coiled DNA. Inherited in pairs in humans; wrapped around histones - Unpackaged = chromatin (waiting to be coiled into chromosomes for division) Genes: Segments of DNA on a chromosome that code for a particular trait - Each gene has a specific location on a 4 certain chromosome Cell Types Somatic cells - Body cells - Diploid (2n) - Pairs of chromosomes inherited from each parent Gametes - Sex cells - Haploid (n) - Half the number of chromosomes - Egg and sperm unite during fertilization to form a diploid zygote - Zygote divides to eventually form a baby with other 2n somatic cells 5  Chromosome Types Autosomes - Chromosomes #1-22 - Found in pairs = homologous chromosomes - Same gene sequences together - One maternal and paternal - NOT connected like sister chromatids - May carry same set of genes, but may have different forms of each gene (alleles) - Chromosomes in pairs = what makes us diploid! Sex Chromosomes - Chromosome # 23 - Technically non-homologous but they are paired together for determining genetic sex of person (male vs female determinants) 6 Humans have 23 pairs so 2n=46 chromosomes! Karyotype How chromosomes are arranged with their homologous pair - to study the sex of the person as well as any issues/abnormalities Chromosome numbers vary from species to species - Thus they will have more or less than 23 pairs like humans - The number does not correlate with the complexity of an organism - ex) dogs = 78 and bananas = 22 7 Polyploidy When there is more than once complete set of chromosomes - Common among plants Strawberries can range from diploid (2n) to octoploid (8n) Example - If a species is 3n = 33, then how many chromosomes are found in the haploid cell? 8 - What about haploid of 8n = 48?  Mitosis vs. Meiosis Tell me everything you know about them 9 Mitosis…why does it happen? Mitosis - Carries on life of cells (maintenance) - Making two genetically identical daughter cells - Performed in all autosomes - Allows growth - How we get from a baby to where we are now - Used for repair and replacement of cells - Are your knees still skinned 10 from when you were a kid?  Cell Cycle Goal: to produce daughter cells with the same DNA as the parent cell Separated in interphase vs mitosis 11 Interphase The non-division portion of the cell cycle - Largest part of cell cycle - Getting the cell ready for division - Consists of three phases of: Gap 1, Synthesis (S phase), and Gap 2 At this time, DNA is uncondensed and looks tangled 12  Interphase Details Gap 1 (G1) - Period of cell growth - Restriction point = where the cell commits to enter cell division Synthesis (S) - DNA replication - So that cell contains double-stranded (replicated) chromosomes - Two sister chromatids to one centromere = one replicated chromosome Gap 2 (G2) - Period of preparation for cell division 13 Mitosis After DNA replication (interphase), cells go through four stages of mitosis before cytokinesis (splitting of cytoplasm) can occur 14 Prophase At this stage, each pair of homologous chromosomes has been duplicated (from S phase) so there is twice as much DNA than normal: - Replicated DNA (held together by cohesin) condense into chromosomes - Nucleus and nucleolus disappear - Centrioles migrate to poles - Spindle fibers start to form - What can a spindle fiber be used 15 for? Metaphase - Spindle fibers attach to chromosomes at centromere - Chromosomes line up in a single file - One on top of the other - Along the metaphase plate (also called equatorial plate) Easy way to tell it is in metaphase? Chromosomes lined up in the middle 16 Anaphase - Centromeres divide/split - Sister chromatids pulled apart from one another toward poles - Once they reach the poles, they are considered single-stranded/unreplicated chromosomes - Still diploid! - 1 replicated = 2 chromatids - 2 chromatids split = 1 unrep 17 Telophase - Chromosomes have reached opposite poles - Spindle fibers disappear - Unraveling into chromatin - Nucleus and nucleolus reappear at each pole - Division of the cytoplasm begins 18 Cytokinesis - Division of cytoplasm - Divides the parent cell into two daughter cell - Identical from parent cell because of the chromosomes that were replicated - Animal cells = a cleavage furrow - Pinches off between two daughter cells - Plant cells = a cell plate forms - Goes between two daughter nuclei - Eventually develops into a cell wall 19 20  21 (please be prepared to analyze a micrograph :) ) If I look at a cell sample, what would the majority of the cells look like? What can happen if cells were constantly dividing? 22  Mitotic Index Lab We must be able to identify stages of mitosis from microscope slides/pictures (why are these cells smaller than many of the others) 23 Cancer Cell Cycle How would the cell cycle differ in terms of time spent in interphase and cell division? What is a tumor? Cancer is when there is a mutation and results in uncontrolled cell division caused by a mutagen - A normal cell would go through apoptosis if there was a mutation - Cancer cells continue to grow and divide and don’t die on their own 24 *HeLa Stem Cells Henrietta Lacks - Who was she? - Where did scientists get her cells? - What research was initially done with her cels? - What is informed consent? - What important discoveries have come from HeLa cells? - How prominent are these cells? Can we get stem cells from any living being? 25 *Mitosis and Aging Telomeres = caps found at the end of each chromosomes - Every time a cell divides, its telomeres shorten Eventually, the telomeres become so short that a cell will no longer be able to divide - Cell death occurs Telomere length = a “molecular clock” for cellular aging - Telomeres not shortening may indicate 26 cancer 27  Mitosis Recap Must be able to tell each part apart and what is happening in each section 28 If we know about mitosis, then how would meiosis be different? 29  Gametes: - Sex cells formed in males and females - Haploid cells from diploid cells Meiosis through spermatogenesis or oogenesis - Meiosis creates gametes The formation of gametes through a cell division that reduces the chromosome number from diploid to haploid (2n to n) 30 Humans as Diploid Organisms In order to have a complete set of chromosomes, we must be able to create gametes with half the genetic content that fuse together - Forming these gametes = meiosis - These gametes fusing together = fertilization - Any sperm can fertilize any egg = increases genetic variability 31  Meiosis Consists of TWO division stages - Meiosis I - Reduction division because during this stage, the chromosome number is divided in half - Meiosis II - Similar to mitosis in that chromosome number is the same at the end of the stage as it is at the start - Results - FOUR daughter cells that are genetically different from the parent cell (same stages of cell division just happening twice) 32  Prophase I Begins with interphase (like mitosis) - DNA is replicated and has two pairs of homologous chromosomes Chromatin condenses to form chromosomes, spindle fibers form, nuclear membrane dissolves, etc. Homologous chromosomes line up for crossing over as a tetrad - To exchange genetic information (recombination) - Increase genetic diversity - Cross over point = chiasma 33 - ONLY happens in Prophase I Crossing Over Synapsis: lining up of the chromosomes Chiasma: the structure where crossing over occurs Between two homologous chromosomes of non-sister chromatids - Exchanging the same gene information, but alleles may end up switching from paternal to maternal (and vice versa) - Comes up again in Chapter 18 - Increases genetic diversity Will line up again for metaphase, but crossing over happens in prophase I 34  Metaphase I Spindle fibers attach to the centromere Chromosomes line up as homologous pairs - Unlike mitosis where they’re single file - So that chromosomes of maternal and paternal origin end up at opposite poles How these homologous chromosomes pair up is independent of other pairs - How each lines up at the plate is random as to what side maternal and paternal will go (independent assortment = many combinations = LOTS of diversity) 35 Anaphase I Spindle fibers shorten = homologous chromosomes are pulled apart Sister chromatids remain together (replicated DNA) - Each new pole has a full set/copy of DNA Cells are now on their way to officially being considered haploid 36 Telophase I Homologous chromosomes begin to uncoil - Spindle fibers disappear Cytoplasm is divided - Nuclear membrane reforms around each new set of chromosomes Each new cell is haploid - They contain EITHER the maternal and/or paternal set of homologous chromosomes and thus half the genetic content 37 Meiosis II Interphase (where replication happens) does NOT happen before the cell goes into prophase II - NO DNA REPLICATION Most like mitosis only in that we go through another round of PMAT with chromosomes lining up in a single file (no tetrad) - Unlike mitosis in that cells are not genetically identical - Result is four haploid daughter cells - Each with an unreplicated/single 38 stranded chromosome 39  Remember! One replicated chromosome = one chromosome (count the centromeres) One replicated chromosome = two chromatids One unreplicated chromosome = one chromatid 40 Amoeba Sisters Chromosome Number Animation  Mitosis vs. Meiosis Meiosis - Produces four daughter cells rather than two - Difference in number of stages - Different alignment at metaphase - Daughter cells have fewer chromosomes than the parent cells - Haploid vs diploid daughter cells - Daughter cells have chromosomes with different combinations of genes compared to those in parent cells - Recombination, independent assortment and (eventually) random fertilization all increase genetic diversity 41 Meiosis Recap Quick review before we move forward 42 Spermatogenesis The product of cytokinesis after telophase II in males to form haploid sperm cells - Splits equally and produces four functional sperm - “True meiosis” Spermatogonium = undifferentiated germ cell and undergoes mitosis to become a spermatocyte 2n 2n n n Spermatids = immature sperm 43 Oogenesis In females, the cytoplasm is not equally divided amongst gametes Again, like spermatogenesis, we start with an oogonium that goes through mitosis to make oocytes - Results in a single viable ovum and three polar bodies - Why unequal division of the cytoplasm? 44 2n 2n n n Nondisjunction Is the failure of the proper separation of chromosomes in either meiosis I or II - Different outcomes depending on where it happens - Both produce gametes with either too few or too many chromosomes - NDJ in MI will have more affected gametes and MII - why? - Results in trisomy or monosomy upon fertilization (aneuploidy) - Only in one location. Whole genome = triploid 45 Monosomy vs. Trisomy 2n-1 cell is 2n+1 cell is fertilized fertilized Zygote missing Zygote has three one chromosome copies of one Only possible with chromosome ← most common 46 X-- why? Why do we not see more incidences of monosomies and trisomies? 47 Aneuploidy Disorders For you to fill in information - Important to be aware of. Will be given in context. - For the sex chromosome related one, be aware of what sex they are genotypically and possibly phenotypically - (Would we know with whom NDJ is happening?) 48 Karyotype Example First, what kind of information can I gather from this karyotype? Second, how can I get this information? 49 Alternation of Generations LIfe cycles of some plants consist of two generations - Diploid sporophyte generation - Produces haploid spores through meiosis - Spread and develop without fertilization - Haploid gametophyte generation - Spores grow into plant body that can make gametes (gametophyte) - The male and female gametes produced then fertilize each other - Develops into a sporophyte 50 - Cycle repeats Common Example Sporophyte (2n) Meiosis to make spores (n) Spores (n) mitosis to make gametophytes (n) Gametophyte makes gametes (n) via mitosis Gametes fuse via fertilization (2n) to form a new sporophyte (through 51 Bonus Examples! 52  Interactive Mitosis Review Games Nondisjunction Animation (Highly Recommended) (ones below do involve LG 7 things, but still informative for this LG) Meiosis and Snurfles 1 Meiosis and Genetics Snurfles 2 53