Lesson 9: Where Babies Come From (BIOL 1441)

Lesson 9: Where Babies Come From BIOL 1441 Cell & Molecular Biology  Learning Objectives (Study Guide) By the end of this lesson, 5. Explain how independent students will be able to: assortment increases diversity in...

Lesson 9: Where Babies Come From BIOL 1441 Cell & Molecular Biology  Learning Objectives (Study Guide) By the end of this lesson, 5. Explain how independent students will be able to: assortment increases diversity in offspring. 1. Explain how asexual reproduction & sexual reproduction are 6. Explain how crossing over different from one another (in increases diversity in offspring. their purposes & in how they 7. Explain how the fertilization work). event increases diversity in 2. Explain how diploid & haploid offspring. cells are different from one 8. Explain why diversity generated another. through sexual reproduction is 3. Explain how mitosis & meiosis are beneficial to offspring. different from one another. 9. Describe the roles of meiosis & 4. Describe what occurs in each mitosis in the life cycle of a phase of meiosis. human.  Sexual Reproduction In sexual reproduction, two parents each pass on half of their genetic information to the offspring This genetic information is shared through cells called gametes (sperm and egg cells) During fertilization, one gamete from each parent fuses to produce a zygote (fertilized egg cell), generating offspring that are genetically different from both parents The gametes used in sexual reproduction are made through the process of meiosis, which produces four haploid cells that are NOT genetically identical to one another. A haploid sperm cell + a haploid egg cell results in a diploid zygote that then divides and divides by mitosis to make the whole multicellular organism.  Meiosis includes two rounds of division. The goal of Meiosis I is to separate the duplicated homologous chromosomes. The goal of Meiosis II is to separate sister  Meiosis Meiosis I I Meiosis I is the first round of cellular division It starts with one diploid (2n) parent cell It ends with two haploid (n) cells Key events of Meiosis I Anapha se Tetrad formation (in Prophase I) Cytokinesis Crossing over (in Prophase I) Tetrad Separation of the duplicated homologous chromosomes (in Anaphase I) Meiosis I is the primary source of the genetic diversity in gametes  Meiosis Prophase I I Tetrads are built in ProphaseCrossing over occurs in Prophase I I Homologous chromosomes connect to A tetrad is a pair of one another & swap genetic homologous chromosomes information (with a total of 4 sister Crossing over generates chromatids) Proteins anchor the recombinant chromosomes (still in homologous chromosomes the form of chromatids), which carry together “mixed” genetic information – a mixture of alleles of the genes. Where crossing over occurs is random.  Prometaphase I & Meiosis I Metaphase I In Prometaphase I, the nuclear envelope around the tetrads breaks down Spindle fibers begin to attach to & pull on the tetrads By Metaphase I, the tetrads have been pulled to the metaphase plate in the middle of the cell The maternal & paternal chromosomes in each tetrad line up randomly This leads to the random separation of these chromosomes in Anaphase I  Anaphase I, Telophase I, & Meiosis I Cytokinesis In Anaphase I, the proteins holding the tetrads together break The homologous duplicated chromosomes in each tetrad are pulled to opposite ends of the dividing cell In telophase I (and cytokinesis), the homologous duplicated chromosomes reach the opposite ends of the dividing cell The plasma membrane of the original cell pinches in, dividing the cytoplasm & organelles into two new cells Each new cell has one copy of each duplicated chromosome, making them haploid cells. Note: The reduction in ploidy level (2n n) occurs during Meiosis I, starting when anaphase I leads to the separation of homologous duplicated chromosomes and finishing when two new cells have formed through cytokinesis.  Meiosis Meiosis II II Meiosis II begins shortly after Meiosis I Both haploid cells, still with duplicated chromosomes, from Meiosis I enter Meiosis II By the end of meiosis II, 4 new haploid cells (gametes) have been generated Note: We are calling the two cells that result from meiosis I “haploid” because each of the duplicated chromosomes still consists of two sister chromatids (that have undergone crossing over) joined at the centromeres. In meiosis II, the sister chromatids will be separated, and one of each will be distributed to one of the four cells that result from meiosis II. The result of meiosis II is four haploid cells, because meiosis I resulted in two haploid cells (still with joined sister chromatids), and meiosis II divides each duplicated chromosome in two. So now each chromatid is its own separate chromosome. Each of the two cells that resulted from meiosis I divides by cytokinesis, to make four haploid cells that no longer have duplicated chromosomes. Each has one of the original four chromatids, each of the chromatids has become its own chromosome, so now there is one of each chromosome per cell. Prophase II, Prometaphase II, Meiosis & Metaphase II II In Prophase II & Prometaphase II, a new set of spindle fibers develops to divide the genetic information The fibers attach to & begin pulling on each of the individual duplicated chromosomes Remember: in Prometaphase I, the fibers attached to tetrads (groups of 2 connected duplicated chromosomes) By Metaphase II, all the duplicated chromosomes in both cells are randomly aligned along the metaphase plate Anaphase II, Telophase II & Meiosis II Cytokinesis In Anaphase II, the sister chromatids (of each duplicated chromosome) are separated from one another Each is pulled toward the opposite ends of the dividing cells New nuclear envelopes form around each set of chromosomes during Telophase II Once cytokinesis finishes splitting the cytosol & organelles, four haploid gametes are created Name of Phase Description Spindle apparatus forms, sister chromatids (no longer identical) begin moving towards metaphase plate Spindle fibers move homologous chromosomes to opposite sides Nuclear membrane reforms, cytoplasm divides, 4 daughter cells formed Chromosomes line up along equator, NOT in homologous pairs Crossing-over occurs Sister chromatids separate Homologous chromosomes line up along equator  Identify the Phases of Meiosis Phase: Phase: Phase: Phase: Phase: Phase: Phase: Phase: Meiosis: Summary In Meiosis I, In Meiosis II, homologous sister chromatids are chromosomes divided. are divided. Four new cells Two new cells (gametes) are generated. are generated. Mitosis vs. Meiosi s Study Tip: Make sure you know the ways that meiosis is similar to & different  Characteristics Mitosis Meiosis Mitosis Number of cell divisions vs. Number of DNA replications Meiosi Do homologous chromosomes s pair to form tetrads? Use this table Does crossing over between to outline non-sister chromatids occur? several critical details about Number of daughter these two cells generated processes. Ploidy of daughter cells (diploid or haploid?) Genetic makeup of daughter cells (identical or different?) Let’s Practice! X X X X X X X X Draw each phase of mitosis & meiosis on the outline on the right. Pay attention to the colors of the chromosomes in each phase!  A Case for Sexual Reproduction Compared to asexual reproduction, sexual reproduction is more challenging It requires two parent organisms Both parents must successfully generate gametes Those gametes must successfully combine Sexual reproduction is extremely important to organisms because it increases genetic diversity Some of this increased diversity comes from meiosis Some of this increased diversity comes through fertilization  Diversity Through Meiosis: Independent Assortment Independent assortment is the random division of genetic information into gametes This is caused by the random Metaphase I arrangement of chromosomes at Both paternal Both maternal Paternal & Paternal & the metaphase plate maternal maternal chromosomes chromosomes chromosomes chromosomes are on the left. are on the are on the left. are on the right. right. In metaphase I, tetrads line up randomly Metaphas e II The number of possible All gametes chromosome combinations for each gamete is 2n (n = number of chromosomes) Human gametes have 23 chromosomes  there are over 8 million (223) different possible  Diversity Through Meiosis: Crossing Over Crossing over is the exchange of genetic information between the non-sister chromatids (in a tetrad) Crossing over generates recombinant chromosomes These chromosomes have a mix of genetic information from maternal & paternal chromosomes Each pair of duplicated human chromosomes typically crosses over 2 – 3 times Diversity Through Meiosis Crossing over occurs during Prophase I of meiosis. Independent assortment occurs during Metaphase I & Metaphase II of meiosis. Both of these processes increase diversity in the offspring.  Diversity Through Fertilization Fertilization is the fusion of two haploid gametes, generating one new diploid cell In sexual reproduction, multiple gametes are typically present at the time of fertilization Each gamete has a different combination of genes Remember: in humans, there are over 8 million possible chromosome combinations for each gamete! Only the exact chromosome combinations in the two successful gametes are passed on to the offspring Your exact chromosome combination had a one in 70 trillion chance of occurring. The chance of two humans being genetically identical (except identical twins) is one in trillions (or more), even for siblings, or parents and offspring. ”Genetically identical” means having all the same alleles of all the genes. This is basically impossible! We’re all genetically different.  Why is Diversity Good? Diversity helps organisms to adapt to their environment Example: the sickle cell allele, which can lead to sickle cell anemia The sickle cell allele protects individuals from malaria, so it is more common in regions where malaria is widespread Diversity decreases the likelihood of passing along genetic diseases Example: Type I diabetes, which is caused by a recessive allele of a gene Because parents each only pass along half of their genetic information, the likelihood of offspring inheriting two copies of a disease allele is decreased  So, Where Do Babies Come From? First, meiosis (in the parents) generates diverse gametes. Then, these gametes randomly combine through fertilization. Then, mitosis of the zygote causes rapid growth, leading to a genetically-distinct baby.  Asexual Reproduction Offspring are made through the process of reproduction, which can be sexual or asexual. In asexual reproduction, offspring receive all of their genetic information from one individual These offspring are genetic clones of that parent and are produced via mitosis Asexual reproduction is used by bacteria and some fungi, plants, and animals. Some can switch back & forth between sexual and asexual reproduction. Humans can only reproduce sexually.  To Prepare for Next Class…  Review your class notes Use the eTextbook & Other Helpful Resources to supplement your lecture notes  Complete the homework assignment Review what you didn’t understand and make another attempt. You can complete the homework as many times as you want!  Print the slides for Lesson #10  Reflect on how much you have learned this semester! You are almost done!