Variation in Traits Two important mechanisms that occur during meiosis introduces variation in traits among offspring:

Crossing Over in Prophase I During prophase I, homologous chromosomes pair up Each chromosome and its homologue ___________ together so that they are tightly aligned Nonsister chromatids exchange segments in a process called _____________ _________ Exchange a segment of the DNA molecule at the same place along their length

Crossing Over in Prophase I Because alleles for the same trait can vary, crossing over allows chromosomes to swap certain genes Results in a new combination of alleles not present in either parental chromosome, which is one source of genetic variation Leads to __________ _______________ Crossing over is a common event In each cycle of meiosis in humans: This rate varies among species and chromosomes

Crossing Over Figure 9.6, pg 144

Crossing Over Figure 9.6, pg 144

Crossing Over Resulting chromosome containing a new combination of alleles

Metaphase I Alignments During metaphase I, homologous chromosomes ____________ line up at the spindle equator Microtubules attach to the first chromosome they contact, regardless of whether it is maternal or paternal During anaphase I, the homologous chromosomes separate into two haploid cells Each cell has a ______________________________ ____________________________________________ This chromosome shuffling is another source of genetic variation for each new generation

Metaphase I Alignment combinations possible 123 or Figure 9.7, pg 145

Metaphase I Alignment For a species with just three pairs of homologues: Eight possible combinations (2 3 ) for possible gametes That means that in humans, a sperm or egg has:

From Gametes to Offspring Every species has a distinctive life cycle Gametes play an important role in these life cycles and bridge generations of individuals For now, we are going to focus on the life cycle of plants and the life cycle of animals

Gamete Formation in Plants Plant cycles usually involve two kinds of multicelled bodies _____________________ are multicelled plant bodies (diploid) that produce haploid spores Spores are haploid cells that undergo mitosis and give rise to multicelled haploid __________________ Haploid gametes-producing bodies are formed in gametophytes, and gametes are formed Gamete-producing bodies and spore-producing bodies develop during the life cycle of plants Example: Pine trees are sporophytes Male and female gametophytes develop in different types of pine cones on each tree

_______ Life Cycle Figure 9.8, pg 146

Plant and Animal Life Cycles Figure 9.8, pg 146

Gamete Formation in Animals: ____ In males: meiosis and gamete formation is called spermatogenesis Sequence of events in sperm formation: 1. Germ cells (2n) 2. Primary spermatocyte (2n) 3. MEIOSIS I 4. Two secondary spermatocytes (n) 5. MEIOSIS II 6. Four spermatids (n) Each __________ develops a tail to become a mature sperm

__________________ : Figure 9.9, pg 147

Gamete Formation in Animals: _______ In females: meiosis and gamete formation are called ________ Sequence of events in oogensis: 1. Germ cell (2n) 2. Primary oocyte (2n) 3. MEIOSIS I 4. Secondary oocyte (n, and large in size) + polar body (n, and small in size) 5. MEIOSIS II 6. One large ovum (n) plus three polar bodies (n, small) The single ovum (egg) is the ONLY cell capable of being fertilized by a sperm The polar bodies wither and die

_________________: Figure 9.9, pg 147

Fertilization The chromosome number of the parent is restored at __________________ Fertilization is the fusion of two haploid nuclei of two gametes Of all the genetically diverse gametes produced, ________ will determine which two will meet at fertilization Many gametes are formed, but only two are needed to produce a zygote Meiosis allows species to maintain their chromosome number; without it, an offspring would have twice as many chromosomes as their parent

Variation in Offspring Three events cause new combinations of alleles in offspring: Crossing over during prophase I (meiosis) Random alignment of maternal and paternal chromosomes at metaphase I (meiosis) Chance meeting of gametes at fertilization All three contribute to variation in traits of the offspring

Comparing Mitosis and Meiosis Results in clones Occurs in single-celled eukaryotes as a means of asexual reproduction All eukaryotes engage in mitosis for growth and tissue repair MitosisMeiosis Occurs in immature reproductive cells giving rise to gametes Sexual reproduction Both processes of meiosis and fertilization give rise to genetic variation

An Ancestral Connection? Despite the differences, there are many similarities in the steps of mitosis and meiosis The molecular machinery of mitosis may have been recruited for meiosis For example: Proteins the recognize and repair breaks in DNA are used in both If the separation of anaphase in mitosis did not occur properly, the result would be that of anaphase I in meiosis Did sexual reproduction result through mutations that caused changes in the mechanisms important before and during mitosis?

Chapter 9 Homework Self Quiz 1-9 Critical Thinking: 1, 3, and 5 (for only the Fruit fly)