AP Biology, Chapter 13
Meiosis and Sexual Life Cycles

Summary
Introduction

AN INTRODUCTION TO HEREDITY
	Offspring acquire genes from parents by inheriting chromosomes
		1. Explain why organisms reproduce only their own kind and why offspring more closely 
		resemble their parents than unrelated individuals of the same species.
			a. We pass coded information in the form of DNA to our offspring
			b. DNA can change in increments
			c. Within species there may be significant variation
			d. The more distantly related, the more different
		2. Explain what makes heredity possible.
			a. DNA is copied and then passed
			b. Inheritance is an orderly, quantitative process
			c. Most often, matched sets of genes are inherited from two parents
	Like begets like, more or less: a comparison of asexual and sexual reproduction
		3. Distinguish asexual and sexual reproduction.
			a. Asexual
				i. One parent
				ii. Offspring = a clone
				iii. DNA passed was grouped by mitosis
				iv. Offspring are much more identical than in sexual reproduction
				v. Advantages: faster, uses less energy, simpler, good in a stable 
				environment
			b. Sexual
				i. Two parents
				ii. Offspring have new combinations of traits
				iii. Alternate method of DNA grouping is required
				iv. Advantages: produces new combinations of traits for changing 
				environments

THE ROLE OF MEIOSIS IN SEXUAL LIFE CYCLES
	Intro
	Fertilization and meiosis alternate in sexual life cycles
		4. Diagram the human life cycle and indicate where in the human body that mitosis and 
		meiosis occur; which cells are the results of meiosis and mitosis; and which cells are 
		haploid.
			a. See text, p. 230, for diagram
			b. Diploid somatic cells arise by mitosis from the zygote
			c. Haploid gametes arise by meiosis
		5. Distinguish among the life cycle patterns of animals, fungi, and plants.
			a. All groups have members with sexual and asexual modes
			b. Differences
				i. Animals most often have a multicellular diploid
				ii. Fungi and some algae have a multicellular diploid
				iii. Plants and some algae
					.Multicellular haploid and diploid
					.Gametophyte alternates with sporophyte
					.= alternation of generations
	Meiosis reduces chromosome number from diploid to haploid: a closer look	
		6. List the phases of meiosis I and meiosis II and describe the events characteristic of 
		each phase. Recognize the phases of meiosis from diagrams or micrographs.
			a. Meiosis I
				i. Prophase I
					.Chromosomes condense
					.Homologues pair during synapsis
					.Pairs are called tetrads and have 4 chromatids
					.Crossing over may result in chiasmata
					.Tetrads attach to spindle
					.Move toward cell center
				ii. Metaphase I
					.Tetrads lined up at center
				iii. Anaphase I
					.Homologues separate
					.Sister chromatids remain attached
					.Number of chromosomes is halved
				iv. Telophase I and cytokinesis
					.Chromosomes may decondense
					.Nuclei may reform
			b. Meiosis II
				i. Prophase II
					.Chromosomes recondense
					.Attach to spindle
					. Move toward cell center
				ii. Metaphase II
					.Lined up
				iii. Anaphase II
					.Centromeres divide
					.Sister chromatids move to opposite poles
				iv. Telophase II and cytokinesis
					.Chromosomes decondense
					.Nuclei reform
		7. Describe the process of synapsis during prophase I and explain how genetic 
		recombination occurs.
			a. Synapsis
				i. Prophase I has phases: leptotene, zygotene, pachytene, diplotene, and 
				diakinesis
				ii. During zygotene, matching sequences on homologs zip together
				iii. Synaptonemal complex forms between homologs
				iv. Pachytene begins when synapsis is complete
				v. During diplotene synaptonemal complexes break down except for 
				chiasmata
				vi. Chromatids cross and remain temporarily attached when they have 
				recombined
			b. Recombination
				i. Matching base sequences align
				ii. DNA helices open up and heteroduplexes form
				iii. DNA strands break precisely and are exchanged
				iv. Breaks are sealed
				v. All of the DNA to the end of the chromosomal arm has been 
				exchanged

		8. Describe the key differences between mitosis and meiosis. Explain how the end result 
		of meiosis differs from that of mitosis.

	Mitosis	Meiosis	
Number of divisions	One	Two	
Synapsis	No	Yes	
Crossing over	Rarely	Frequently; required for segregation	
Number of cells	Two	Four	
Number of chromosomes	Same	Halved	
Role	Somatic cells from zygote	Produce gametes	

ORIGINS OF GENETIC VARIATION
	Intro
	Sexual life cycles produce genetic variation among offspring
		9. Explain how independent assortment, crossing over, and random fertilization 
		contribute to genetic variation in sexually reproducing organisms.
			a. Independent assortment
				i. Different pairs of homologues aren't physically connected
				ii. Maternal and paternal chromosomes separate randomly
				iii. Gametes get random sets of parental chromosomes
			b. Crossing over
				i. Genetic recombination shuffles chromosome parts
				ii. New combination of maternal and paternal genes are made
			c. Random fertilization
				i. A random sperm with a random set with randomly shuffled parts
				ii. A random egg with a random set with randomly shuffled parts
				iii. Ensures many new combinations
	Evolutionary adaptation depends on a population's genetic variation
		10. Explain why inheritable variation was crucial to Darwin's theory of evolution.
			a. Individuals with heritable variations show differential survival
			b. The variations are passed on to their offspring