AP Biology Chapter 13 Notes I. Chapter 13: Meiosis and Sexual Life Cycles a. Overview: i. Heredity: = inheritance = the transmission of traits from one generation to the next ii. Variation: differences between parents and offspring and siblings iii. Genetics: the scientific study of heredity and hereditary variation. II. Chapter 13.1: Offspring acquire genes from parents by inheriting chromosomes. a. Inheritance of Genes: i. Genes= hereditary units= segments of DNA= passes on inherited information ii. Genome= our genetic constitution iii. Gametes= reproductive cells = sperm or egg= transmit genes form one generation to the next b. Every species has a characteristic number of chromosomes i. Each chromosome is a single strand of DNA 1. Contains hundreds to thousands of genes 2. Locus- a genes specific location on a chromosome c. Comparison of Asexual and Sexual Reproduction: i. Asexual reproduction: organisms reproducing exact replicas of themselves- or clones 1. Example: budding- localized mass of mitotically dividing cells forming a new organism ii. Sexual reproduction: two parents give rise to unique offspring with unique combinations of genes III. Chapter 13.2: Fertilization and Meiosis Alternate in Sexual Life Cycles i. Life Cycle- is the generation to generation sequence of stages in the reproductive history of an organisms, form conception to production of its own offspring. b. Sets of Chromosomes in Human Cells: i. Somatic cells- body cells any cell that is not a gamete ii. Karyotype- the ordered display of chromosomes 1. Micrographs of chromosomes reveals that there are two of each type a. Chromosomes are visible under a microscope when they condense during mitosis b. They can also be stained revealing the different locations of the centromere giving each chromosome a unique size and shape i. Colored bands are produced from different stains
2. Homologous chromosomes: a pair of chromosomes that share the same length,, centromere position and staining pattern. a. One exception: the X and Y chromosome i. Only small parts of the X and Y are homologous ii. Most genes on the X are not found on the Y, and the Y chromosome has genes that are lacking on the X iii. Y chromosome is much smaller than the X iv. They determine an individuals sex, so they are sex chromosomes b. Autosomes: all chromosomes that are not sex chromosomes c. Homologous pairs of chromosomes in somatic cells, are a consequence of our sexual origins. i. One chromosome from each parent ii. Of our 46 chromosomes, 23 come from our mother and 23 come from our father d. Number of chromosomes is represented by n i. A cell with two chromosome sets is diploid (2n) ii. For humans: 2n = 46 = the number of chromosomes in our somatic cells iii. A cell that has synthesized its DNA has duplicated its chromosomes and has two sister chromatids 3. Gamete cells contain a single chromosome set and are considered haploid (n = 23), 22 autosomes and a sex chromosome a. Egg cell contains a X chromosome b. Sperm cell may contain a X or Y c. Behavior of Chromosomes Sets in the Human Life Cycle: i. Fertilization: Human life cycle begins when a haploid sperm cell fertilizes a haploid egg cell 1. Fertilized egg is called a zygote is diploid 2. What would happen if the gametes divided by mitosis? ii. Meiosis: type of cell division that reduces the number of sets of chromosomes from two to one in the gametes 1. Occurs in the ovaries and testes. 2. Produces haploid sex cells, sperm or egg 3. Diploid condition restored when sperm and egg unite d. The Variety of Sexual Life Cycles: differ in the timing of meiosis and fertilization, but all contain a cycle that results in the halving and doubling of chromosomes that contributes to genetic variation i. Three Main types of life cycles:
IV. 1. Animals: diploid organism- haploid gametes produced by meiosis a. Fertilization produces a diploid organisms which divides by mitosis to form a multicellular organism 2. Plants and some Algae: Alternation of generations a. Includes both diploid and haploid multicellular organisms i. Multicellular diploid stage = sporophyte ii. Meiosis in the sporophyte produces haploid spores iii. Spores give rise to multicellular individuals iv. Spore then divides mitotically to produce a multicellular gametophyte v. Gametophytes produce haploid gametes vi. Fertilization of haploid gametes produces a diploid zygote which develops into the next sporophyte generation b. Summary: sporophyte produces a gametophyte as its offspring, and the gametophyte produces the next sporophyte generation 3. Most fungi and some protists: a. Diploid zygote is formed from haploid gametes b. Meiosis produces haploid cells that divide by mitosis to produce cells that develop into gametes Chapter 13.3: Meiosis reduces the number of chromosome sets from diploid to haploid. meiosis is preceded by the replication of chromosomes- single replication is followed by two consecutive cell divisions, Meiosis I and Meiosis II. Results: four daughter cells each with half as many chromosomes as the parent cell. a. Meiosis I: separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes b. Meiosis II: separates sister chromatids just like in mitosis c. The Stages Of Meiosis: i. Interphase: chromosomes replicate during S phase ii. Centrosomes replicate forming two centrosomes d. Prophase I: i. 90% of meiosis is spent in this phase ii. chromosomes condense iii. homologous chromosomes loosely pair along their lengths iv. crossing over occurs 1. DNA of non- sister chromatids break at corresponding places then rejoin on the opposite DNA
v. Synapsis 1. Synaptonemal complex forms between homologues, holding them tightly together along their lengths vi. Synaptonemal complex disassembles in late prophase 1. Chromosomes become visible as tetrads- a group of four chromatids vii. Each tetrad has one or more chiasmata 1. Criss- crossed regions where crossing over has occurred 2. Hold homologues together until anaphase I viii. Movement of centrosomes 1. Formation of spindle fibers 2. Break down of nuclear envelop 3. Dispersal of nucleoli ix. Late prophase x. Kinetochores of each homologue attach to microtubules from one pole or the other 1. Then move toward the metaphase plate e. Metaphase I: i. Tetrads are arranged on the metaphase plate 1. One chromosome facing each pole ii. Chromatids are attached to kinetochores from their respective poles f. Anaphase I: i. Chromosomes move towards the poles ii. Sister chromatids remain attached at the centromere and move as a unit iii. Homologous chromosomes move towards opposite poles g. Telophase I and Cytokinesis: i. Each half of the cell has a complete set of haploid set of chromosomes but is still composed of two sister chromatids ii. Cytokinesis occurs simultaneously iii. Animals- cleavage furrow forms, plants- a cell plate forms h. Prophase II: i. Spindle apparatus forms ii. Chromosomes move towards metaphase plate(late prophase) i. Metaphase II: i. Chromosomes are positioned on metaphase plate ii. Sister chromosomes at this point are not identical because of crossing over iii. Kinetochores are attached to microtubules j. Anaphase II: i. Centromeres of chromosomes separate and the sister chromatids come apart ii. Sister chromatids move as two individual chromosomes towards the opposite poles k. Telophase II and Cytokinesis:
i. Nuclei form, chromosomes begin decondensing 1. Cytokinesis occurs ii. One parent cell produces four daughter cells with a haploid set of unreplicated chromosomes iii. Each daughter cell is genetically distinct from the other and from the parent cell l. Spermatogenesis- process of producing sperm cells i. Spermatogonia: (2n) are the cells in the testes that will undergo meiosis. 1. Spermatocytes: (2n) - [Meiosis I] 23 pairs of homologues including X and Y 2. Spermatids: (n) - [Meiosis II] 23 chromosomes - one of which is an X or a Y chromosome 3. Spermatozoa: (n): 'streamlined' - cell membrane, nucleus, acrosome, mitochondria, flagella m. Oogenesis: i. Oogonia: (2n) 2 million are formed in a baby girl before birth! ii. Oocytes: (2n) - [Meiosis I] 23 pairs of homologues including 2Xs iii. Oocyte: (n) - [Meiosis II] 23 chromosomes - one of which is an X iv. 1 ovum (n) + 3 polar bodies (n) - the 3 polar bodies disintegrate. The 1 ovum gets all the resources (cytoplasm, mitochondria) and may get fertilized. V. Chapter 13.4: Genetic variation produced in sexual life cycles contributes to evolution: a. Different versions of genes are caused by changes in an organisms DNA which is a mutation. i. Genes are reshuffled during sexual reproduction to produce variations b. Origins of Genetic Variation Among Offspring: i. Behavior of chromosomes during meiosis is responsible for variation ii. Independent assortment of alleles: 1. Homologous pairs of chromosomes are randomly arranged during metaphase I a. 50% chance the offspring will get the mothers or fathers chromosome b. each chromosome is sorted independent of the others during meiosis one 2. each daughter cell represents one outcome of a possible four a. 2n= 4 b. the number of possible combinations when chromosome sort independently during meiosis is 2 n, where n = the haploid number of the organism
i. example: n = 3 there are eight combinations of chromosomes ii. example: humans= n= 23= 2 23 = 8 x 10 6 c. each gamete that you produce in life contains roughly one in 8 x 10 6 iii. Crossing Over: 1. Recombinant chromosomes: individual chromosomes that carry genes derived from two different parents a. Genetic linkage- the tendency of genes on the same chromosome to be inherited together b. Some genetic traits segregate together 2. Begins very early in prophase I when homologous chromosomes pair a. Each gene is aligned with the corresponding gene on the other chromosome 3. Occurs when one maternal and paternal chromatid of a homologous pair are broken at the same place and then rejoined to each others DNA a. This occurs on non- sister chromatids b. Breaks in the chromosome are random c. Two homologous sections trade places d. Chromosomes with new combinations of maternal and paternal genes 4. In humans an average of one to two cross overs occurs per chromosome pair a. Depends on the size of the chromosome b. Depends on the position of the centromere 5. In some species, crossing over may be essential for synapsis and proper assortment of chromosomes in meiosis I 6. Crossing over is used for gene mapping, measuring the position or locus of a gene on the chromosome a. The more frequently recombination occurs between to genes, the further they are apart b. Recombination frequency: the frequency of exchange between two points along the length of a chromosome i. Proportional to the distance in base pairs separating the two points iv. Random Fertilization: the fusion of a male and female human gamete will produce about 64 trillion diploid combinations 1. Not including the recombinant process which would increase this number tremendously c. Evolutionary Significance of Genetic Variation Within Populations d. Summary of mitosis vs meiosis:
VI. Comparing Prokaryotic and Eukaryotic genetic variation: a. Prokaryotes: i. Transformation or the uptake of DNA: 1. Griffins experiment a. S strain vs. R strain ii. Transduction: viral transmission of genetic information 1. Transferring bacterial DNA via a virus from one host cell to another iii. Transposition: movement of DNA segments between and with DNA molecules 1. Involves DNA intermediate