1 Chapter 13 –Is Sex Necessary? Well, is it??

2 The Benefits of Sex A novel assortment of genes Starting with meiosis, cell fusion, the diploid state, and meiosis again in the new individual, old combinations of genes are broken up and new combinations are created. Diploid Phase (well, I guess this isn’t really a benefit but it is worth remembering). A competitive advantage in a variable environment. And whatever the benefits of sex really are, all complex, present-day organisms have evolved through sexual rather than asexual means. In Vitro Human Fertilization

3 Diploid zygote is very transient in lower eukaryotes as opposed to the higher euks which have a dominant diploid state. Primitive plants such as mosses and ferns, euks, have a dominant haploid state.

4 That’s right!!! It’s a clam egg with hopeful sperm.

5 Summary: ___________ _______________ has been favored by evolution probably because the _________________ recombination of genetic information improves the chances of producing at least some offspring that will ________________ in an unpredictably variable environment. The sexual reproductive cycle involves the alternation of ______________ and _____________ states; diploid cells divide by _____________ to form haploid cells, and the haploid cells from two individuals fuse in pairs at fertilization to form new _________ cells. In the process, genomes are mixed and recombined to produce individuals that inherit novel assortments of genes. Most of the life cycle of higher plants and animals is spent in the _________ phase; only a small proportion of the diploid cells undergo _________ to produce haploid cells (the gametes) and the haploid phase is very _________.

6 Chromosome Theory of Heredity and Meiosis

7 Compare and Contrast one more nuclear division is required in meiosis for a total of 2 to produce the haploid gametes. each diploid cell under- going meiosis produces 4 genetically different haploid cells; mitosis produces 2 genetically identical diploid cells.

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10 Things to note: sister chromatids in each pair are tightly connected along their length by the cohesins. sister chromatids are connected at their centromeres so it is the chiasma plus the connection of the sister chromatids that keep these two duplicated chromosomes together.

11 Bivalent with 3 chiasmata What to note: crossing over does not occur between sister chromatids of the same chromosome.

12 Importance of crossing over: mutants that have no chiasmata fail to separate normally and so resulting gametes contain too many or too few chromosomes.

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14 Well, I can see how similarly shaped chromosomes can pair up but... What about those X and Y chromosomes? Do they pair up also and form chiasmata?  X chromosomes of females: well, they already are of similar size so yes, they pair and segregate normally.  X and Y

15 The Crossover Event requires a “meiotic endonuclease.”  Crossover Interference

16 Summary: The formation of both eggs and sperm begins in a similar way, with the process called ______________. In this process ______ (a number) successive cell divisions following one round of DNA replication give rise to ______ (a number) haploid cells from a single diploid cell. Meiosis is dominated by prophase of meiotic division I, which can occupy 90% or more of the total meiotic period. As it enters this prophase, each chromosome consists of two tightly joined _________ _____________. The two replicated homologs present in each diploid nucleus then pair to form a bivalent, consisting of ______ ___________________. Chromosomal crossover events occur during this time. Each results in the formation of a ______________, which helps hold each pair of homologs together during ______________ I. Crossing-over has an important role in _____________ genes during gamete formation, and it allows geneticists to map the relative positions of genes on chromosomes. The pairing of homologs culminates in the formation of a structure called a synaptonemal complex, which somehow serves to spread out the crossover events along the chromosomes. At anaphase of the first meiotic cell division, the _____ of the sister chromatids suddenly become unglued, causing one member of each chromosome pair, still composed of a pair of sister chromatids linked at their _________________, to be distributed to each daughter nucleus. A _____________ cell division cycle, without DNA replication, then rapidly ensues; in anaphase II, each sister chromatid separates from its sister and is segregated into a separate haploid nucleus.

17 Sex Determination in Mammals Primordial Germ Cells Migrate into the Developing Gonad Most animals, the unfertilized egg is asymmetrical. Upon fertilization and development of the embryo, specific molecules in the egg’s cytoplasm become primordial germ cells. In mammals... Totipotent Genital Ridge of the gonad

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19 So It’s Not Temperature After All? The Sry gene on the Y chromosome can redirect a female embryo to become a male. Individuals with a Y chromosome will as males no matter how many X chromosomes they have. Individuals without a Y chromosome develop as females, even if they have only one X chromosome. The Y chromosome induces the cells of the genital ridge to develop into a testis instead of an ovary. Sry gene or Sex-determining region of Y When this gene is introduced into the genome of an XX mouse zygote, the transgenic embryo produced a male; but no sperm production.

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