Biologych9

Biologych9 - 9.1 An Evolutionary View Asexual reproduction...

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Unformatted text preview: 9.1 An Evolutionary View Asexual reproduction one alone produces offspring; each inherits same number and kinds of genes as parent = clones Genes stretches of chromosomes DNA molecules; contain all heritable info necessary to make new individuals Sexual reproduction process involving meiosis, formation of gametes, and fertilization union of nuclei of two gametes - In m o s t s e x u al r e pr o d u c e r s, first c ell of n e w in divid u al c o n t ai n s p air s of g e n e s o n p air s of c hr o m o s o m e s (o n e m a t e r n al, o n e p a t e r n al) - Two g e n e s of a p air m a y n o t b e id e n tic al b/ c g e n e' s m ol e c ul ar s t r u c t ur e c a n m u t a t e s o t h a t t w o g e n e s p air e d in a p e r s o n's c ells m a y " s a y" diff er e n t t hi n g s Allel e e a c h u ni q u e m ol e c ul ar for m of t h e s a m e g e n e * By s e x u al r e pr o d u c tio n, offs prin g in h e rit n e w c o m b o s of all el e s, le a di n g t o * Vari a tio n in tr ait s is a fou n d a tio n for e v ol u tio n a r y c h a n g e 9.2 Overview of Meiosis Partitions chromosomes into parcels TWICE prior to cytoplasmic division First step leading to formation of gametes Homologous chromosomes pair of chromosomes with same length, shape and assortment of genes that line up with each other at meiosis - Body c ells of h u m a n s h a v e (2 3 + 2 3 ) h o m olo g o u s c hr o m o s o m e s ; s o d o g e r m c ells t h a t giv e r human gametes - Aft er g e r m c ell finis h e s m ei o sis, (2 3) c hr o m o s o m e s will e n d u p in t h o s e g a m e t e s , givin g it a h a ploid n u m b e r (n) As in mi t o sis, d u plic a t e s DNA in int e r p h a s e a n d ( 2) DNA m ol e c ul e s a n d a s s o ci a t e d p r o t ei n s a t t a c h e d a t c e n tr o m e r e , d e e mi n g t h e m sis t e r c hr o m a ti d s. Also si mil arly t o m i t o sis, mi cro t u of s pi n dl e a p p a r a t u s m o v e c hr o m o s o m e s in pr e s crib e d dir e c tio n s. DIFFERENTLY h o w e v e r, in m ei o sis c hr o m o s o m e s g o t hr o u g h TWO c o n s e c u tiv e divisio n s t h a t e n d wit h t h e for m a tio n of four h a ploid n u cl ei - Th er e is n o int e r p h a s e b e t w e e n m ei o sis I a n d m ei o sis II In m ei o sis I, e a c h d u plic a t e d c hr o m o s o m e alig n s wit h p a r t n e r, h o m olo g u e t o h o m olo g u e . A t h e t w o c hr o m o s o m e s of e v e r y p air h a v e lin e d u p wit h e a c h o t h e r, t h e y a r e m o v e d a p a r t. - Th e cy t o pl a s m t y pic ally s t a r t s t o divid e a t s o m e p oin t aft e r e a c h h o m olo g u e d e t a c h e s fro m p artner - Th e t w o d a u g h t e r c ells for m e d t his w a y a r e h a ploid, wit h o n e of e a c h t y p e of c hr o m o s o m e In m ei o sis II, t h e sis t e r c hr o m a ti d s of e a c h c hr o m o s o m e a r e s e p a r a t e d fro m e a c h o t h e r - Ea c h c hr o m a ti d is n o w a s e p a r a t e c hr o m o s o m e - ( 4) n u cl ei for m a n d cy t o pl a s m divid e s o n c e m o r e - Ou t c o m e is ( 4) h a ploid c ells 9.3 Visual Tour of Meiosis 1. Prophase I: chromosomes become visible as threadlike forms each pairs with its homologue and usually swaps segments with it microtubules forming bipolar spindle and one pair of centrioles moved to opposite side of nuclear envelope 2. Metaphase I: microtubules of each spindle have tethered one of each type of chromosome; tug of war results, aligning chromosomes midway between poles 3. Anaphase I: microtubules attached to each chromosome shorten and move toward poles; other microtubules overlap at equator and push poles father apart (driven by motor proteins) 4. Telophase I: one of each type of chromosomes has now arrived at poles; cytoplasm divides, forming two haploid cells (all chromosomes still duplicated) 5. Prophase II: new bipolar spindle forms in each haploid cell; microtubules have moved one pair of centrioles to opposite end of cell; chromatids of each chromosome becomes tethered to spindle poles 6. Metaphase II: microtubules from both poles have assembled/disassembled in tug-of-war that ends at metaphase II, when all chromosomes positioned midway between poles 7. Anaphase II: attachment b/w sister chromatids breaks; each is now separate chromosome but tethered to microtubules that move it toward spindle pole; one of each type of chromosome is present in each parcel of unduplicated chromosome near each pole 8. Telophase II: four nuclei form as nuclear envelope encloses each cluster of chromosomes; after cytoplasmic division, each of resulting daughter cells has haploid (n) number of chromosomes 9.4 How Meiosis Puts Variation in Traits Pairs of homologous chromosomes swap parts during prophase I and become aligned with partner at metaphase I contribute enormously to variation in traits - Intro d u c e n o v el c o m b o s of all el e s int o g a m e t e s t h a t for m aft e r m ei o sis In p r o p h a s e I, h o m olo g u e s d r a w n clo s e a n d m ol e c ul ar int e r a c tio n s s tit c h t h e m t o g e t h e r p oi p oi nt alo n g le n g t h w/ littl e s p a c e b e t w e e n t h e m - Th e inti m a t e , p a r all el ori e n t a tio n fav or s cr o s si n g o v e r - Th e t w o NON-si s t e r c hr o m a ti d s b r e a k t a t s a m e pl a c e s al o n g le n g t h , t h e n t h e t w o e x c h a n g e c orr e s p o n di n g s e g m e n t s; t h e y s w a p g e n e s (g e n e s c o m e in slig h tly diff er e n t for m s: all el e s) o Nu m b e r of all el e s o n o n e c hr o m o s o m e a r e al m o s t g u a r a n t e e d t o b e diff er e n t fro m p a r t n all el e s o n h o m olo g u e * Ev ery cr o s s ov e r is a c h a n c e t o s w a p slig h tly diff er e n t v e r sio n s of h e r e dit a ry in s tr u c tio n s fo g e n e pr o d u c t s Major s h uffling of int a c t c hr o m o s o m e s s t a r t d u rin g tr a n sitio n fro m pr o p h a s e I t o m e t a p h a s e - As mi cr ot u b ul e s gr ow o u t w ar d fro m p ol e s , t h e y lat c h o n t o t h e first c hr o m o s o m e t h e y c o n t a d u e t o r a n d o m t e t h e rin g, t h e r e is n o p a r tic ul ar p a t t e r n t o m e t a p h a s e I p o sitio n s of m a t e r n a p a t e r n al c hr o m o s o m e s - Aft er a p air of h o m olo g o u s c hr o m o s o m e s a r e m o v e d a p a r t d u rin g a n a p h a s e I, eit h e r o n e of c a n e n d u p a t ei t h e r s pi n dl e p ol e 9.5 From Gametes to Offspring Gametes are not all the same in their details Seasons vary for plants so fertilization must coincide with spring rains and other conditions that favor growth of the new individual life cycles of plants generally alternate between spore and gamete production Spores haploid resting cells, often walled, that develop after meiosis - Origin a t e in r e pr o d u c tiv e s tr u c t ur e s of s p or o p h y t e s (i. e. pi n e tr e e s , c or n pl a n t s) - St a y d or m a n t in d ry/c old s e a s o n s - Upo n r e s u mi n g gr ow t h (g e r mi n a tio n) t h e y u n d e r g o mi t o sis a n d for m g a m e t o p h y t e s g a m e pr o d u ci n g h a ploid b o di e s In a ni m al s, g e r m c ells giv e ris e t o g a m e t e s - In m al e s , a di ploid g e r m c ell d e v elo p s int o a lar g e , i m m a t ur e c ell: p ri m a r y s p e r m a t o c y t e ; t h e n t e r s m ei o si s a n d c yt o pl a s mic divisio n s. Four h a ploid c ells r e s ult a n d d e v elo p int o s p e r m a t h e s e u n d e r g o c h a n g e s t h a t inclu d e for m a tio n of a t ail; e a c h b e c o m e s a s p e r m - In fe m al e s , a g e r m b e c o m e s a n o o c y t e , or im m a t ur e e g g t h a t s t o c k pil e s m a n y cy t o pl a s mic c o m p o n e n t s a n d its four d a u g h t e r c ells diff er in siz e a n d fu n c tio n; divid e s aft e r m ei o si s I, w o n e d a u g h t e r c ell (t h e s e c o n d a r y o o c y t e ) g e t s n e a rly all of t h e cy t o pl a s m w hil e t h e o t h e r c e first p ol ar b o d y, is s m all; lat e r b o t h e n t e r m ei o sis II, t h e n cy t o pl a s mic divisio n; o n e of s e c o n o o c y t e' s d a u g h t e r c ells d e v elo p s int o a s e c o n d p ol ar b o d y w hil e t h e o t h e r r e c eiv e s m o s t of - cytoplasm and develops into a gamete; a ma ture fe male ga mete is called an ovu m or an eg t hree polar bodies t hat for med don't function as gametes and degenerate) Chromosome # restored at fer tilization when m ale/female gametes unite and haploid nucle Fertilization also adds variation among offspring: which ga metes will join is unpredictable During prophase I, each hu man chromosome undergoes average of 2 to 3 crossovers Even without crossovers, random positioning of pairs of paternal/ maternal chromosomes at m e taphase I results in one of m illions of possible combos in each ga mete ...
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