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Transcript
INTRODUCTION TO HEREDITY • HEREDITY = CONTINUITY OF BIOLOGICAL TRAITS FROM ONE GENERATION TO THE NEXT • VARIATION = INHERITED DIFFERENCES AMONG INDIVIDUALS OF THE SAME SPECIES • GENETICS = THE STUDY OF HEREDITY AND VARIATION HEREDITY • OFFPRING ACQUIRE GENES FROM PARENTS BY INHERITING CHROMOSOMES • DNA = TYPE OF NUCLEIC ACID • GENES = UNITS OF HEREDITARY INFO THAT ARE MADE OF DNA AND ARE LOCATED ON CHROMOSOMES HEREDITY • THE ACTUAL TRANSMISSION OF GENES FROM PARENTS TO OFFSPRING DEPENDS ON THE BEHAVIOR OF CHROMOSOMES • CHROMOSOMES = ORGANIZATIONAL UNIT OF HEREDITY MATERIAL IN THE NUCLEUS OF EUKARYOTIC ORGANISMS • LOCUS = SPECIFIC LOCATION ON A CHROMOSOME THAT CONTAINS A GENE THE ROLE OF MEIOSIS IN SEXUAL LIFE CYCLES • FERTILIZATION AND MEIOSIS ALTERNATE IN SEXUAL LIFE CYCLES • ALTERNATION BETWEEN HAPLOID (n) AND DIPLOID (2n) CONDITION • SOMATIC CELLS - CONTAIN 46 CHROMOSOMES • KARYOTYPE = A DISPLAY OF AN INDIVIDUAL’S SOMATIC-CELL METAPHASE CHROMOSOMES SEXUAL LIFE CYCLES • HOMOLOGOUS CHROMOSOMES = A PAIR OF CHROMOSOMES THAT HAVE THE SAME SIZE, CENTROMERE POSITION AND STAINING PATTERN • AUTOSOME - A CHROMOSOME THAT IS NOT A SEX CHROMOSOME (22 PAIRS IN HUMANS) • SEX CHROMOSOMES = IT IS THE 23RD CHROMOSOME PAIR IN HUMANS • FEMALES (XX) • MALES (XY) ALTERNATION OF GENERATIONS • DIPLOID = CELLS CONTAIN TWO SETS OF CHROMOSOMES (2n);ALL AUTOSOMES • HAPLOID = CELLS CONTAIN ONE SET OF CHROMOSOMES(n); GAMETES • GAMETE = SEX CELLS; SPERM AND OVA FERTILIZATION • THE DIPLOID NUMBER IS RESTORED DURING FERTILIZATION, THE UNION OF TWO GAMETES • ZYGOTE = A DIPLOID CELL FORMED FROM THE UNION OF TWO HAPLOID GAMETES SEXUAL LIFE CYCLE VIDEO QuickTime™ and a Cine pak decomp ress or are nee ded to s ee this picture. ASEXUAL LIFE CYCLE VIDEO QuickTime™ and a Cine pak decomp ress or are nee ded to s ee this picture. MEIOSIS: A CLOSER LOOK • MEIOSIS AND SEXUAL REPRODUCTION GREATLY CONTRIBUTE TO GENETIC VARIATION AMONG OFFSPRING • MEIOSIS INCLUDES STEPS THAT CLOSELY RESEMBLE MITOSIS, SO YOU ALREADY KNOW A LOT OF THIS! MEIOSIS V. MITOSIS • LIKE MITOSIS, MEIOSIS IS PRECEDED BY REPLICATION OF CHROMOSOMES • MEIOSIS DIFFERS IN THAT THIS SINGLE REPLICATION IS FOLLOWED BY TWO CONSECUTIVE CELL DIVISIONS • FOUR DAUGHTER CELLS ARE FORMED DURING MEIOSIS • THE DAUGHTER CELLS ARE HAPLOID, THEY ONLY HAVE HALF THE NUMBER OF CHROMOSOMES AS THE ORIGINAL CELL MEIOSIS-INTERPHASE I • CHROMOSOMES REPLICATE • EACH DUPLICATED CHROMOSOME CONSISTS OF TWO IDENTICAL SISTER CHROMATIDS ATTACHED AT THEIR CENTROMERES • CENTRIOLE PAIRS IN ANIMAL CELLS ALSO REPLICATE INTO TWO PAIRS MEIOSIS I • THIS CELL DIVISION SEGREGATES THE TWO CHROMOSOMES OF EACH HOMOLOGOUS PAIR AND REDUCES THE CHROMOSOME NUMBER BY ONE-HALF. IT INCLUDES THE FOLLOWING FOUR PHASES: – PROPHASE I, METAPHASE I, ANAPHASE I, TELOPHASE I PROPHASE I • THIS PHASE TAKES UP 90% OF THE TIME REQUIRED FOR MEIOSIS • CHROMOSOMES CONDENSE • SYNAPSIS OCCURS: HOMOLOGOUS CHROMOSOMES COME TOGETHER AS PAIRS • SINCE EACH CHROMOSOME HAS TWO CHROMATIDS, EACH HOMOLOGOUS PAIR IN SYNAPSIS APPEARS AS A COMPLEX OF FOUR CHROMATIDS, OR A TETRAD PROPHASE I (CON’T) • IN EACH TETRAD, SISTER CHROMATIDS OF THE SAME CHROMOSOME ARE ATTACHED AT THEIR CENTROMERES. NONSISTER CHROMATIDS ARE LINKED BY X-SHAPED CHAISMATA, SITES WHERE HOMOLOGOUS STRAND EXCHANGE OR CROSSING OVER OCCURS. • CROSSING OVER IS VERY IMPORTANT. THIS EXCHANGE OF GENETIC MATERIALS ALLOWS FOR GENETIC VARIATION AMONG OFFSPRING PROPHASE I VIDEO QuickTime™ and a Cine pak decomp ress or are nee ded to s ee this picture. METAPHASE I • TETRADS ARE ALIGNED ON THE METAPHASE PLATE • EACH SYNAPTIC PAIR IS ALIGNED SO THAT CENTROMERES OF HOMOLOGUES POINT TOWARD OPPOSITE POLES • EACH HOMOLOGUE IS ATTACHED TO KINETOCHORE MICROTUBULES EMERGING FROM THE POLE IT FACES, SO THAT THE TWO HOMOLOGUES SEPARATE IN ANAPHASE AND MOVE TOWARD OPPOSITE POLES METAPHASE I VIDEO QuickTime™ and a Cine pak decomp ress or are nee ded to s ee this picture. ANAPHASE I • HOMOLOGUES SEPARATE AND ARE MOVED TOWARD THE POLES BY THE SPINDLE • SISTER CHROMATIDS REMAIN ATTACHED AT THEIR CENTROMERES AND MOVE AS A UNIT TOWARD THE SAME POLE, WHILE THE HOMOLOGUE MOVES TO OTHER POLE • THIS DIFFERS FROM MITOSIS DURING WHICH CHROMOSOMES LINE UP INDIVIDUALLY ON METAPHASE PLATE, AND SISTER CHROMATIDS ARE SPLIT APART ANAPHASE I VIDEO QuickTime™ and a Cine pak decomp ress or are nee ded to s ee this picture. TELOPHASE I AND CYTOKINESIS • EACH POLE NOW HAS A HAPLOID SET OF CHROMOSOMES THAT ARE STILL MADE OF TWO SISTER CHROMATIDS ATTACHED AT THE CENTROMERE • CYTOKINESIS OCCURS, FORMING TWO HAPLOID DAUGHTER CELLS. CLEAVAGE FURROWS FORM IN ANIMAL CELLS TELOPHASE I VIDEO QuickTime™ and a Cine pak decomp ress or are nee ded to s ee this picture. MEIOSIS II • THERE IS NO INTERPHASE II. THE REASON IS THAT THE DNA WILL NOT BE DUPLICATED AGAIN BEFORE THE SECOND CELL DIVISION • THIS SECOND MEIOTIC DIVISION SEPARATES SISTER CHROMATIDS OF EACH CHROMOSOME PROPHASE II • SPINDLE APPARATUS FORMS AND CHROMOSOMES MOVE TOWARD THE METAPHASE II PLATE METAPHASE II • CHROMOSOMES ALIGN SINGLY ON THE METAPHASE PLATE • KINETOCHORES OF SISTER CHROMATIDS POINT TOWARD OPPOSITE POLES ANAPHASE II • CENTROMERES OF SISTER CHROMATIDS SEPARATE • SISTER CHROMATIDS OF EACH PAIR (NOW INDIVIDUAL CHROMOSOMES) MOVE TOWARD OPPOSITE POLES OF THE CELL TELOPHASE II • NUCLEI FORM AT OPPOSITE POLES OF THE CELL • CYTOKINESIS OCCURS PRODUCING FOUR HAPLOID DAUGHTER CELLS MEIOSIS II VIDEO QuickTime™ and a Cine pak decomp ress or are nee ded to s ee this picture. KEY DIFFERENCES: MEIOSIS AND MITOSIS • MEIOSIS IS A REDUCTION DIVISION. CELLS PRODUCED BY MITOSIS HAVE THE SAME # AS THE ORIGINAL CELL, WHEREAS CELLS MADE BY MEIOSIS HAVE HALF THE # OF CHROMOSOMES • MEIOSIS CREATES GENETIC VARIATION DUE TO CROSSING OVER, WHICH IS ABSENT IN MITOSIS • MEIOSIS IS TWO SUCCESSIVE NUCLEAR DIVISIONS, WHEREAS MITOSIS IS JUST ONE DIVISION COMPARISON: MEIOSIS I AND MITOSIS MEIOSIS I MITOSIS • PROPHASE: SYNAPSIS OCCURS TO FORM TETRADS • METAPHASE: HOMOLOGOUS PAIRS (TETRADS) ALIGN • ANAPHASE: PAIRS OF CHROMOSOMES SEPARATED. CENTROMERE DO NOT DIVIDE, SISTER CHROMATIDS STAY TOGETHER, MOVE TO SAME POLE • PROPHASE: NO SYNAPSIS OR CROSSING OVER • METAPHASE: INDIVIDUAL CHROMOSOMES ALIGN • ANAPHASE: SISTER CHROMATIDS OF INDIVIDUAL CHROMOSOMES SEPARATE AND MOVE TO OPPOSITE POLES ORIGINS OF GENETIC VARIATION • SEXUAL REPRODUCTION CONTRIBUTES TO GENETIC VARIATION BY: – INDEPENDENT ASSORTMENT OF CHROMOSOMES – CROSSING OVER IN MEIOSIS I – RANDOM FUSION OF GAMETES DURING FERTILIZATION INDEPENDENT ASSORTMENT • DURING METAPHASE I, EACH HOMOLOGOUS PAIR OF CHROMOSOMES ALIGNS. EACH PAIR CONSISTS OF ONE MATERNAL AND ONE PATERNAL CHROMOSOME • THERE IS A 50-50 CHANCE THAT A DAUGHTER CELL WILL RECEIVE EITHER MATERNAL OR PATERNAL CHROMOSOME • EACH HOMOLOGOUS PAIR ORIENTS INDEPENDENTLY INDEPENDENT ASSORT (CON’T) • SINCE EACH PAIR ASSORTS INDEPENDENTLY, THE PROCESS PRODUCES 2n POSSIBLE COMBINATIONS OF MATERNAL AND PATERNAL CHROMOSOMES IN GAMETES, WHERE n IS THE HAPLOID # • IN HUMANS, THE POSSIBLE COMBINATIONS WOULD BE 223, OR ABOUT 8 MILLION • THUS, EACH HUMAN GAMETE CONTAINS ONE OF EIGHT MILLION POSSIBLE ASSORTMENTS OF CHROMOSOMES INHERITED FROM BOTH PARENTS INDEPENDENT ASSORTMENT CROSSING OVER • OCCURS DURING PROPHASE I, CHAISMATA BECOME VISIBLE AT PLACES WHERE THE EXCHANGE OCCURS • PRODUCES CHROMOSOMES THAT CONTAIN GENES FROM BOTH PARENTS • IN HUMANS, THERE IS AN AVERAGE OF TWO OR THREE CROSSOVER PER CHROMOSOME PAIR • SYNAPSIS IS PRECISE, HOMOLOGUES ALIGN GENE BY GENE CROSSING OVER RANDOM FERTILIZATION • IN HUMANS, WHEN INDIVIDUAL OVUM REPRESENTATIVE OF ONE OF EIGHT MILLION POSSIBLE COMBINATIONS IS FERTILIZED BY A SPERM CELL WITH THE SAME POSSIBILITIES, THE RESULTING ZYGOTE CAN HAVE ONE OF 64 TRILLION POSSIBLE DIPLOID COMBINATIONS