Post on 02-Jan-2016
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
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ASEXUAL LIFE CYCLE VIDEO
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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
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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
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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
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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
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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
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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
• 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
MEIOSIS I MITOSIS
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