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 Cinepak decompressor are needed to see this picture.

ASEXUAL LIFE CYCLE VIDEO

QuickTime™ and a Cinepak decompressor are needed to see 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 Cinepak decompressor are needed to see 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 Cinepak decompressor are needed to see 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 Cinepak decompressor are needed to see 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 Cinepak decompressor are needed to see 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 Cinepak decompressor are needed to see 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

• 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