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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
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