Variation in TraitsTwo important mechanisms that occur
during meiosis introduces variation in traits among offspring:
Crossing Over in Prophase IDuring prophase I, homologous
chromosomes pair upEach chromosome and its homologue
___________ together so that they are tightly aligned
Nonsister chromatids exchange segments in a process called _____________ _________Exchange a segment of the DNA
molecule at the same place along their length
Crossing Over in Prophase IBecause alleles for the same trait can vary,
crossing over allows chromosomes to swap certain genes
Results in a new combination of alleles not present in either parental chromosome, which is one source of genetic variation
Leads to __________ _______________Crossing over is a common event
In each cycle of meiosis in humans:
This rate varies among species and chromosomes
Metaphase I AlignmentsDuring metaphase I, homologous
chromosomes ____________ line up at the spindle equatorMicrotubules attach to the first chromosome
they contact, regardless of whether it is maternal or paternal
During anaphase I, the homologous chromosomes separate into two haploid cellsEach cell has a __________________________________________________________________________
This chromosome shuffling is another source of genetic variation for each new generation
Metaphase I AlignmentFor a species with just three pairs of
homologues:Eight possible combinations (23 ) for
possible gametes That means that in humans, a sperm or
egg has:
From Gametes to OffspringEvery species has a distinctive life cycleGametes play an important role in these
life cycles and bridge generations of individuals
For now, we are going to focus on the life cycle of plants and the life cycle of animals
Gamete Formation in PlantsPlant cycles usually involve two kinds of
multicelled bodies_____________________ are multicelled plant bodies
(diploid) that produce haploid sporesSpores are haploid cells that undergo mitosis
and give rise to multicelled haploid __________________
Haploid gametes-producing bodies are formed in gametophytes, and gametes are formed
Gamete-producing bodies and spore-producing bodies develop during the life cycle of plants
Example: Pine trees are sporophytesMale and female gametophytes develop in
different types of pine cones on each tree
Gamete Formation in Animals: ____In males: meiosis and gamete formation is
called spermatogenesisSequence of events in sperm formation:
1. Germ cells (2n)2. Primary spermatocyte (2n)3. MEIOSIS I4. Two secondary spermatocytes (n)5. MEIOSIS II6. Four spermatids (n)
Each __________ develops a tail to become a mature sperm
Gamete Formation in Animals: _______In females: meiosis and gamete formation are called
________Sequence of events in oogensis:
1. Germ cell (2n)2. Primary oocyte (2n)3. MEIOSIS I4. Secondary oocyte (n, and large in size) +
polar body (n, and small in size)
5. MEIOSIS II6. One large ovum (n) plus three polar bodies (n,
small)The single ovum (egg) is the ONLY cell capable of
being fertilized by a spermThe polar bodies wither and die
FertilizationThe chromosome number of the parent is
restored at __________________Fertilization is the fusion of two haploid nuclei of
two gametesOf all the genetically diverse gametes produced,
________ will determine which two will meet at fertilization
Many gametes are formed, but only two are needed to produce a zygote
Meiosis allows species to maintain their chromosome number; without it, an offspring would have twice as many chromosomes as their parent
Variation in OffspringThree events cause new combinations of
alleles in offspring: Crossing over during prophase I (meiosis)Random alignment of maternal and paternal
chromosomes at metaphase I (meiosis)Chance meeting of gametes at
fertilization
All three contribute to variation in traits of the offspring
Comparing Mitosis and Meiosis
Results in clonesOccurs in single-celled
eukaryotes as a means of asexual reproduction
All eukaryotes engage in mitosis for growth and tissue repair
Mitosis MeiosisOccurs in
immature reproductive cells giving rise to gametes
Sexual reproduction
Both processes of meiosis and fertilization give rise to genetic variation
An Ancestral Connection?Despite the differences, there are many similarities
in the steps of mitosis and meiosisThe molecular machinery of mitosis may have been
recruited for meiosisFor example:
Proteins the recognize and repair breaks in DNA are used in both
If the separation of anaphase in mitosis did not occur properly, the result would be that of anaphase I in meiosis
Did sexual reproduction result through mutations that caused changes in the mechanisms important before and during mitosis?
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