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

Crossing Over

Figure 9.6, pg 144

Crossing Over

Figure 9.6, pg 144

Figure 9.6, pg 144

Crossing Over

Resulting chromosome containing a new combination of alleles

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 Alignment

combinations possible

1 2 3

or

or

or

Figure 9.7, pg 145

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

_______ Life Cycle

Figure 9.8, pg 146

Plant and Animal Life Cycles

Figure 9.8, pg 146

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

__________________ :

Figure 9.9, pg 147

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

_________________:

Figure 9.9, pg 147

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?

Chapter 9 HomeworkSelf Quiz 1-9Critical Thinking: 1, 3, and 5 (for

only the Fruit fly)