Transcript Fig. 10-5, p. 158
Chapter 10
Meiosis and Sexual Reproduction
Objectives
1. Contrast asexual and sexual types of reproduction that occur on the cellular and multicellular organism levels.
2. Understand the effect that meiosis has on chromosome number.
3. Describe the events that occur in each meiotic phase.
Objectives
4. Compare mitosis and meiosis; cite similarities and differences.
5. Contrast meiosis in plant and animal life cycles.
10.0
Why Sex
Asexual reproduction is easier and faster One parent alone transmits genetic information to offspring. (all clones) Sexual reproduction can be an alternative adaption in changing environments. (survival) Male and female must find each other and exchange genetic material.
Why Sex
Sexual reproduction has advantages when other organisms change. (Predators and prey, Hosts and pathogens) The outcome of sexual reproduction is offspring that display novel combinations of traits.(diversity)
10.1 Alleles and Sexual Reproduction Sexual Reproduction involves Meiosis Gamete production Fertilization Produces genetic variation among offspring
Introducing Alleles
Allele – each unique molecular form of the same gene.
Such tiny differences affect thousands of traits.
Alleles are one reason why individuals do not all look alike.
Sexual reproduction leads to new alleles
Homologous Chromosomes Carry Different Alleles
Cell has two of each chromosome One chromosome in each pair from mother, other from father Paternal and maternal chromosomes carry different alleles
Homologous Chromosomes
Fig. 10-2, p.156
Sexual Reproduction Shuffles Alleles
Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits This variation in traits is the basis for evolutionary change
Alleles
Section 10.2: What Meiosis Does Meiosis is a nuclear division process that divides a parental chromosome number by half in specialized reproductive cells. Sexual reproduction will not work without it.
Unlike mitosis, meiosis sorts out chromosomes into parcels two times.
Germ cells undergo meiosis and cytoplasmic division Meiosis involves only the sex cells.
Cellular descendents of germ cells become gametes, (sperm and egg) Gametes meet at fertilization
Fig. 10-3, p.156
Chromosome Number
Sum total of chromosomes in a cell Germ cells are diploid (2
n
), they have a pair of each type of chromosome. We call them homologous chromosomes.
Gametes are haploid (
n
) Meiosis halves parental chromosome number
Meiosis: Two Divisions
Two consecutive nuclear divisions Meiosis I Meiosis II DNA is not duplicated between divisions – NO Interphase Four haploid nuclei form
Meiosis I – Prophase I, Metaphase I, Anaphase I, Telophase I
Each homologue (matching chromosome) in the cell pairs with its partner, then the partners separate p. 158
Meiosis II - Prophase II, Metaphase II, Anaphase II, Telophase II The two sister chromatids of each duplicated chromosome are separated from each other
two chromosomes (unduplicated) one chromosome (duplicated) p. 158
10.3 Meiosis I -Prophase I
Each duplicated chromosome pairs with homologue Homologues swap segments (crossing over).
Each chromosome becomes attached to spindle
Fig. 10-5, p. 158
Metaphase I
Chromosomes are pushed and pulled into the middle of cell by microtubules The spindle is fully formed
Fig. 10-5, p. 158
Anaphase I
Homologous chromosomes separate and begin to move toward pole.
The sister chromatids remain attached
Fig. 10-5, p. 158
Telophase I
The chromosomes arrive at opposite poles Usually followed by cytoplasmic division.
Now have two haploid cells (n).
Chromosomes are still duplicated.
Fig. 10-5, p. 158
Prophase II
In each daughter cell microtubules attach to the kinetochores of the duplicated chromosomes.
One chromatid of each chromosome becomes tethered to one spindle pole.
Fig. 10-5, p. 158
Metaphase II
In each daughter cell duplicated chromosomes line up at the spindle equator, midway between the poles
Fig. 10-5, p. 158
Anaphase II II
In each daughter cell sister chromatids separate and move toward opposite poles to become independent chromosomes.
Fig. 10-5, p. 158
Telophase II II
The chromosomes arrive at opposite ends of the cell A nuclear envelope forms around each set of chromosomes, each cell divides in half.
Four haploid (n) cells.
Fig. 10-5, p. 158
Section 10.4: How Meiosis Introduces Variations in Traits Crossing over partner.
– a molecular interaction between a chromatid of one chromosome and a chromatid of the homologous This really is gene swapping.
Crossing Over
• During Prophase I each chromosome becomes zippered to its homologue •All four chromatids are closely aligned •Nonsister chromosomes exchange segments
Effect of Crossing Over
After crossing over, each chromosome contains both maternal and paternal segments Breaks up old combinations of alleles and creates new allele combinations in offspring
Random Alignment
During transition between prophase I and metaphase I, microtubules from spindle poles attach to kinetochores of chromosomes. Initial contacts between microtubules and chromosomes are random, there is no particular pattern to the metaphase position of chromosomes.
Random Alignment
Either the maternal or paternal member of a homologous pair can end up at either pole. This can also lead to different traits in each new generation.
The chromosomes in a gamete are a mix of chromosomes from the two parents.
Possible Chromosome Combinations
As a result of random alignment, the number of possible combinations of chromosomes in a gamete is: 2
n
(
n
is number of chromosome types)
Possible Chromosome Combinations
Thus, every time a human sperm or egg forms, there is a total of 8,388,608 or 2 23 Possible combinations of maternal and paternal chromosomes.
Section 10.5: From Gametes to Offspring The life cycle of most plant species alternates between sporophyte and gametophyte stages.
A sporophyte is a spore producing body that makes spores by the process of meiosis.
A spore is a haploid reproductive cell that undergoes mitosis and gives rise to a gametophyte.
A gametophyte gives rise to gametes, which can then be fertilized and form the zygote.
Plant Life Cycle
sporophyte zygote fertilization gametes
diploid haploid
gametophytes meiosis spores Fig. 10-8a, p.162
Gamete formation in animals
In the male reproductive system, a germ cell develops into four haploid cells, each becoming a sperm.
In the female reproductive system, a germ cells develops into one haploid ovum, or egg, and three polar bodies. The polar bodies eventually degenerate.
When fertilization occurs the diploid number is restored.
Animal Life Cycle
zygote fertilization multicelled body
diploid haploid
gametes meiosis Fig. 10-8b, p.162
Fertilization
Male and female gametes unite and nuclei fuse Fusion of two haploid nuclei produces diploid nucleus in the zygote Which two gametes unite is random Adds to variation among offspring
Factors Contributing to Variation among Offspring
Crossing over during prophase I (average of 2 or 3 in every human chromosome) Random alignment of chromosomes at metaphase I Random combination of gametes at fertilization
10.6 Mitosis & Meiosis Compared
Mitosis Functions Asexual reproduction Growth, repair Occurs in somatic cells Produces clones Meiosis Function Sexual reproduction Occurs in germ cells Produces variable offspring
Prophase vs. Prophase I
Prophase (Mitosis) Homologous pairs do not interact with each other Prophase I (Meiosis) Homologous pairs become zippered together and crossing over occurs
Anaphase, Anaphase I, and Anaphase II
Anaphase I (Meiosis) Homologous chromosomes separate from each other Anaphase/Anaphase II (Mitosis/Meiosis) Sister chromatids of a chromosome separate from each other
Results of Mitosis and Meiosis
Mitosis Two diploid cells produced Each identical to parent Meiosis Four haploid cells produced Differ from parent and one another
Repair of DNA breaks
Checkpoint genes code for proteins that can recognize and repair breaks in the double-stranded DNA molecules of chromosomes.
If they detect a problem, there is a pause in the cycle until the DNA is repaired.
Review of Meiosis
http://highered.mcgraw hill.com/sites/0072437316/student_view0/ch apter12/animations.html#
An Ancestral Connection
Was sexual reproduction a giant evolutionary step from aseuxal reproduction?
Giardia intestinalis – single-celled parasite, does not have a mitochondria, does not form a spindle during mitosis, and has never been observed to reproduce sexually.
Chlamydomonas – a single-celled alga, haploid cells reproduce asexually by mitosis. They can also fuse and form diploid individuals.