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Meiosis and Sexual Life Cycles

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1 Meiosis and Sexual Life Cycles
Chapter 13 Meiosis and Sexual Life Cycles

2 Inheritance/Heredity
When traits are passed down from one generation to the next, we say they are inherited. The offspring differ from one generation to the next and we use the term variation to describe these differences.

3 Genetics When people study genetics, they are studying the heredity and inherited variations. For example, when we say we have our parent’s nose or eyes, we are actually saying that we have their segments of DNA (genes)

4 Genes Genes are regions of our genome that program the specific traits we see (phenotype) such as eye color or nasal appearance.

5 So, how does this happen? The exact replication of our genes/DNA in our gametes which produces copies that can be passed from one generation to the next.

6 So, how does this happen? Remember, our gametes are the sperm and egg that unite during fertilization and give rise to the new individuals.

7 So, how does this happen? The new individuals now have traits of both parents. Remember, the chromosome that is inherited is tightly coiled and contains a few hundred to a few thousand genes.

8 The Locus The locus is the region of the chromosome where a particular gene is located.

9 Modes of Reproduction When discussing heredity, there are a couple of modes of reproduction that need to be discussed. Sexual and asexual.

10 Sexual Reproduction 2 parents “mate” and give rise to offspring that have a variety of combinations of genes which are inherited by the new offspring. They are not genetically identical to the parents.

11 Asexual Reproduction A single parent gives rise to the offspring.
In the process, all of the parent’s genes are passed on to the offspring and a clone is formed.

12 Asexual Reproduction A hydra budding is an example of asexual reproduction. Movie

13 The Sexual Lifecycle--Humans
Somatic cells are the cells other than the sperm and egg. Each one contains 46 c-somes (23 pairs, 1 pair from each parent). Each parent contributes 22 autosomes and 1 sex c-some.

14 Homologous Chromosomes
Homologous c-somes are c-somes of the same number. Each homologous c-some contains genes that control the same inherited characteristic. For example, if the gene that codes for height is found on one c-some, it is found at the exact same locus on the homologous c-some.


16 Homologous Chromosomes--Sort Of…
The only two c-somes that don’t really fit the idea of being homologous are the sex c-somes, the X and the Y. The two sets of genes on these c-somes are more different than they are alike. They determine the sex of the individual: XX-female; XY-male

17 Chromosome Number-Autosomes
When discussing c-some number, we use an “n” to represent this idea. Any cell having both copies of c-somes from the parent are said to be diploid which is 2n. In humans, 2n=46.

18 Chromosome Number-Sex Chromosomes
The gametes of sexually reproducing organisms contain a single c-some set and are said to be haploid. n=23 (22 autosomes + 1 sex c-some).


20 Fertilization When 2 haploid cells fuse together, a diploid zygote is formed. The zygote now contains a mix of genes from the parents. It will divide mitotically and give rise to a mature organism.

21 Sexual Maturity As the organism reaches sexual maturity, its gonads will begin producing haploid cells through a process called meiosis.

22 Meiosis Meiosis will allow the chromosome number to remain constant throughout the generations.

23 Meiosis and Fertilization
The process of meiosis and fertilization are common to sexually reproducing organisms.

24 3 Main Types of Life Cycles in Sexually Reproducing Organisms
1. Animals 2. Plants and some algae 3. Fungi and some protists

25 Animals In animals, meiosis occurs in the production of gametes and the gametes are the only haploid cells. The gametes unit during fertilization. Mitosis gives rise to a diploid, multicellular organism.

26 Plants and Some Algae Alternation of Generations
A complex reproductive event where a plant gives rise to spores which gives rise to gametophytes. The gametophytes give rise to gametes that fertilize forming a zygote that becomes a new plant.

27 Most Fungi and Some Protists
The adult is a haploid, multicellular organism. Mitosis produces gametes that fertilize forming a zygote. Meiosis then produces haploid cells which divide mitotically to form another haploid, multicellular organism.


29 3 Main Types of Life Cycles in Sexually Reproducing Organisms
All 3 result in meiosis and mitosis. All 3 accomplish the same thing: chromosome halving and doubling which results in genetic variation among offspring.

30 Meiosis Meiosis reduces the ploidy. Provides for genetic variation.
Keeps the c-some number constant through the generations.

31 Meiosis Meiosis can be broken into two parts: Meiosis I and Meiosis II. Meiosis I separates homologous c-somes. Meiosis II is essentially a mitotic division.

32 Meiosis Meiosis halves the number of chromosomes.


34 Meiosis I Prophase I: Homologous c-somes condense; crossing-over takes place (a source of variation; nuclear envelope breaks down Movie

35 Meiosis I Metaphase I: Homologous pairs of c-somes line up on the metaphase plate. Movie

36 Meiosis I Anaphase I: The c-somes (each now having 2 chromatids) of each homologous pair separate and move to the opposite poles of the “cell.” Movie

37 Meiosis I Telophase I: C-somes arrive at the spindle poles.
Cytokinesis: Cytoplasm divides producing 2 cells, each having 1/2 the original number of c-somes. Movie

38 Interkinesis In some cells, the c-somes relax and the nuclear envelope reforms. Other cells go directly to meiosis II.


40 Meiosis II Sister Chromatids Separate
Prophase II: Chromosome condense again, the nuclear envelope disappears and the mitotic spindle reforms. Movie

41 Meiosis II Sister Chromatids Separate
Metaphase II: C-somes line along the metaphase plate. Movie

42 Meiosis II Sister Chromatids Separate
Anaphase II: Sister chromatids separate and migrate as individual c-somes to the spindle poles. Movie

43 Meiosis II Sister Chromatids Separate
Telophase II: C-somes arrive at the poles, the spindle breaks down, the nuclear envelope reforms. Cytokinesis: The cytoplasm divides. Movie

44 Mitosis and Meiosis, A Comparison
The biggest difference is that in mitosis, genetically identical cells are formed and ploidy is conserved. In meiosis, genetically different cells are formed and the ploidy is reduced.


46 3 Main Events Important to Meiosis
1. Synapsis and Crossing Over 2. Tetrad Formation 3. Separation of Homologues

47 1. Synapsis and Crossing Over
In prophase I, duplicated homologues line up and become physically attached. Crossing over occurs where genetic information is rearranged between the non-sister chromatids comprising the tetrad. An “X” shaped region appears called the chiasma at the site of crossing over.


49 2. Tetrad Formation The paired tetrads align along the metaphase (I) plate rather than the individual replicated c-somes (as in mitosis).

50 3. Separation of Homologues
At anaphase I of meiosis, the duplicated c-somes of each homologous pair move to opposite poles, sister chromatids remain attached (instead of separating as they do in mitosis).

51 3 Mechanisms that Produce Genetic Variation
1. Independent Assortment 2. Crossing Over 3. Random Fertilization

52 1. Independent Assortment
Homologous c-somes can be positioned independently at the metaphase plate. Maternal and paternal c-somes sort independently of every other pair.

53 1. Independent Assortment
Independent assortment give a large number of possible daughter cells. The cells are dividing in two and are sorting independently. The number of combinations is 2n. “n” is the haploid number (23 in humans) 223 possible combinations (8,000,000+)

54 2. Crossing Over Gene mixing. Non-sister chromatids trade places.
Taking crossing over into account makes the number of possible combinations endless.

55 3. Random Fertilization The different combinations of gametes produced during meiosis will randomly fertilize upon mating. This produces the endless amount of gamete combinations.

56 Movie


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