Chapter 13Chapter 13

Meiosis and Sexual Life Cycles

Inheritance/HeredityInheritance/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.

GeneticsGenetics

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)

Genes

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

So, how does this happen?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.

So, how does this happen?

Remember, our gametes are the sperm and egg that unite during fertilization and give rise to the new individuals.

So, how does this happen?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.

The LocusThe Locus

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

Modes of ReproductionModes of Reproduction

When discussing heredity, there are a couple of modes of reproduction that need to be discussed.

Sexual and asexual.

Sexual ReproductionSexual 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.

Asexual ReproductionAsexual 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.

Asexual ReproductionAsexual Reproduction

A hydra budding is an example of asexual reproduction.

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The Sexual Lifecycle--HumansThe 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.

Homologous ChromosomesHomologous 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.

Homologous Chromosomes--Sort Of…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

Chromosome Number-AutosomesChromosome 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.

Chromosome Number-Sex ChromosomesChromosome 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).

FertilizationFertilization

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.

Sexual MaturitySexual Maturity

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

MeiosisMeiosis

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

Meiosis and FertilizationMeiosis and Fertilization

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

3 Main Types of Life Cycles in Sexually Reproducing Organisms3 Main Types of Life Cycles in Sexually Reproducing Organisms 1. Animals 2. Plants and some algae 3. Fungi and some protists

AnimalsAnimals

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.

Plants and Some AlgaePlants 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.

Most Fungi and Some ProtistsMost 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.

3 Main Types of Life Cycles in Sexually Reproducing Organisms3 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.

MeiosisMeiosis

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

through the generations.

MeiosisMeiosis

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.

MeiosisMeiosis

Meiosis halves the number of chromosomes.

Meiosis IMeiosis I

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

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Meiosis IMeiosis I

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

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Meiosis IMeiosis 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.”

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Meiosis IMeiosis 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.

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InterkinesisInterkinesis

In some cells, the c-somes relax and the nuclear envelope reforms.

Other cells go directly to meiosis II.

Meiosis II Sister Chromatids SeparateMeiosis II Sister Chromatids Separate Prophase II:

Chromosome condense again, the nuclear envelope disappears and the mitotic spindle reforms.

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Meiosis II Sister Chromatids SeparateMeiosis II Sister Chromatids Separate Metaphase II: C-

somes line along the metaphase plate.

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Meiosis II Sister Chromatids SeparateMeiosis II Sister Chromatids Separate Anaphase II: Sister

chromatids separate and migrate as individual c-somes to the spindle poles.

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Meiosis II Sister Chromatids SeparateMeiosis II Sister Chromatids Separate Telophase II: C-

somes arrive at the poles, the spindle breaks down, the nuclear envelope reforms.

Cytokinesis: The cytoplasm divides.

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Mitosis and Meiosis, A ComparisonMitosis 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.

3 Main Events Important to Meiosis3 Main Events Important to Meiosis 1. Synapsis and Crossing Over 2. Tetrad Formation 3. Separation of Homologues

1. Synapsis and Crossing Over1. 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.

2. Tetrad Formation2. Tetrad Formation

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

3. Separation of Homologues3. 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).

3 Mechanisms that Produce Genetic Variation3 Mechanisms that Produce Genetic Variation 1. Independent Assortment 2. Crossing Over 3. Random Fertilization

1. Independent Assortment1. Independent Assortment

Homologous c-somes can be positioned independently at the metaphase plate.

Maternal and paternal c-somes sort independently of every other pair.

1. Independent Assortment1. 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+)

2. Crossing Over2. Crossing Over

Gene mixing. Non-sister chromatids trade places.

Taking crossing over into account makes the number of possible combinations endless.

3. Random Fertilization3. Random Fertilization

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

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