Biology II

Chapter 12 notes

Reproduction is an emergent property associated with life. The fact that organisms reproduce their own kind is a consequence of heredity

Heredity= Continuity of biological traits from one generation to the next

Þ Results from the transmission of heredity units, or genes, from parents to offspring

Þ Because they share similar genes, offspring more closely resemble their parents to close relatives than unrelated individuals of the same species

Variation= Inherited differences among individuals of the same species

Þ Though offspring resemble their parents and siblings, they also diverge somewhat as a consequence of inherited differences of the same species

Þ The development of genetics in this century has increased our understanding about the mechanisms of variation and heredity

Genetics= The scientific study of heredity and variation

Note: Beginning students often compartmentalize their knowledge, which makes it difficult to transfer & apply information learned in one context to a new situation. Be forewarned that unless the instructor points it out, some students will never make the connection that meiosis, sexual reproduction and heredity are all aspects of the same process

I. Offspring acquire from parents by inheriting chromosomes

DNA- Type of nucleic acid that is a polymer of four different kinds of nucleotides

Genes= Units of hereditary information that are made of DNA and are located on chromosomes

Þ Have specific sequence of nucleotides, the monomers of DNA

Þ Most genes program cells to synthesize specific proteins; the action of these proteins produce an organism’s inherited traits

Inheritance if possible because:

Þ DNA is precisely replicated producing copies of genes that can be passes along from parents to offspring

Þ Sperm and ova carry each parent’s genes are combined in the nucleus of the fertilized egg

The actual transmission of genes from parents to offspring depends of the behavior of chromosomes

Chromosomes _ Threadlike structures in eukaryotic nuclei that are made of DNA and protein

Þ Consist of a single long DNA molecule that is highly folded and coiled along with proteins

Þ Contain genetic information arranged in a linear sequence

Þ Contain hundreds of thousands of genes. Each of which is a specific region of the DNA molecule, or locus

Locus= Specific location on a chromosome that contains a gene

Þ Each species has a chromosome number; humans have 46

II. Like begets like, more or less: a comparison of asexual versus sexual reproduction

Asexual Reproduction	Sexual Reproduction
Single individual is the sole parent Single parent passes on all its genes to its offspring Offspring are genetically identical to the parent Results in a clone, or genetically identical individual. Rarely, genetic differences occur as a result of mutation, a change in DNA	Two parents give rise to offspring. Each parent passes on half its genes to its offspring Offspring have a unique combination of genes inherited from both parents Results in greater genetic variation; offspring vary genetically from their siblings and parents

What generates this genetic variation during sexual reproduction? The answer lies in the process of meiosis.

III. Fertilization and meiosis alternate in sexual life cycles: an overview

The human life cycle follows the same pattern found in all sexually reproducing organisms; meiosis and fertilization result in alternation between the haploid and diploid condition.

Life Cycle- Sequence of stages in an organism’s reproductive history, from conception to production of its own offspring

Somatic Cell- Any cells other than a sperm or an egg cell.

Þ Human somatic cells contain 46 chromosomes distinguishable by differences in size, position of the centromere, and staining or banding pattern

Þ Using these criteria, chromosomes from a photomicrograph can be matched into homologous pairs and arranged in a standard sequence to produce a karyotype

Homologous chromosomes (homologous)- A pair of chromosomes that have the same size, centeromere position and staining pattern.

Þ With one exception, homologues carry the same genetic loci

Þ Homologous autosomes carry the same genetic loci; however, human sex chromosomes carry different loci even though they pair during prophase of Meiosis I.

Autosome- A chromosome that is not a sex chromosome

Sex Chromosome- Dissimilar chromosomes that determine an individual’s sex

Þ Females have a homologous pair of X chromosomes

Þ Males have one X and one Y chromosomes

Þ Thus, humans have 22 pairs of autosomes and 1 pair of sex chromosomes

Chromosomal pairs in the human karyotype are a result of our sexual origins

Þ One homologue is inherited from each parent

Þ Thus, the 46 somatic-cell chromosomes are actually two sets of 23 chromosomes; one a maternal set and the other a paternal set

Þ Somatic cells in humans and most other animals are diploid

Diploid- Condition in which cells contain two sets of chromosomes; abbreviated as 2n

Haploid- Condition in which cells contain one set of chromosomes; it is the chromosomes number of gametes and is abbreviated as n

Gamete- A haploid reproductive cell

Þ Sperm cells and ova are gametes, and they differ from somatic cells in their chromosome number. Gametes only have one set of chromosomes

Þ Human gametes contain a single set of 22 autosomes and 1 sex chromosomes (either an X or a Y)

Þ Thus, the haploid number of humans is 23

The diploid number is restored when two haploid gametes unite in the process of fertilization. Sexual intercourse allows a haploid sperm cell from the father to reach and fuse with an ovum from the mother.

Fertilization- The union of 2 gametes to form a zygote

Zygote- A diploid cell that results from the union of two haploid gametes

Þ Contains the maternal and parental haploid sets of chromosomes from the gametes and is diploid (2n)

Þ As humans develop from a zygote to a sexually mature adult, the zygote’s genetic information is passed with precision to all somatic cells by mitosis

Gametes are the only cell in the body that are NOT produced by mitosis

Þ Gametes are produced in the ovaries or testes by the process of meiosis

Þ Meiosis is a special type of cell division that produces haploid cells and compensates for the doubling of chromosome number that occurs at fertilization

Þ Meiosis in humans produces sperm cells and ova which contain 23 chromosomes

Þ When fertilization occurs, the diploid conditions (2n=46) is restored in the zygote

A. The Variety of Sexual Life Cycles

Alternation of meiosis and fertilization is common to all sexually reproducing organisms; however, the timing of these two events in the life cycle varies among species. There are three basic patterns of sexual life cycles:

Animal: In animals, including humans, gametes are the only haploid cells.

Þ Meiosis occurs during gamete production. The resulting gametes undergo no further cell division before fertilization

Þ Fertilization produces a diploid zygote that divides by mitosis to produce a diploid multicellular animal

Fungi and Some Protists: In many fungi and some protist, the only diploid stage is the zygote.

Þ Meiosis occurs immediately after the zygote forms

Þ Resulting haploid cells divide by mitosis to produce a haploid multicellular organism

Þ Gametes are produced by mitosis from the already haploid organism

Plants and Some Algae: Plants and some species of algae alternate between multicellular haploid and diploid generations

Þ This type of life cycle is called an alternation of generations

Þ The multicellular diploid stage is called a sporophyte, or spore-producing plant. Meiosis in this stage produces haploid cells called spores

Þ Haploid spores divide mitotically to generate a multicellular haploid stage called a gametophyte, or gamete-producing plant

Þ Haploid gametophytes produce gametes by mitosis

Þ Fertilization produces a diploid zygote which develops into the next sporophyte generation

IV. Meiosis reduces chromosomes number from diploid to haploid: a closer look

Meiosis and sexual reproduction significantly contribute to genetic variation among offspring.

Meiosis includes steps that closely resemble corresponding steps in mitosis.

Þ Like mitosis, meiosis is preceded by replication is followed by two consecutive cell divisions: Meiosis I and Meiosis II

Þ These cell division produce four daughter cells instead of two as in mitosis

Þ The resulting daughter cell have half the number of chromosomes as the original cell; whereas, daughter cells of mitosis have the same number of chromosomes as the parent cell

A. The Stages of Meiotic Cell Division

Interphase I: Interphase I precedes meiosis.

Þ Chromosomes replicates as in mitosis

Þ Each duplicated chromosome consists of two identical sister chromatids attached at their centromeres

Þ Centriole pairs in animal cells also replicate into two pairs

Meiosis I: This cell division segregates the two chromosomes of each homologous pair and reduces the chromosome number by one-half. It includes the following four phases:

Prophase I. This is a longer and more complex process than prophase of mitosis

Þ Chromosomes condense

Þ Synapsis occurs. During this process, homologous chromosomes come together as pairs

Þ Chromosomes condense further until they are distinct structures that can be seen with a microscope. Since each chromosome has two chromatids, each homologous pair in synapsis appears as a complex of four chromatids or a tetrad.

Þ In each tetrad, sister chromatids of the same chromosome are attached at their centromeres. Nonsister chromatids are linked by X-shaped chiasmata, sites where homologous strand exchange or crossing-over occurs

Þ Chromosomes thicken further and detach from the nuclear envelope

As prophase I continues, the cell prepares for nuclear division.

Þ Centriole pairs move apart and spindle micortubules form between them.

Þ Nuclear envelope and nucleoli disperse

Þ Chromosomes begin moving to the metaphase plate, midway between the two poles of the spindle apparatus

Þ Prophase I typically occupies more than 90% of the time required for meiosis

Metaphase I. Tetrads are aligned on the metaphase plate.

Þ Each synaptic pair is aligned so that centromeres of homologues point towards opposite poles

Þ Each homologue is thus attached to kinetochore mircotubules emerging from the pole it faces, so that the two homologues are destined to separate in anaphase and move towards opposite poles

Anaphase I. Homologues separate and are moved towards the poles by the spindle apparatus

Þ Sister chromatids remain attached at their centromeres and move as a unit towards the same pole, while the homologue moves towards the opposite pole.

Þ This differs from mitosis during which chromosomes line up individually on the metaphase plate (rather than in pairs) and sister chromatids are moved apart towards opposite poles of the cell

Telophase I and Cytokinesis. The spindle apparatus continues to separate homologous chromosome pairs until the chromosomes reach the poles

Þ Each pole now has a haploid set of chromosomes that are each still composed of two sister chromatids attached at the centromere

Þ Usually, cytokinesis occurs simultaneously with Telophase I, forming two haploid daughter cells. Cleavage furrows form in animal cells, and cell plates form in plant cells.

Þ In some species, nuclear membranes and nucleoli reappear and the cell enters a period of interkinesis before meiosis II. In other species, the daughter cells immediately prepare for meiosis II.

Þ Regardless of whether a cell enters interkinesis, no DNA replication occurs before meiosis II.

Meiosis II: This second meiotic division separates sister chromatids of each chromosome

Prophase II.

Þ If the cell entered interkinesis, the nuclear envelope and nucleoli disperse

Þ Spindle apparatus forms and chromosomes move towards metaphase II plate

Metaphase II. Chromosomes align singly on the metaphase plate

Þ Kinetochores of sister chromatids point towards opposite poles

Anaphase II. Sister chromatids separate

Þ Centromeres of sister chromatids separate

Þ Sister chromatids of each pair (now individual chromosomes) move toward opposite poles of the cell

Telophase II and Cytokinesis.

Þ Nuclei form at opposite poles of the cell

Þ Cytokinesis occurs producing four haploid daughter cells

B. A Comparison of Mitosis and Meiosis

NOTE: Spending time on a comparison of mitosis and meiosis is really worth the effort. It not only brings closure to the topic, but also provides and opportunity to check for understanding. One check is to ask students to identify unlabeled diagrams of various stages in mitosis and meiosis. The ability to distinguish metaphase of mitosis from metaphase of meiosis I, is particularly diagnostic of student understanding .

Though the processes of mitosis and meiosis are similar in some ways, there are some key differences:

Þ Meiosis is a reduction division. Cells produced by mitosis have the same number of chromosomes as the original cell, whereas cells produced by meiosis have half the number of chromosomes as the parent cell.

Þ Meiosis creates genetic variation. Mitosis produces two daughter cells genetically identical to the parent cell and to each other. Meiosis produces four daughter cells genetically different from the parent cell and from each other.

Þ Meiosis is two successive nuclear divisions. Mitosis, on the other hand, is characterized by just one nuclear division

Comparison of Meiosis I and Mitosis

Meiosis I Mitosis

Prophase Synapsis occurs to form tetrads Neither Synapsis nor crossing

Chiasmata appear as evidence that over occurs

crossing over has occurred

Metaphase Homologous pairs (tetrads) align Individual chromosomes

on the metaphase plate align on the metaphase plate

Anaphase Meiosis I separates pairs of Mitosis separates sister

chromosomes. Centromeres chromatid of individual

do NOT divide and sister chromosomes. Centromeres

chromatids of each divide and sister chromatids

chromosome move to the same move to OPPOSITE poles of

pole of the cell; only the the cell.

homologues separate

Meiosis II is virtually identical in mechanism to mitosis, separating sister chromatids.

V. Sexual life cycles produce genetic variation among offspring

Meiosis and fertilization are the primary sources of genetic variation in sexually reproducing organisms. Sexual reproduction provides genetic variation by:

Þ independent assortment

Þ crossing over during prophase I of meiosis

Þ random fusion of gametes during fertilization

A. Independent Assortment of Chromosomes

At Metaphase I, each homologous pair of chromosomes aligns on the metaphase plate.

Each pair consist of one maternal and one paternal chromosome.

Þ The orientation of the homologous pair to the poles is random, so there is a fifty-fifty chance that a particular daughter cell produced by meiosis I will receive the maternal chromosome of the homologous pair, and fifty-fifty chance that it will receive the paternal chromosome

Þ Each homologous pair of chromosomes orients independently of the other pairs at metaphase I; thus, the first meiotic division results in independent assortment of maternal and paternal chromosomes

Þ A gamete produced by meiosis contains just one of all the possible combinations of maternal and paternal chromosomes

Independent Assortment- The random distributions of genetic material to the gametes. (In a more specific sense, assortment refers to the random distribution of genes located on different chromosomes.)

Þ Since each homologous pair assorts independently from all the others the process produces 2n possible combinations of maternal and paternal chromosomes in gametes, where n is the haploid number

Þ In humans, the possible combinations would be 223, or about 8 million

Þ Thus, each human gamete contains one of eight million possible assortments of chromosomes inherited from that person’s mother and father

Þ Genetic variation results from this reshuffling of chromosomes, because the maternal and paternal homologues will carry different genetic information at many of their corresponding loci

B. Crossing Over

Another mechanism that increases genetic variation is the process of crossing over, during which homologous chromosomes exchange genes

Crossing over- The exchange of genetic material between homologues; occurs during prophase of meiosis I. This process:

Þ Occurs when homologous portions of two nonsister chromatids trade places. During prophase I, X-shaped chiasmata become visible at places where this homologous strand exchange occurs

Þ Produces chromosomes that contain genes from both parents

Þ In humans, there is an average of two or three crossovers per chromosome pair

Þ Synapsis during prophase I is precise, so that homologous align gene by gene. The exact mechanism of synapsis is still unknown, but involves the formation of the snynaptonemal complex, a protein structure that brings the chromosome into close association

C. Random Fertilization

Random fertilization is another source of genetic variation in offspring.

Þ In humans, an egg cell that is one of eight million different possibilities will be fertilized by a sperm cell that is also one of eight million possibilities. Thus, the resulting zygote can have one of 64 trillion possible diploid combinations

To summarize, there are three important sources of genetic variability in sexually reproducing organisms:

Þ Independent assortment of homologous chromosomes pairs during meiosis I

Þ Crossing over between homologous chromosomes during prophase of meiosis I

Þ Random fusion of gametes

VI. Evolutionary adaptation depends on a population’s genetic variation

Inheritable variation is the basis for Charles Darwin’s theory that natural selection is the mechanism for evolutionary change. Natural selection

Þ Increases the frequency of inheritable variations that favor the reproductive success of some individuals over others

Þ Results in adaptation, the accumulation of inheritable variations that are favored by the environment

Þ In the face of environmental change, genetic variation increases the likelihood that some individuals in a population will have inheritable variations that help them cope with the new conditions

There are two sources of genetic variation:

Þ Sexual reproduction: independent assortment in meiosis I, crossing over in prophase of meiosis I, and random fusion of gametes during fertilization

Þ Mutation, which is rare structural change in a gene.