BIS 1A Handout 13 - Meiosis

Heredity: Similarity and Variation

Living organisms are distinguished by their ability to reproduce their own kind.

The transmission of traits from one generation to the next is called heredity or inheritance.

However, offspring differ somewhat from parents and siblings, demonstrating variation.

Farmers have bred plants and animals for desired traits for thousands of years, but the mechanisms of heredity and variation eluded biologists until the development of genetics in the 20th century.

Genetics is the scientific study of heredity and variation.

Offspring acquire genes from parents by inheriting chromosomes

In genetic terminology, a character is a heritable feature, and a trait is the variation in a character. Hair color is a character and black is a trait.

The composition of genes, or genotype, are responsible for characters, and the physical appearance of traits is the phenotype.

Your genome is comprised of the tens of thousands of genes that you inherited from your mother and your father.

Genes program specific traits that emerge as we develop from fertilized eggs into adults.

Genes are segments of DNA. Genes have specific nucleotide sequences.

Most genes program cells to synthesize specific enzymes and other proteins whose emergent properties produce an organism's inherited traits.

In plants and animals, sperm and ova (unfertilized eggs) transmit genes from one generation to the next.

After fertilization (fusion of a sperm cell and an ovum), genes from both parents are present in the nucleus of the fertilized egg, or zygote.

DNA in the nucleus of a eukaryotic cell divided among chromosomes.

A species has a characteristic chromosome number.

Humans have 46 chromosomes in almost all of their cells.

Each chromosome has hundreds or thousands of genes, each at a specific location, its locus.

A comparison of asexual and sexual reproduction.

Only organisms that reproduce asexually can produce offspring that are exact copies of themselves.

In asexual reproduction, a single individual is the sole parent to donate genes to its offspring.

An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals.

Members of a clone may be genetically different as a result of mutation.

In sexual reproduction, two parents produce offspring that have unique combinations of genes inherited from the two parents.

Unlike a clone, offspring produced by sexual reproduction always vary genetically from their siblings and their parents.

Fertilization and meiosis alternate in sexual life cycles

A life cycle is a sequence of stages in the reproductive history of an organism.

It starts at the conception of an organism and continues until the organism produces its own offspring.

Somatic cells contain sets of chromosomes.

In humans, each somatic cell (all cells other than sperm or ovum) has 46 chromosomes.

Images of the 46 human chromosomes can be arranged in pairs in order of size to produce a karyotype display.

The two chromosomes comprising a pair have the same length, centromere position, and staining pattern.

These homologous chromosome pairs carry genes that control the same inherited characters.

Two distinct sex chromosomes, the X and the Y, are an exception to the general pattern of homologous chromosomes in human somatic cells.

The other 22 pairs are called autosomes.

The pattern of inheritance of the sex chromosomes determines an individual's sex.

Human females have a homologous pair of X chromosomes (XX).

Human males have an X and a Y chromosome (XY).

Only small parts of the X and Y are homologous.

Most of the genes carried on the X chromosome do not have counterparts on the tiny Y.

The Y chromosome also has genes not present on the X.

The occurrence of homologous pairs of chromosomes is a consequence of sexual reproduction.

We inherit one chromosome of each homologous pair from each parent.

The 46 chromosomes in each somatic cell are two sets of 23, a maternal set (from your mother) and a paternal set (from your father).

The number of chromosomes in a single set is represented by n.

Any cell with two sets of chromosomes is called a diploid cell and has a diploid number of chromosomes, abbreviated as 2n.

Sperm cells or ova (gametes) have only one set of chromosomes-22 autosomes and an X (in an ovum) and 22 autosomes and an X or a Y (in a sperm cell).

A gamete with a single chromosome set is haploid, abbreviated as n.

Let's discuss the role of meiosis in the human life cycle.

The human life cycle begins when a haploid sperm cell fuses with a haploid ovum.

These cells fuse (syngamy), resulting in fertilization.

The fertilized egg (zygote) is diploid because it contains two haploid sets of chromosomes bearing genes from the maternal and paternal family lines.

As an organism develops from a zygote to a sexually mature adult, mitosis generates all the somatic cells of the body.

Gametes, which develop in the gonads (testes or ovaries), are not produced by mitosis.

Instead, gametes undergo the process of meiosis in which the chromosome number is halved.

Fertilization restores the diploid condition by combining two haploid sets of chromosomes.

Organisms display a variety of sexual life cycles.

The timing of meiosis and fertilization varies among species.

These variations can be grouped into three main types of life cycles.

In most animals, including humans, gametes are produced directly from a diploid adult.

Meiosis occurs after a diploid, multicellular adult develops from the zygote.

Plants and some algae have a second type of life cycle called alternation of phases.

The multicellular, diploid phase produces haploid spores from the adult after meiosis.

The multicellular, haploid phase produces gametes from the adult by mitosis.

Most fungi and some protists have a third type of life cycle.

The zygote undergoes meiosis to produce haploid cells.

These haploid cells grow by mitosis to an adult, producing gametes by mitosis.

Although the three types of sexual life cycles differ in the timing of meiosis and fertilization, they share a fundamental feature: each cycle of chromosome halving and doubling contributes to genetic variation among offspring.

Meiosis reduces the number of chromosome sets from two to one

Many steps of meiosis resemble steps in mitosis.

However, in meiosis, there are two consecutive cell divisions, meiosis I and meiosis II, resulting in four daughter cells.

The first division, meiosis I, separates homologous chromosomes, halving the number.

The second, meiosis II, separates sister chromatids (duplicated chromosomes).

The four daughter cells have only half as many chromosomes as the parent cell.

Meiosis I is preceded by interphase, in which the chromosomes are replicated to form sister chromatids.

These are genetically identical and joined at the centromere.

The single centrosome is replicated, forming two centrosomes.

Division in meiosis I occurs in four phases: prophase I, metaphase I, anaphase I, and telophase I.

Prophase I

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

During prophase I, the chromosomes begin to condense.

Homologous chromosomes loosely pair up along their length, precisely aligned gene for gene.

In crossing over, DNA molecules in nonsister chromatids break at corresponding places and then rejoin the other chromatid.

In late prophase, each chromosome pair becomes visible as a tetrad, or group of four chromatids.

Each tetrad has one or more chiasmata, sites where the chromatids of homologous chromosomes have crossed and segments of the chromatids have been traded.

Spindle microtubules form from the centrosomes, which have moved to the poles.

The breakdown of the nuclear envelope and nucleoli take place.

Kinetochores of each homologue attach to microtubules from one of the poles.

Metaphase I

At metaphase I, the tetrads are all arranged at the metaphase plate.

Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad, while those from the other pole are attached to the other.

Anaphase I

In anaphase I, the homologous chromosomes separate. One chromosome moves toward each pole, guided by the spindle apparatus.

Telophase I and cytokinesis

In telophase I, movement of homologous chromosomes continues until there is a haploid set at each pole.

Cytokinesis usually occurs simultaneously, by the same mechanisms as mitosis.

In animal cells, a cleavage furrow forms. In plant cells, a cell plate forms.

The chromosome number is now haploid.

No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II, as the chromosomes are already replicated.

Meiosis II

Meiosis II is very similar to mitosis.

During prophase II, a spindle apparatus forms and attaches to kinetochores of each sister chromatid.

Spindle fibers from one pole attach to the kinetochore of one sister chromatid, and those of the other pole attach to kinetochore of the other sister chromatid.

At metaphase II, the sister chromatids are arranged at the metaphase plate.

Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical.

At anaphase II, the centomeres of sister chromatids separate and travel toward opposite poles.

In telophase II, nuclei form around the chromosomes, and cytokinesis separates the cytoplasm.

There are four haploid daughter cells.

There are key differences between mitosis and meiosis.

Meiosis produces cells that are genetically distinct from the parent cell and from each other.

Three events, unique to meiosis, occur during the first division cycle.

During prophase I of meiosis, replicated homologous chromosomes line up and become physically connected along their lengths by a zipperlike protein complex, the synaptonemal complex, in a process called synapsis. Genetic rearrangement between nonsister chromatids called crossing over also occurs. Once the synaptonemal complex is disassembled, the joined homologous chromosomes are visible as a tetrad. X-shaped regions called chiasmata are visible as the physical manifestation of crossing over.

At metaphase I of meiosis, homologous pairs of chromosomes align along the metaphase plate. In mitosis, individual replicated chromosomes line up along the metaphase plate.

At anaphase I of meiosis, it is homologous chromosomes, not sister chromatids, that separate and are carried to opposite poles of the cell. Sister chromatids of each replicated chromosome remain attached.

Genetic variation produced in sexual life cycles contributes to evolution

What is the origin of genetic variation?

Mutations are the original source of genetic diversity.

Once different versions of genes arise through mutation, genetic exchange during meiosis and fertilization produce offspring with their own unique set of traits.

Sexual life cycles produce genetic variation among offspring.

Three mechanisms contribute to genetic variation:

Independent assortment of chromosomes.

Crossing over.

Random fertilization.

Independent assortment of chromosomes contributes to genetic variability due to the random orientation of homologous pairs of chromosomes at the metaphase plate during meiosis I.

Therefore, the first meiotic division results in independent assortment of maternal and paternal chromosomes into daughter cells.

The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number of the organism.

For humans with n = 23, there are 223, or more than 8 million possible combinations of chromosomes.

Crossing over produces recombinant chromosomes, a combination of genes inherited from each parent.

In crossing over, homologous portions of two nonsister chromatids trade places.

For humans, this occurs an average of one to three times per chromosome pair.

At metaphase II, nonidentical sister chromatids sort independently from one another, increasing by even more the number of genetic types of daughter cells that are formed by meiosis.

The random nature of fertilization adds to the genetic variation arising from meiosis.

Any sperm can fuse with any egg.

The ovum is one of more than 8 million possible chromosome combinations.

The successful sperm is one of more than 8 million possibilities.

The resulting zygote could contain any one of more than 70 trillion chromosomal combinations.

Crossing over adds even more variation to this.

The three sources of genetic variability in a sexually reproducing organism are:

Independent assortment of homologous chromosomes during meiosis I and of nonidentical sister chromatids during meiosis II.

Crossing over between homologous chromosomes during prophase I.

Random fertilization of an ovum by a sperm.

All three mechanisms recombine the various genes carried by individual members of a population.

Evolutionary adaptation depends on a population's genetic variation.

Darwin recognized the importance of genetic variation in evolution.

A population evolves through the differential reproductive success of its variant members.

Those individuals best suited to the local environment leave the most offspring, transmitting their genes in the process.

This natural selection results in adaptation, the accumulation of favorable combinations of genetic variation.

If the environment changes or a population moves to a new environment, different genetic combinations that work best in the new conditions will produce more offspring, and these combinations will increase.

Sex and mutation continually generate new genetic variability.

Although Darwin realized that heritable variation makes evolution possible, he did not have a theory of inheritance.

Gregor Mendel, a monk and teacher read Darwin's work. Mendel published a theory of inheritance that supported Darwin's theory.

However, this work was largely unknown until 1900, after Darwin and Mendel had both been dead for more than 15 years.