Biology 151 - Lecture 11: Meiosis

Metaphase chromosomes

A chromosome in the stage of the cell cycle (the sequence of events in the life of a cell) when a chromosome is most condensed and easiest to distinguish and so to study. Metaphase chromosomes are often chosen for karyotyping and for chromosome analysis because they are readily seen.

Genes

A sequence of nucleotides in DNA or RNA that encodes the synthesis of a gene product, either RNA or protein.

Loci

A specific, fixed position on a chromosome where a particular gene or genetic marker is located.

How is genetic variation increased?

1. Crossing Over 2. Independent Assortment (Random Alignment) of Chromosomes 3. Random Fertilization Gene duplication, mutation, or other processes can produce new genes and alleles and increase genetic variation. New genetic variation can be created within generations in a population, so a population with rapid reproduction rates will probably have high genetic variation.

Diploid cell (2n)

A cell that contains two complete sets of chromosomes. This is double the haploid chromosome number. Each pair of chromosomes in a diploid cell is considered to be a homologous chromosome set.

Anaphase I

Anaphase I begins when the two chromosomes of each bivalent (tetrad) separate and start moving toward opposite poles of the cell as a result of the action of the spindle. Notice that in anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is that sister chromatids remain joined after metaphase in meiosis I, whereas in mitosis they separate.

Asexual vs. Sexual Reproduction

Asexual reproduction involves one parent and produces offspring that are genetically identical to each other and to the parent. Sexual reproduction involves two parents and produces offspring that are genetically unique.

Mitosis

Asexual reproduction, get an exact copy A type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.

Prophase I

At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope. The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. The sites of crossing-over are seen as crisscrossed nonsister chromatids and are called chiasmata (singular: chiasma). The nucleolus disappears during prophase I. In the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell. The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus. Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.

How does meiosis increase the genetic diversity of gametes?

Because of recombination and independent assortment in meiosis, each gametecontains a different set of DNA. This produces a unique combination of genes in the resulting zygote. ... The result is 4 haploid daughter cells known as gametes.

How are genes/DNA passed on?

Cell division is the mechanism by which DNA is passed from one generation of cells to the next and ultimately, from parent organisms to their offspring. ... During meiosis, the cells needed for sexual reproduction divide to produce new cells called gametes.

Independent assortment (random alignment) of chromosomes

During Anaphase. Homologous pairs of chromosomes orient randomly at metaphase I of meiosis Each pair of chromosomes sorts maternal and paternal homologues into daughter cells independently of the other pairs The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the number of unique chromosomes (haploid number).

Meiosis II

During meiosis II, the sister chromatids within the two daughter cells separate, forming four new haploid gametes. The mechanics of meiosis II is similar to mitosis, except that each dividing cell has only one set of homologous chromosomes.

Telophase II and Cytokinesis (Meiosis II)

During telophase II, the fourth step of meiosis II, the chromosomes reach opposite poles and begin to decondense, cytokinesis occurs,Cytokinesis separates the two cells into four unique haploid cells. At this point, the newly formed nuclei are both haploid. the two cells produced by meiosis I divide to form four haploid daughter cells.

Metaphase II

Each of the daughter cells completes the formation of a spindle apparatus. Single chromosomes align on the metaphase plate, much as chromosomes do in mitosis. This is in contrast to metaphase I, in which homologous pairs of chromosomes align on the metaphase plate. For each chromosome, the kinetochores of the sister chromatids face the opposite poles, and each is attached to a kinetochore microtubule coming from that pole.

What is the Evolutionary Significance of Genetic Variation Within a Population?

Genetic variation is an important force in evolution as it allows natural selection to increase or decrease frequency of alleles already in the population.

Haploid cell (n)

Having a single set of chromosomes. Organisms that reproduce asexually are haploid. Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent). In humans, only their egg and sperm cells are haploid.

Why must mammalian gametes be haploid

If they were not haploid, the number of chromosomes in each offspring will keep doubling. Haploid cells have half the # of chromosomes so since they will combine with another haploid cell it will create something new with the correct # of chromosomes.

Meiosis I

In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity. DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis.

What are the similarities between meiosis and mitosis?

Mitosis and meiosis both involve duplication of a cell's DNA content. Each strand ofDNA, or chromosome, is replicated and remains joined, resulting in two sister chromatids for each chromosome. A common goal of mitosis and meiosis is to split the nucleus and its DNA content between two daughter cells.

How does meiosis and mitosis differ from each other?

Mitosis- (Metaphase)Homologous chromosomes do not align, sister chromatids separate, produces two diploid (2n) somatic cells that are genetically identical to each other and the original parent cell Meiosis- (Metaphase)Homologous chromosomes do align, homologous chromosomes separate, produces four haploid (n) gametes that are genetically unique from each other and the original parent (germ) cell.

Recombinant chromosomes

Mix of maternal and paternal alleles. (Crossing over). Parental chromosomes. Retain allele combination from each parent.

Principle of natural selection: Individuals within a species vary! Which variation is most fit...?

Natural selection is an inevitable outcome of three principles: most characteristics are inherited, more offspring are produced than are able to survive, and offspring with more favorable characteristics will survive and have more offspring than those individuals with less favorable traits.

Random Fertilization

Refers to the fact that if two individuals mate, and each is capable of producing over 8million potential gametes, the random chance of any one sperm and egg coming together is a product of these two probabilities Any of the 8.4 million possible sperm can fertilize any of the 8.4 million possible eggs 8.4 million possibilities of sperm X 8.4 million possibilities of egg You are a 1 in 70 trillion chance combination!

Sister Chromatids

Refers to the identical copies (chromatids) formed by the DNA replication of a chromosome, with both copies joined together by a common centromere. In other words, a sister chromatid may also be said to be 'one-half' of the duplicated chromosome. A pair of sister chromatids is called a dyad.

Metaphase I

The centrioles are at opposite poles of the cell. The pairs of homologous chromosomes (the bivalents), now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate. Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids).

Anaphase II

The centromeres separate, and the two chromatids of each chromosome move to opposite poles on the spindle. The separated chromatids are now called chromosomes in their own right.

Crossing over

The exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes, a mixture of parental characteristics in offspring.

Telophase I and Cytokinesis (Meiosis I)

The homologous chromosome pairs complete their migration to the two poles as a result of the action of the spindle. Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids. A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows. In animal cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells. After cytokinesis, each of the two progeny cells has a nucleus with a haploid set of replicated chromosomes. Many cells that undergo rapid meiosis do not decondense the chromosomes at the end of telophase I. Other cells do exhibit chromosome decondensation at this time; the chromosomes recondense in prophase II.

Centromere

The region of a chromosome to which the microtubules of the spindle attach, via the kinetochore, during cell division.

Homologous chromosomes

Two pieces of DNA within a diploid organism which carry the same genes, one from each parental source. In simpler terms, both of your parents provide a complete genome. Each parent provides the same 23 chromosomes, which encode the same genes.

Prophase II

While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II. The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole. The two pairs of centrioles separate into two centrosomes. The nuclear envelope breaks down, and the spindle apparatus forms.

Meiosis

sexual reproduction, get offspring with gene combinations not present in the parents - unique - Requires a mechanism for giving your progeny only half of your genes A type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.