How Cells Reproduce Part 3

Introducing Alleles  Asexual reproduction involves the simple inheritance of genes from one parent and produces daughter cells genetically identical to the parent and to each other.  The offspring are essentially clones of the parent (unless a mutation occurs).  With sexual reproduction, however, inheritance of genes by offspring is much more complicated.

Introducing Alleles  Sexual reproduction involves the inheritance of genes from two parents.  Why does nature favor sexual reproduction?  Put simply, sexual reproduction allows for variation, diversity, which can produce combinations of genes that make an organism and/or its offspring better suited to survive in a changing environment.

Introducting Alleles  All offspring produced by asexual reproduction are adapted to the same environment in the same way.  If the environment changes such that one organism cannot survive, all the offspring that were asexually produced from that organism will also be vulnerable to that changed environment and will die also.

Asexual Reproduction Black environment camoflages these genetically identical black butterflies Environment changes to white. All genetically identical black butterflies are eaten by predators

Introducing Alleles  However, organisms that have been produced as a result of sexual reproduction have received genetic information that has been “mixed up” such that the offspring that are produced are genetically different from one another.  If the environment changes such that one organism cannot survive, since the organisms that were produced from this organism are genetically different, chances are that at least some of the offspring that were sexually produced from that organism will be able to survive that change in the environment.  In this way, sexual reproduction brings together adaptive traits and separates adaptive traits from maladaptive traits, increasing the odds of at least some offspring surviving, even if the enviornment changes.

Sexual Reproduction Genetically different butterflies. Even if the environment changes, no matter which environment they are on, at least some will be camoflaged and survive

Introducing Alleles  The somatic cells (body cells) of multicellular organisms contain homologous pairs of chromosomes.  One of each pair is paternal and one f each pair is maternal.  Both chromosomes of each pair carry the same set of genes (such as genes that code for eye color).  However, these genes may not code for the exact same proteins (one may have eye color genes that code for brown eyes, while the other contains genes that code for blue eyes).

Introducing Alleles  The two different forms of a gene (such as the gene that codes for brown eyes and the gene that codes for blue eyes) are called alleles.  There can be more than two alleles for a particular gene.  For example, the mutated alleles that code for sickle-cell anemia and beta thalassemia are two of over 700 known alleles of the human beta globin gene.

Introducing Alleles  Different alleles for genes are one reason that individuals of a sexually reproducing species do not all look identical.  Since each organism produced as a result of sexual reproduction contain different combinations of alleles from the two parents, this produces new combinations of traits in each offspring.

Different Combinations of Alleles in Sexually Reproducing Organisms

Meiosis Halves the Chromosome Number  The process of sexual reproduction begins with meiosis, a type of nuclear division that halves the chromosome number in the daughter cells.  Since sexual reproduction involves the fusion of reproductive cells from two different parents, the chromosome number of the reproductive cells must be half that (haploid) of the parent so that the organism resulting from the fusion of these cells will be the same (diploid) as that of the parent organisms from which the cells came.

Meiosis Halves the Chromsome Number Haploid + Haploid  Diploid  46 Products of meiosis (n)

Meiosis Halves the Chromosome Number  The cells that are produced as a result of meiosis are haploid (1n) reproductive cells called germ cells or gametes.  A sperm cell is a male gamete.  An egg cell is a female gamete.  During fertilization, the haploid sperm and haploid egg fuse to form a diploid zygote, which is the first cell of a new individual.

Similarities of Mitosis and Meiosis  DNA is replicated before both processes.  Chromosomes are composed of two sister chromatids attached at the centromere at the beginning of both processes.  A spindle apparatus moves the chromosomes during both processes.

Differences Between Meiosis and Mitosis  Meiosis involves two nuclear divisions (called Meiosis I and Meiosis II), mitosis involves only one nuclear division.  Meiosis produces four genetically distinct haploid reproductive cells, mitosis produces two genetically identical somatic cells.  There is no Interphase between Meiosis I and Meiosis II, Interphase occurs after every mitotic division.  During Meiosis I, homologous chromosomes pair up, mitosis does not involve the pairing of homologous chromosomes.  Meiosis involves crossing over between the two chromosomes in each homologous pair resulting in the production of genetically different daughter cells, mitosis does not involve crossing over.

Differences Between Meiosis and Mitosis

Prophase I  During Prophase I, homologous chromosomes pair during a process called synapsis.  This pair of paired homologous chromosomes is called a tetrad.  This pairing allows a process called crossing over to occur. This crossing over occurs at sites where the two chromosomes’ DNA twists around each other called chiasma.  During crossing over, the pairs of homologous chromosomes are able to exchange corresponding segments of their DNA.  This produces chromosomes that are genetically different from either parent from which they originally came.

Prophase I  Crossing over is a normal and frequent event that occurs during meiosis.  In humans, anywhere from 46 to 95 crossovers occur every time meiosis happens, meaning that every chromosome crosses over at least once.  Each crossover is an opportunity for homologous chromosomes to exchange alleles, introducing unique combinations of alleles in both members of a pair of homologous chromosomes, which results in unique combinations of genes and traits in offspring.

Prophase I 2n = 4

Prophase I  The centrosome with its centrioles has duplicated and one centrosome moves to each “pole” of the cell as the nuclear membrane breaks down.  By the end of Prophase I, microtubules of the spindle apparatus attach to the paired chromosomes.  These microtubules attach such that one chromosome of each pair is attached to ooposite poles of the cell.

Metaphase I  The microtubules attached to the chromosomes grow and shrink, pushing and pulling the chromosomes.  At metaphase I, all of the microtubules have become the same length so that the pairs of chromosomes are lined up at the equator of the cell.

Metaphase I

Anaphase I  During Anaphase I, the microtubules of the spindle apparatus separate the pairs of chromosomes, pulling them toward opposite ends of the cell.

Anaphase I

Telophase I  During Telophase I, the chromosomes reach the opposite poles of the cell.  New nuclear envelopes form around the two clusters of chromosomes at the opposite ends of the cell.  At this point, the chromosome number has been halved as each new nucleus contains half the number of chromosomes as the original cell did.  At this point, the results are two genetically different haploid reproductive cells.

Telophase I n = 2

Meiosis II  There will be NO DNA replication before Meiosis II.  Meiosis II is basically just a mitotic division, resulting in the separation of the sister chromatids of the chromosomes in the cells that were produced during Meiosis I.  At the end of Meiosis II, there will be four haploid reproductive cells that are all genetically distinct.

Meiosis II Prophase II: Chromosomes condense, nuclear envelope breaks down, centrosomes move to opposite poles Metaphase II: spindle micro- tubules become some length causing chromomes to line up on metaphase plate Anaphase II: sister chromatids Are pulled apart to opposite Ends of the cell as microtubules shorten Telophase II : nuclear envelope reforms, chromosomes unwind 4 genetically different haploid reproductive daughter cells result