Meiosis I

Cell growth and preparation for division

End of previous mitotic event

G1 I Cell growth wdttk S and G phases ^^^ similar to the S and G phases of mitosis

DNA is copied

End of previous mitotic event

DNA is copied

G1 I Cell growth wdttk S and G phases ^^^ similar to the S and G phases of mitosis

Figure 6.7 Interphase and meiosis.

Interphase consists of G1, S, and G2 and is followed by two rounds of nuclear division, meiosis I and meiosis II.

Dna Replication Adding Nucleotides

DNA Replication When a DNA molecule is to be replicated, or copied, it is split up the middle of the helix, and new nucleotides are added to each side of the original parent molecule, maintaining the A:T and G:C pairings. This results in two daughter DNA molecules, each composed of one strand of parental nucleotides and one newly synthesized strand (Figure 6.8a).

To replicate the DNA, an enzyme that facilitates DNA synthesis is required. This enzyme, DNA polymerase, moves along the length of the unwound helix and helps bind incoming nucleotides to each other on the newly forming daughter strand (Figure 6.8b). Nucleotides on the daughter strand are complementary to those across from them on on the parent strand. When free nucleotides floating in the nuclear fluid have an affinity for each other (A for T and G for C), they bind to each other across the width (rungs) of the helix. The DNA polymerase catalyzes the formation of the phosphodiester bond along the length (handrails)

Duplicated Chromosome Picture

Figure 6.9 Unduplicated and duplicated chromosomes. Uncondensed chromosomes are not linear, but are drawn this way here to illustrate that duplicated chromosomes are exact copies of the unduplicated chromosome and are attached at the centromere.

of the helix. The two complementary nitrogenous bases (A,C,G, and T) are joined, and the DNA polymerase advances along the parental DNA strand to the next unpaired nucleotide.

Just as happens in mitosis, when an entire chromosome has been replicated, the duplicated copies are identical to each other, attached at the centromere, and called sister chromatids (Figure 6.9).

Once the cell's DNA has been duplicated, it can enter the meiotic phase of the cell cycle, consisting of two cell divisions called meiosis I and meiosis II.

Meiosis I The first meiotic division, meiosis I, consists of prophase I, metaphase I, anaphase I, and telophase I (Figure 6.10).

During prophase I of meiosis, the nuclear envelope starts to break down, and the microtubules begin to assemble. The previously replicated chromosomes condense, so they can be moved around the cell without becoming tangled; now the chromosomes are linear and can be viewed under a microscope. At this time, the homologous pairs of chromosomes can exchange genetic information, a process called crossing over.

At metaphase I the homologues line up at the equator, or middle, of the cell. Microtubules bind to the metaphase chromosomes near the centromere. The arrangement of homologous pairs is haphazard with regard to which member of a homologous pair faces which pole. This is called random alignment. A detailed description of crossing over and random alignment, along with their impact on genetic diversity, is given below.

At anaphase I the homologues are separated from each other by shortening of the microtubules, and at telophase I the nuclear envelopes reform around the chromosomes. The DNA is then partitioned into each of the two daughter cells by cytokinesis. Because each daughter cell contains only one copy of each type of chromosome, at this point the cells are haploid. Now both of these daughter cells are ready to undergo meiosis II.

Meiosis II Meiosis II consists of prophase II, anaphase II, metaphase II, and telophase II. This second meiotic division is virtually identical to mitosis. At prophase II of meiosis, the cell is readying for another round of division, and www

Media Activity 6.1 Meiosis

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  • silke
    What happens when uncondensed chromosomes are viewed through the microscope?
    7 years ago

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