The Cell Cycle Back to Top Despite differences between prokaryotes and eukaryotes, there are several common features in their cell division processes. Replication of the DNA must occur.
More About Meiosis Background At a genetic level, sexual reproduction is all about mixing up genes and putting together new combinations. Most of the action during meiosis centers on the chromosomes. Cells in the human body have 46 chromosomes, including 22 pairs of autosomes and one pair of sex chromosomes XX in females, XY in males.
Because there are two sets of chromosomes, one from each parent, the cells are considered diploid. Meiosis starts with a diploid cell and turns it into four haploid cells, cells with only one set of chromosomes. This means that when the chromosomes of egg and sperm cells combine at fertilization, the embryo regains the normal diploid number.
Meiosis mixes up the parental genes in two ways. First, the members of each chromosome pair come together and swap segments in a process known as crossing over, or recombination. Second, because each gamete gets only half the parental chromosomes, the exact combination in each egg or sperm can and does vary.
This is because during meiosis the chromosomes assort independently, with a random member of each pair going to each daughter cell. Because males have one X and one Y chromosome, half the cells get an X and half get a Y during the meiosis that leads to sperm production.
In females, all the eggs will get one or the other X. In a general sense, the sex of the offspring is determined by the particular sex chromosome carried by the sperm.
However, in the early weeks of development, all fetuses have preliminary structures for both sexes, and the immature gonads can become either testes or ovaries. In the seventh week of fetal development, a gene on the Y chromosome, if present, activates, and the bipotential gonads commit to becoming testes.
In the absence of a Y chromosome, and the signal to form testes, the fetus develops as a girl. At least that's the way it usually happens. In rare cases, an XX individual becomes a male or an XY individual becomes female. Researchers realized that studying the genes of these sex-reversed people could lead them to the master switch for sex determination.
They subsequently identified a gene called SRY sex-determining region on the Y chromosome. Introduction Meiosis, the form of cell division unique to egg and sperm production, sets the stage for sex determination by creating sperm that carry either an X or a Y sex chromosome.
But what is it about the X or Y that determines sex? Before a meiotic cell divides, its two sets of chromosomes come together and cross over, or swap, segments.
The first animation shows normal crossing over, where the X and Y chromosomes exchange pieces only at their tips. The second animation shows a rare mistake in which the Y chromosome transfers a gene called SRY to the X chromosome, resulting in sex-reversed babies.
Studies of sex-reversed individuals led researchers to identify the master switch for sex determination, the SRY gene, which tells a fetus to become a boy. Normal male meiosis In the cell nucleus, chromosomes contributed by this male's mother in red and father in blue pair up.
For clarity, only the X and Y sex chromosomes and 5 of the 22 pairs of autosomes nonsex chromosomes are shown. Each chromosome has replicated and consists of two identical chromatids. Crossing over can occur anywhere along the autosomes, and here, they swap segments at each end. The X and Y chromosomes normally cross over only at their tips indicated in blue on the Y.
Note that SRY lies below this region. The nuclear membrane breaks down, and the chromosomes line up along the cell's equatorial plane and then move to the poles. A random member of each chromosome pair goes to each haploid daughter cell.
A second division separates the chromatids and produces four cells, which develop into sperm. In the top panel, a sperm with an X chromosome fertilizes the egg; in the bottom panel, a sperm with a Y chromosome fertilizes the egg. The XX and XY fetuses develop along the same pathway through week six.
Atypical male meiosis resulting in sex-reversed individuals Meiosis begins just as in the previous example. This produces two sperm with abnormal sex chromosomes. They are sex reversed. Autosomes Autosomes are chromosomes other than sex chromosomes and are the same in both sexes.
Chromosomes Mammalian chromosomes are DNA molecules bound up with proteins, particularly proteins known as histones, which form a core for the strand of DNA to wrap around.
Chromosomes become visible in the cell nucleus only during cell division when they are condensed into tightly coiled rods. They typically have two arms on either side of a centromere, a condensed region critical for the movement and sorting of chromosomes during cell division.
The two halves of a replicated chromosome are called chromatids.Name _____ Date _____ BIO SOL Review 13 - Reproduction - Mitosis, Meiosis (12 questions) 1. () Which phase of mitosis would be seen next? Mitosis. Mitotic recombination is primarily a result of DNA repair processes responding to spontaneous or induced damages.
(Also reviewed in Bernstein and Bernstein, pp –). Homologous recombinational repair during mitosis is largely limited to interaction between nearby sister chromatids that are present in a cell subsequent to DNA replication but prior to cell division.
Mitosis and meiosis have different purposes, but share common features in how they work. Knowing their similarities is the beginning of understanding how they are different. The fundamental difference between mitosis and meiosis is that mitosis produces two daughter cells with the same number of.
Paul Andersen compares and contrasts mitosis and meiosis. He shows how you can count cells in various phases of mitosis to construct a cell cycle pie chart. In G1, each chromosome is a single chromatid. In G2, after DNA replication in S phase, as cell enter mitotic prophase, each chromosome consists of a pair of identical sister chromatids, where each chromatid contains a linear DNA molecule that is identical to the joined sister.
Prokaryotes - Missing a Nucleus If you're looking to learn about cells with a nucleus, this is the wrong place. Prokaryotes do not have an organized nucleus. Their DNA is kind of floating around the cell.