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Transcript of Lesson 11
Think About It!
How many sets of genes do multicellular organisms inherit?
Chromosomes—those strands of DNA and protein inside the cell nucleus—are the carriers of genes.
The genes are located in specific positions on chromosomes.
A cell that contains both sets of homologous chromosomes is diploid, meaning “two sets.”
The diploid number of chromosomes is sometimes represented by the symbol 2N
All the cells in your body except for your sex cells are diploid
Humans have 23 homologous chromosome pairs.
Each of the 23 chromosomes from the male parent has a corresponding chromosome from the female parent.
Some cells contain only a single set of chromosomes, and therefore a single set of genes.
Such cells are haploid, meaning “one set.”
The gametes of sexually reproducing organisms are haploid.
Meiosis is a process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
Phases of Meiosis
Meiosis usually involves two distinct divisions, called meiosis I and meiosis II.
By the end of meiosis II, the diploid cell becomes four haploid cells.
Phases of Meiosis
Each replicated chromosome consists of two identical chromatids joined at the center.
The cells begin to divide, and the chromosomes pair up, forming a structure called a tetrad, which contains four chromatids.
As homologous chromosomes pair up and form tetrads, they undergo a process called crossing-over.
During metaphase I of meiosis, paired homologous chromosomes line up across the center of the cell.
During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell.
During telophase I, a nuclear membrane forms around each cluster of chromosomes.
Cytokinesis follows telophase I, forming two new cells.
Meiosis I results in two cells, called daughter cells, each of which has 46 chromatids, as it would after mitosis.
Because each pair of homologous chromosomes was separated, neither daughter cell has the two complete sets of chromosomes that it would have in a diploid cell.
Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II.
The chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I
During metaphase of meiosis 2, chromosomes line up in the center of each cell.
As the cell enters anaphase, the paired chromatids separate.
Telophase 2 and Cytokinesis
Each of the four daughter cells produced in meiosis II receives two chromatids.
These four daughter cells now contain the haploid number (n)—just two chromosomes each
Gametes to Zygotes
The haploid cells produced by meiosis II are gametes.
In male animals, these gametes are called sperm.
The female gamete is called an egg in animals.
Gametes to Zygotes
Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote.
The zygote undergoes cell division by mitosis and eventually forms a new organism.
Comparing Meiosis and Mitosis
Mitosis is a form of asexual reproduction, whereas meiosis is an early step in sexual reproduction.
The principle of independent assortment states that genes for different traits can segregate independently during gamete formation.
Independent assortment accounts for the many genetic variations observed in plants and animals.
Something to Remember
A graphic representation of all the autosomes of an organisms body arranged from largest to smallest followed by the allosomes