BIOL 2112 Chapter Notes - Chapter 13: Sister Chromatids, Y Chromosome, Somatic Cell
3 May 2018
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1) A. How many pairs of homologous chromosomes are in the nucleus of a normal
human cell? In humans, each somatic cell has 46 chromosomes. During mitosis, the
chromosomes become condensed. At this point they can become distinguished from one
another by their size and the pattern of the colored bands produced by certain chromatin
binding stains. There are two chromosomes of each 23 types. The resulting ordered
display is called a karyotype. Two chromosomes of a pair have the same length,
centromere position, and staining patters – these are called homologous chromosomes
or homologs. Both chromosomes of each pair carry genes controlling the same inherited
characters. B. How many of these pairs are autosomes? The two chromosomes referred
to as X and Y are an important exception to the general pattern of homologous
chromosomes in human somatic cells. Typically, female humans have a homologous pair
of X chromosomes (XX) while males have one X and one Y chromosome (XY). Only small
parts of the X and Y are homologous. Most of the genes carried on the X chromosome do
not have counterparts on the Y chromosome, and the Y chromosome has genes lacking
on the X. Due to their role in sex determination, the X and Y chromosomes are called sex
chromosomes and the other chromosomes are called autosomes. We inherit one
chromosome from each parent, thus, the 46 chromosomes in our somatic cells are
usually 2 sets of 23 chromosomes – a maternal set and a paternal set. 2) A. In the
human body, what types of cells are diploid and what types of cells are haploid? Any cell
with two chromosome sets is diploid and has a diploid number of chromosomes
abbreviated 2n. In a cell in which DNA synthesis has occurred, all the chromosomes are
duplicated and therefore each consist of 2 identical sister chromatids associated closely
at the centromere and along the arms. Even though the chromosomes are duplicated,
we still say the cell is diploid because it has only 2 sets of information regardless of the
number of chromatids, which are just copies of the information from one set. Gametes
contain a single set of chromosomes and are called haploid cells. Each has haploid
number of chromosomes. For humans, the haploid number is 23. The set of 23 consists of
22 autosomes plus a single sex chromosome. An unfertilized egg contains an X
chromosome, and a sperm contains either an X or a Y chromosome. B. What is the
process that reduces the number of sets of chromosomes from two to one and where
does this occur? Meiosis reduces the number of chromosome sets from diploid to
haploid counterbalancing the doubling that occurs in fertilization. C. How is the diploid
condition restored in humans? By combining two sets of chromosomes. 3) Summarize
the process of meiosis in three sentences. Meiosis is preceded by the duplication of
chromosomes. However, this single duplication is followed by not one, but two
consecutive cell divisions called meiosis I and meiosis II. These two divisions result in 4
daughter cells each with half as many chromosomes as the parent cell. 4) A primary
difference between mitosis and meiosis is the pairing of homologous chromosomes in
prophase I of meiosis. A. What is this process called and what promotes this process?
During early prophase I, the chromosomes have been duplicated and the sister
chromatids are held together by proteins called cohesins. Each chromosome pair with its
homolog aligns gene by gene and crossing over occurs: the DNA molecules of non-sister
chromatids are broken (by proteins) and are rejoined together.
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