Name _______________________________
Block 1 3 4 5
Unit 4 Introduction: Chromosome Reading
Directions: Annotate this reading as you go.
As usual: Highlight definitions, underline examples.
New procedure: Write questions or summaries in the margins. You need at least 1 for
each heading.
Coiled bundles of DNA and proteins, containing
hundreds or thousands of genes. What are these
things?
So far, we have talked about DNA at the molecular level—
What are nucleotides made of? What order do they come
in?
Our next unit begins with chromosomes. DNA does not
come in one long strand. Instead, our genome is broken up
into pieces. Specifically, our DNA in almost every cell is
arranged as 46 pieces, 23 from each parent.
Chromosomes and Genes
Each species has a characteristic number of
chromosomes. Chromosomes are coiled structures
made of DNA and proteins called histones (Figure to
the right). Chromosomes are the form of the genetic
material of a cell during cell division.
Each chromosome has many specific genes on it.
Chromosome 1 always has the gene that
determines eye color at the top, a gene that helps
determine height in the middle, and at the bottom,
the gene that determines hair color. Another
chromosome will have genes for other traits.
The human genome has 23 pairs of chromosomes located in the nucleus of most cells. Each chromosome
is composed of genes and other DNA wound around histones (proteins) into a tightly coiled molecule.
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The human species is characterized by 23 pairs of chromosomes, as shown in the figure below.
Human Chromosomes. Humans have 23 pairs of chromosomes. Pairs 1-22 are autosomes. Females have two X chromosomes,
and males have an X and a Y chromosome.
Chromosome numbers
Most Chromosomes are numbered from largest to smallest. In the human chromosomes in the
figure above, chromosome 1 is the largest. One copy came from the mom, and one came from
the dad of this human.
Autosomes
Of the 23 pairs of human chromosomes, 22 pairs are autosomes (numbers 1–22
in Figure above). Autosomes are chromosomes that contain genes for characteristics that are
unrelated to sex. These chromosomes are the same in males and females. The great majority of
human genes are located on autosomes.
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Sex Chromosomes
The remaining pair of human chromosomes consists of the sex chromosomes, X and Y.
Females have two X chromosomes, and males have one X and one Y chromosome.
As you can see from the figure above the X chromosome is much larger than the Y chromosome.
The X chromosome has about 2,000 genes, whereas the Y chromosome has fewer than 100,
none of which are essential to survival. Virtually all of the X chromosome genes are unrelated to
sex. Only the Y chromosome contains genes that determine sex. A single Y chromosome gene,
called SRY (which stands for sex-determining region Y gene), triggers an embryo to develop into
a male. Without a Y chromosome, an individual develops into a female, so you can think of
female as the default sex of the human species.
Human Genes
Humans have an estimated 20,000 to 22,000 genes. This may sound like a lot, but it really isn’t.
Far simpler species have almost as many genes as humans, or more!
Summary
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Humans have 23 pairs of chromosomes. Of these, 22 pairs are autosomes.
The X and Y chromosomes are the sex chromosomes. Females have two X
chromosomes, and males have one X and one Y.
The Y chromosome makes you male. Without a Y, you are female.
Practice
1. What new biology vocabulary words have you found in this reading? Write each one
below and either copy or write your own definition. (Hint: The three most important are
in bold)
2. How many total chromosomes do humans have?
3. Chromosomes can be looked at as pairs numbered 1-22 and X and Y. For example, you
have 2 chromosome 18s. Where does each copy come from?
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4. What is an autosome?
5. What is a sex chromosome?
6. Genetically, what is the difference between a human male and female?
7. What determines the sex of a baby?
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