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Chromosome Tutorial
Chromosome Description
A chromosome is a molecule of DNA that is
supercoiled around proteins so it can fit into the
nucleus of a cell. In humans, the DNA molecule of a
single chromosome ranges between 14 and 73 mm in
length.
atsdr.cdc.gov
All eukaryotic
chromosomes consist of three distinct regions, the telomeres (tips of the
chromosome), the centromere (the constricted region of the chromosome),
and several hundred to several thousand genes found along the “body” of
the chromosome.
Use the following link to learn more about the packaging of DNA in
chromosomes.
http://learn.genetics.utah.edu/content/begin/traits/tour_chromosome.html
Answer questions 1 and 2 on the student worksheet.
Humans have 46 chromosomes (or 23 pairs) arranged by size. Use the
following link to learn about the size and number of genes on the largest and smallest human chromosomes.
http://useast.ensembl.org/Homo_sapiens/Location/Genome?r=6:133017695-133161157
Answer question 3 on the student worksheet
Chromosome Structure
THE TELOMERES
The telomeres are a repetitive base sequence
on the tips of the chromosome. In humans,
this sequence is TTAGGG and may be
repeated hundreds or thousands of times.
The telomeres prevent adjacent
chromosomes from fusing together and
prevent the loss of the essential genes in the
body of the chromosome during DNA
replication.
At the end of the tutorial you will
learn about the link between
telomeres and aging.
THE CENTROMERE
The constricted (narrowed area) on the chromosome is called the centromere. The centromere is usually composed of
about several million base pairs with a repetitive base sequence. The centromere has two functions. Prior to cell division,
every chromosome in a cell is replicated. The identical DNA copies are held together at the centromere. (Each identical
DNA copy attached at the centromere is called a sister chromatid. Two sister chromatids attached at the centromere is
called a duplicated chromosome.
The centromere also serves as an
attachment site for the spindle
microtubules during cell division so
the chromosomes can be moved and
distributed between daughter cells
Answer question 4 on the
student worksheet
CENTROMERE POSITION
The centromere divides the
chromosome into two arms, the “p”
arm and the “q” arm. The position of
the centromere determines the length
and identity of each arm. If the arms
are unequal in length, the shorter arm is
always designated “p” and the longer
arm is designated “q”. If they are
equal in length, the area above the
centromere is designated “p” and the
area below the centromere is
designated “q”. Human chromosomes
are classified into three types based on
the position of the centromere. In a
metacentric chromosome, the
centromere is roughly centered causing
the “p” and “q” arms to be about equal
in length. In a submetacentric
chromosome, the centromere is off
center resulting in a shorter “p” and
longer “q” arm. In an acrocentric
chromosome, the centromere is very
close to one end of the chromosome
resulting in a very short “p” and very
long “q” arm.
THE CHROMOSOME BODY
Chromosomes are usually stained with a chemical called Giemsa to increase their visibility. This produces a
unique pattern of light and dark bands along the body of the chromosome. Each band may contain hundreds
of genes. A gene is a section of DNA that can be transcribed into RNA. Active genes (those that are
transcribed frequently) are located in the light bands. The dark bands contain less active genes and genes
that are tissue specific (only expressed in very specific cell types).
Answer question 5-7 on the student worksheet
DIPLOID AND HAPLOID CELLS
Humans (and most other sexually reproducing organisms) contain two types of cells; diploid and haploid. These cells
differ in their chromosome content. Diploid cells contain homologous chromosomes; pairs of chromosomes that carry
the same complement of genes with one member of the pair inherited from the father and the other from the mother.
Chromosomes that are homologous are almost always the same size, have their centromeres in the same position and carry
the same number and type of genes. (An exception to this rule will be described later in the tutorial.)
Homologous chromosomes are not identical because the DNA sequence of a gene at a particular locus may vary from its
homolog. Alternative versions of a gene are called alleles.
All somatic cells (cells in the body excluding the sperm and egg cells) are diploid. Diploid cells are designated 2N.
Haploid cells do not contain homologous chromosome pairs. A haploid cell only contains one member of each
homologous chromosome pair. The gamete cells (sperm and egg) are haploid. Haploid cells thus contain half the number
of chromosomes of diploid cells. Haploid cells are designated N. Haploid cells are produced by meiotic cell division.
Human Diploid Cell
Human Haploid Cell
www.biologyjunction.com
www.learngenetics.utah
During sexual reproduction, a diploid cell from each parent undergoes a specialized form of cell division to produce
gamete cells. The gamete cells are haploid and specialize to become sperm or eggs. Fertilization fuses the haploid sperm
and egg to produce a single diploid cell (called a zygote), which
becomes the first cell of the offspring.
Answer questions 8-14 on the student
worksheet
Check Your Understanding of
Diploid and Haploid Cells with the
Smurfs
Smurfs are simple, sexually
reproducing organisms. Their
genome consists of nine genes
distributed on three different
chromosomes. The table below shows the distribution of genes on the chromosomes and the
alleles Papa Smurf has for each gene.
Trait
Skin Color
Hat Color
Facial Hair
Foot Size
Belly Button
Pants
Tail
Digits
Eyes
Location of Gene
Chromosome #1
Chromosome #1
Chromosome #1
Chromosome #1
Chromosome #2
Chromosome #2
Chromosome #2
Chromosome #3
Chromosome #3
Allele on First Homolog
S = blue
r = white
H= beard
f = large feet
B= outie
p = red
T= snub
D= four
E= black
Allele on Second Homolog
s = white
R = red
h = no beard
f= large feet
B= outie
p = red
t = long
d = five
e= red
Answer questions 15-17 on the student worksheet
Autosomes and Sex Chromosomes
In sexually reproducing organisms, there is one pair of chromosomes involved with gender determination. These are
called the sex chromosomes. In humans, the sex chromosomes of a female are perfectly homologous and designated XX.
In males, the sex chromosomes are NOT homologous and designated XY. The X chromosome contains roughly 2000
genes while the Y chromosome contains roughly 300. A small portion of the two chromosomes are homologous and in
males they behave as a homologous pair.
XY chromosomes
The homologous chromosome pairs in sexually reproducing organisms that are not involved in gender determination are
called autosomes.
Answer questions 18-19 on the student worksheet
Click on this link to learn about telomeres and aging. After you have read the
article, answer the questions at the end of the article on your own piece of
paper.
http://sciencenetlinks.com/science-news/science-updates/grandfathers-and-telomeres/
Name: __________________________________________________
Honors Biology
Chromosome Tutorial Student Work Guide


Go to the following webpage: http://www1.ccs.k12.in.us/teachers/tkons, Click on Honors Biology, click on the
Chromosome Tutorial Link.
Read and answer the questions.
1. Use arrows to mark the length of the DNA molecule of the smallest human chromosome and the largest human
chromosome on the ruler below.
2.
Label the three major components of the eukaryotic chromosome on the diagram below.
3.
Complete the following table for the largest and smallest human chromosome using the link provided in the
tutorial. You will need to identify the largest and smallest by the picture. Once you do this, click on
Chromosome summary in the left column. Under “change chromosome”, select the largest chromosome and its
data will appear. Repeat for the smallest chromosome.
.
Largest Human Chromosome
(identify it by number)
Total number of base pairs (length)
Total number of genes (add together
protein coding,noncoding and
psuedogenes)
Smallest Human Chromosome
(identify it by number)
Total number of base pairs
Total number of genes
4. Explain the relationship between a sister chromatid and a duplicated chromosome. Circle and label a sister
chromatid and duplicated chromosome on the diagram below.
5. Describe the function of each chromosome component in the table below.
Chromosome Component Function(s)
Gene
Telomere
Centromere
6 . Identify the following chromosomes as metacentric, parametacentric or submetacentric. Label the “p” and “q” arms
of each chromosome. (The line represents the position of the centromere.)
6.
How many genes might a band stained with giemsa dye on a chromosome contain?
7. Label a band on the metacentric chromosome above that contains genes which are transcribed frequently.
8. Complete the following table comparing and contrasting diploid and haploid cells.
Description
Homologous chromosome pairs
Designated N
Formed by mitotic cell division
Designated 2N
Has one member of each
homologous chromosome pair
Includes gametes
Includes somatic cells
Formed by meiotic cell division
Heart and liver cells
Sperm and Egg cells
Diploid
9. What are homologous chromosomes?
10. Provide three features of shared by homologous chromosomes.
Haploid
11. Why are homologous chromosomes NOT identical to one another?
12. How are homologous chromosomes different from a duplicated chromosome?
13. Identify the following cells as diploid or haploid.
14. Label a homologous chromosome pair in one of the diploid cells.
15. Use Papa Smurf’s Table of Genetics to complete the diagram showing the chromosomes in one of Papa Smurf’s
cells.
 Identify the homologous chromosome pairs (1-4). Write the chromosome # above each chromosome.
 Write in the correct alleles (letters) for each gene (box) on the chromosomes.
16. Use the same procedure to complete the diagram for another of Papa Smurf’s cells.
17. Label each of Papa Smurf’s cells as diploid or haploid.
18. What are sex chromosomes? What is the sex chromosome combination for human males and females?
19. What are autosomes?
Return to the Link on Telomeres and Aging. Answer questions here