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.

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.

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 cellscontain 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

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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 / Location of Gene / Allele on First Homolog / Allele on Second Homolog
Skin Color / Chromosome #1 / S = blue / s = white
Hat Color / Chromosome #1 / r = white / R = red
Facial Hair / Chromosome #1 / H= beard / h = no beard
Foot Size / Chromosome #1 / f = large feet / f= large feet
Belly Button / Chromosome #2 / B= outie / B= outie
Pants / Chromosome #2 / p = red / p = red
Tail / Chromosome #2 / T= snub / t = long
Digits / Chromosome #3 / D= four / d = five
Eyes / Chromosome #3 / E= black / 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.

Name: ______

Honors Biology

Chromosome Tutorial Student Work Guide

  • Go to the following webpage: 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.
  1. Label the three major components of the eukaryotic chromosome on the diagram below.

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  1. 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
  1. Explain the relationship between a sister chromatid and a duplicated chromosome. Circle and label a sister chromatid and duplicated chromosome on the diagram below.
  1. 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.)

  1. How many genes might a band stained with giemsa dye on a chromosome contain?
  1. Label a band on the metacentric chromosome above that contains genes which are transcribed frequently.
  1. Complete the following table comparing and contrasting diploid and haploid cells.

Description / Diploid / Haploid
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
  1. What are homologous chromosomes?
  1. Provide three features of shared by homologous chromosomes.
  1. Why are homologous chromosomes NOT identical to one another?
  1. How are homologous chromosomes different from a duplicated chromosome?
  1. Identify the following cells as diploid or haploid.
  1. Label a homologous chromosome pair in one of the diploid cells.
  1. 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.
  1. Use the same procedure to complete the diagram for another of Papa Smurf’s cells.
  1. Label each of Papa Smurf’s cells as diploid or haploid.
  1. What are sex chromosomes? What is the sex chromosome combination for human males and females?
  1. What are autosomes?

Return to the Link on Telomeres and Aging. Answer questions here