Chromosomes & Chromosome Number
Monday, March 29, 2010
8:36 PM
Drill ‐ March 28, 2011
Answer the following on a new sheet in your notebook.
1. Each normal human body cell (called a somatic cell) has 46 chromosomes. By what process does each somatic cell get a copy of these chromosomes?
2. If you began as a single cell, then where did the chromosomes for this first cell come from?
3.
Organisms that reproduce sexually (like humans) join two cells together to produce the first cell of their new offspring. If two somatic cells joined together in this process how would the number of chromosomes in the offspring compare to the number of chromosomes in its parents cells?
4. Propose a mechanism that would allow for sexual reproduction, but would keep the number of chromosomes from generation to generation the same. The Problem of Chromosome Number
Types of Reproduction
Organisms can reproduce in two ways:
○ Sexual reproduction ‐ type of reproduction in which genetic material from two* parents is combined to produce an offspring that is genetically similar but not identical to either parent
○ Asexual reproduction ‐ type of reproduction in which a single parent produces offspring that are genetically identical to itself (clones)
Significance of Chromosomes
Everyone has traits.
○ A trait is a characteristic that can be passed from parent to offspring.
Each trait is the result of proteins that are created by a person's cells.
The production of these proteins is controlled by genes
○ A gene is a segment of DNA on a chromosome that codes for a particular protein.
Every chromosome consist of hundreds of genes.
All of the DNA that exists in the nuclei of an organism on all of its chromosomes is called its genome.
Unit 8 - Intro to Genetics Page 1
Significance of Chromosomes
Everyone has traits.
○ A trait is a characteristic that can be passed from parent to offspring.
Each trait is the result of proteins that are created by a person's cells.
The production of these proteins is controlled by genes
○ A gene is a segment of DNA on a chromosome that codes for a particular protein.
Every chromosome consist of hundreds of genes.
All of the DNA that exists in the nuclei of an organism on all of its chromosomes is called its genome.
Homologous Chromosomes
Human body cells have 46 chromosomes.
Each parent contributes 23 chromosomes to his/her offspring. Therefore every person has 23 pairs of chromosomes.
Each pair‐ one from the female parent and one from the male parent ‐ are called homologous chromosomes.
Homologous chromosomes have three things in common:
○ The carry the same kinds of genes.
○ They are the same size.
○ Their centromeres are in the same location.
Other Links
An Introduction to DNA and Chromosomes
LabBench
http://www.phschool.com/science/biology_place/labbench/lab3/homologs.html
Screen clipping taken: 3/29/2010, 9:46 PM
Unit 8 - Intro to Genetics Page 2
Karyotypes
Geneticists are scientists who study the inheritance of traits from one generation to the next and how these traits result in variation. One tool geneticists have are karyotypes.
○ A karyotype is a photo of an organisms chromosomes.
 Chromosomes are stained.
 A photomicrograph is taken of the chromosomes during metaphase of mitosis.
 Homologous pairs of chromosomes are arranged from largest to smallest.
Haploid and Diploid Cells
In order to keep the number of chromosomes constant from one generation to the next, sexually producing organisms must first divide their homologous pairs of chromosomes in half before reproduction.
Specialized sex cells produced by sexually reproducing organisms are called gametes. Male gametes are called sperm.
Female gametes are called eggs.
In humans, each gamete contains 23 chromosomes ‐ half of the total number of 46.
The number of chromosomes in an organism's gametes is called its haploid number.
○ The symbol n is used to represent the haploid number.
○ For humans n = 23.
○ The value of n is different for different organisms.
When two haploid gametes (egg and sperm) combine with one another it is called fertilization.
As a result of fertilization, the resulting cell has n + n chromosomes or 2n chromosomes.
○ A cell with 2n chromosomes is said to be diploid.
○ All of the cells of your body, except the gametes, are diploid.
Homework
Read pp. 275‐278 in your textbook.
Complete the corresponding workbook pages.
Unit 8 - Intro to Genetics Page 3
Sperm fertilizes an egg
http://carnegieinstitution.org/first_light_case/horn/lessons/images/sperm%2Begg.JPG
Screen clipping taken: 3/29/2010, 10:05 PM
Meiosis
Monday, March 29, 2010
10:39 PM
Drill 1. List two differences between sexual reproduction and asexual reproduction.
2. Complete the table. Organism
Haploid Number Diploid Number n
2n
Armadillo
House fly
64
6
Apple tree
34
Camel
35
Bat
22
Salamander
Corn
Human
24
10
46
3. In an organism, if 2n = 30, then n = ______.
4. The haploid chromosome number of an alligator is 16. What is the value of ‘2n’ for this organism?
5. A cell is taken from the body of a male chimpanzee. It contains 48 chromosomes. How many chromosomes would be in a chimpanzee sperm cell?
Meiosis
Meiosis is a type of reduction division that produces gametes in the gonads of sexually reproducing organisms.
○ Reduces the chromosome number in a cell by half.
○ Separates homologous pairs of chromosomes
○ Involves two successive cell divisions ‐ meiosis I and meiosis II
○ Results in greater genetic variation
Unit 8 - Intro to Genetics Page 4
Synapsis ‐ Homologous pairs line up together. They are held together tightly.
Homologous pairs of chromosomes line up at the equator side by side.
Formation is called a tetrad because it consists of four chromatids.
Sometimes pieces of homologous chromosomes are exchanged. This is called crossing over.
Crossing Over
Unit 8 - Intro to Genetics Page 5
Homologous pairs of chromosomes are separated. At this point the resulting daughter cells have become
haploid.
The activities of Meiosis II are no different than mitosis. However, cells are already haploid as compared to the parent cell so four haploid cells result.
To verify chromosome number count the centromeres in the new cells.
Meiosis Animations
Meiosis in Motion ‐ John Kyrk Site
How Meiosis Works ‐ very comprehensive
Cells Alive ‐ Meiosis Animation
Gamete Production in Humans
Meiosis occurs differently in males and females. ○ In males…
 the cytoplasm is equally divided after meiosis I and meiosis II.
 Results in four similar‐sized sperm cells
 Spermatogenesis (sperm production by meiosis) occurs in the testes from adolescence until age 70+
○ In females…
 The cytoplasm is not divided evenly after meiosis I and meiosis II.  One cell during each division retains the majority of cytoplasm.
 Results in one large egg and three smaller polar bodies.
 Polar bodies are reabsorbed by the gonad and not used in reproduction.
 Oogenesis (egg production) occurs in the ovaries prior to a female's birth. Immature egg cells will mature starting in adolescence.
Unit 8 - Intro to Genetics Page 6
Homework
• Read pp. 271‐276 in your textbook.
• Complete the Meiosis Concept Map and Study Guide.
• Add the following to your flash cards: meiosis, synapsis, crossing over, tetrad, gonad, polar body, spermatogenesis, oogenesis
Unit 8 - Intro to Genetics Page 7
Meiosis vs. Mitosis
Wednesday, March 31, 2010
12:40 PM
Drill 1. Each laboratory group should obtain an envelope from the class manager.
2. Take out the meiosis cards and spread them out on their desk. The pictures show the steps of the process of meiosis.
3. Put the pictures in order. When your group has the pictures in order, label each phase with a transparency marker. 4. Finally, match the descriptions of each phase to the appropriate labeled picture. 5. Check your work with the teacher when you think that you have properly completed the task. 6. When instructed, place all materials back in the envelope. Meiosis Video and Animation
Meiosis ‐ From John Kyrk
Comparing Mitosis and Meiosis
Mitosis
Meiosis
One division occurs during mitosis.
Two sets of divisions occur during meiosis: meiosis I and meiosis II.
DNA replication occurs during interphase
DNA replication occurs once before meiosis I.
Synapsis of homologous chromosomes does not occur.
Synapsis of homologous chromosomes occurs during prophase I.
Two identical cells are formed per cell cycle.
Four haploid cells (n) are formed per cell cycle.
The daughter cells are genetically identical.
The daughter cells are not genetically identical because of crossing over.
Mitosis occurs only in body cells.
Meiosis occurs in reproductive cells.
Mitosis is involved in growth and repair.
Meiosis is involved in the production of gametes and providing genetic variation in organisms.
Unit 8 - Intro to Genetics Page 8
Homework
Enjoy your Easter Break.
You may want to review Meiosis before you return on April 12th.
Unit 8 - Intro to Genetics Page 9