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Introduction to the lesson
Your hair colour, your height, your ability to roll your tongue and even certain aspects of your personality are all determined by genes. Genes are made up of deoxyribonucleic acid---DNA. The human genome is made up of 3 billion DNA base pairs which are divided among 46 chromosomes. In this lesson you will begin to study the structure and function of chromosomes.
Reading
Read the section “Why grow old?” in your biology text book on p.94.
Thinking it through
- Would you like to live to old age? Why / why not?
- Why do you think living organisms grow old?
- The idea of prolonging life by preventing ageing raises some important moral and ethical questions. What questions does it raise in your mind?
- What do you know about telomeres and their function in the cell?
Reading
Read the untitled section on p.94 of your text book.
Thinking it through
- Did you realize the importance of the nucleus in controlling the cell’s activity?
- Did you realize that it is the genetic material contained in the DNA that directs all life?
- Do you remember the cell principle from IG? [All living organisms are composed of living units called cells and of cell products and that all cells come only from pre-existing cells]
Chromosomes
Read the section under the heading “Chromosomes” on p.94-96 of the text book.
Notes:
Study Figure 5.2 on p.95 of the text book.
- Notice the x-shaped objects – these are sets of chromosomes.
- Notice that each chromosome is composed of two chromatids held at the centromere.
- Notice the different sizes of the chromosomes.
- Notice the different positions of the centromeres.
- (You may want to refer back to the IG book and look at figures 18.3 and 18.4 on p.231-232 of the Cambridge IG Biology Coursebook Third edition or Fig.14.2 and 14.3 on p.295 of the Cambridge IG Biology Coursebook Second edition).
Study Figure 5.3 on p.95 of the text book
- Notice the two identical chromatids making up one chromosome.
- Notice the position of the telomeres and the centromere.
Learning Activity 1
Answer the following questions in your exercise book.
- Draw and label a simplified diagram of the structure of a chromosome. Include the following:
- Telomeres
- Centromeres
- Genes
- Chromatids
- The number of chromosomes is characteristic of the species. In humans there are 46 chromosomes and in fruit flies there are only 8. Do a quick internet research and find out how many chromosomes there are in:
- Dogs
- Cats
- Mice
- Horses
- Monkeys
- Any other eukaryotic organism that you are interested in
- Do you think the number of chromosomes says anything about the intelligence or complexity of the organism? Why/why not?
Chromatin
Learning Activity 2
Answer the following questions in your exercise book.
- Why is protein necessary to package the DNA?
- Write a short paragraph describing chromatin. Mention the following:
- Euchromatin and heterochromatin
- 2 essential features for nuclear division
- A single strand of a cell's DNA can be up to 2-3 metres long. How can such a long molecule fit inside a cell which is only a few micrometres big? Which protein is involved in compacting the DNA strand?
- Study Fig. 5.4 (p.96) which shows how nucleosomes are involved in DNA coiling. Although you are not required to learn about nucleosomes at AS Level, it will help you to understand the structure of DNA.
Definitions and review
Definitions
- Gametes
- Zygote
- Chromosomes
- Chromatids
- Centromere
- Chromatin
- Euchromatin
- Heterochromatin
- Nucleosome
- Interphase
- Telomere
Mindmap
Start a new mindmap for chapter five and add this section to it.