Page 7 - CB1.3 Genetic Varation sciPAD

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CB 1.3 GENETIC VARATION
PRINCIPLES OF GENETIC VARIATION
Cells and Chromosomes
Immediately after fertilisation (when a sperm cell joins an ovum), most organisms consist of a
single cell. That cell contains all the necessary instructions to grow into a multicellular organism,
how that organism will carry out its life functions, and even whether it will get certain diseases. The
development of an organism from a single fertilised egg cell is very complicated and all of the
instructions on how to do so were, at one point, contained in that single fertilised egg.
These instructions are called the ‘genetic code’. The genetic code is found within DNA. DNA
occurs as chromosomes contained within the nucleus of each cell.
Your body is
made of cells.
The nucleus contains
46 chromosomes.
Chromosomes are
a length of DNA.
Inside every cell*
is a nucleus.
*except for your red blood cells
Cell
Organism Chromosome DNA
1. Place the following terms in order from largest to smallest.
DNA Nucleus Chromosome Cell Organism

2. From a single cell (i.e. a fertilised egg), an average human grows to contain an estimated
50-100 trillion cells.
(a) Describe where the single cell gets the information to ultimately produce an adult
human. 

(b) Outline where this information is stored. 

3. Most cells in the human body contain 46 chromosomes, occurring as 23 pairs.
(a) How does the term ‘chromosome’ relate to ‘DNA’? 


(b) Outline the role of the nucleus. 

(c) Define the term ‘homologous pair’
. 

(d) Predict how many of these chromosomes come from:
(i) the person’s mother: 
(ii) the person’s father: 
Each species has its own unique number of chromosomes. Humans have 46 chromosomes. These
chromosomes occur in pairs called homologous pairs (‘homos’ is Greek for ‘the same’). One
chromosome of the pair comes from your mother, the other chromosome comes from your father.
Homologous chromosomes are identical in size and shape.
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