Introduction to Genetics and DNA

Outcomes:

- Recall the basic structure of the cell, including the location and function of DNA
- Draw a labelled diagram to help explain the structure of DNA.
- Construct a model of DNA and identify the important features of the model

Learning goals

To describe the Watson and Crick model of the structure of DNA
To define the terms chromosome and gene
To describe the relationship between DNA, genes and chromosomes
To briefly describe the role of proteins in the body
To describe the role of genes which code for a specific protein
To outline gene expression as the grouping of three bases in DNA to determine the type of amino acids that join together to form a protein strand

Genetics is the study of inheritance. Genetic information called deoxyribonucleic acid (DNA), which is contained in the nucleus of the cell, is passed on from generation to generation. This results in traits or specific characteristics being passed on from parents to offspring, and occurs in all species – including humans.

DNA is a double-stranded molecule that forms a ‘double helix’ shape, like a twisted ladder.

DNA is a polymer (poly means ‘many’) as it is made up of numerous subunits called nucleotides.

A nucleotide of DNA has three major components:

a five-carbon deoxyribose sugar
a nitrogenous base – one of adenine (A), guanine (G), cytosine (C) or thymine (T)
a negatively charged phosphate group.

The nucleotides bind together to form two long strands. The two strands then bind together (using hydrogen bonds between the nitrogenous bases) and twist to form the double helix (twisted ladder shape). The sugars and phosphates form what would be the side of the ladder (and are called the sugar–phosphate backbone), while the nitrogenous bases from one strand align towards the centre and bind to complementary bases on the other strand to form the rungs of the ladder.

The structure of DNA: the subunits called nucleotides all join up to form the double-stranded DNA. Note the sugar–phosphate backbone on the edge of the DNA molecule and the nitrogenous bases joining in the centre.

The bases on one strand pair with the bases on another strand: adenine (A) always pairs with thymine (T) and guanine (G) always pairs with cytosine (C). This pairing is called complementary pairing and has to do with the shape of the nitrogenous bases and the number of hydrogen bonds they need to form to hold the strands together.

For example, you can see in Figure that guanine and cytosine both form three hydrogen bonds, while adenine and thymine only form two hydrogen bonds.

What is a chromosome?

Just prior to cell replication, the DNA winds up tightly around proteins called histones to form structures called chromosomes. Each molecule of DNA condenses to form one chromosome. Along the chromosome are genes, which are the units of inheritance.

Genes are actually just sections of DNA or chromosome that hold the specific instructions required for making each and every one of the proteins in our bodies! Proteins are fundamental to all of the chemical reactions in an organism, and also form much of the structure of a chromosome due to the histones that DNA wraps around.

The relationship between DNA and chromosomes

You may be familiar with the X-shape of a chromosome, but this shape is only seen when DNA replication (when DNA is copied) is occurring.

Before replication, the chromosome appears as a single condensed DNA molecule. After replication, this produces two identical chromatids that are joined together by centromeres to form the distinct X-shape; these are then known as sister chromatids. See below

A scanning electron micrograph of a human chromosome. Notice the two identical chromatids joined at a centromere.

Questions:

Write out the full name for DNA.
What is a gene?
Where in the cell are chromosomes located?
What two scientists established the structure of DNA?
What is the shape of DNA?
What are the sides of the DNA ladder made of?
What are the "rungs" of the DNA ladder made of?
What sugar is found in DNA?
How do the bases bond together? A with _______, G with ________.
Why is DNA called the "Blueprint of Life"
Write a sentence explaining the relationship between a cell's nucleus, genes, chromosomes and DNA.
What important function does DNA have in a cell?
Explain why it is important for DNA to replicate exactly.
What process do chromosomes go through before a cell divides?

Quick Questions

1. The structure of DNA could be described as

a. a large polymer molecule
b. a double helix
c. a large protein molecule
d. two of the above

2. DNA stands for

a. diribose nuclear acid
b. deoxyribose nucleic acid
c. deoxyribose nuclear acid
d. deoxyribulose nitrogenous acid

3. In the DNA molecule the nitrogen bases

a. form the spiral backbone
b. alternate with phosphate groups to form the ‘railings’ of the spiral
c. form the connecting links that hold the two chains together
d. bond so that adenine always links with cytosine

4. Genes are

a. the triplet code of nitrogen bases
b. segments of the DNA molecule that code for a particular protein
c. found attached to one of each pair of chromosomes
d. composed of five consecutive triplet codes

5. The features of an organism

a. are determined only by the genes they inherit
b. are determined only by the environment in which they live
c. that are acquired during their life can readily be inherited by their children
d. are determined both by its inherited genes and its interaction with the environment

ANSWERS: B, B, C, B, D