Coding for Life

IntroductionNucleic acids are very special chemicals in cells which have the information ...• to control cellular activities, and• concerned with the transmission of inherited characteristics.All nucleic acids are made of simple repeating units (monomers), linked together to form lengthy strands, called nucleotides.

Coding for Life

Nucleic acids There are two types of nucleic acids ... 1Deoxyribonucleic acid (DNA).

This central nucleic acid is a major component of chromosomes. Found primarily in the nucleus, a small amount is found in mitochondria and chloroplast.2Ribonucleic acid (RNA).

Is involved in the ‘reading’ of the DNA information. Found in the nucleus and cytoplasm of cells.

General structure of nucleotidesNucleotides are the building blocks (monomers) of the two different nucleic acids (RNA and DNA). The best way to understand the structure of DNA and RNA is to study the nucleotides. Each nucleotide consists of three parts:• a sugar molecule,• a phosphate group,• a nitrogenous base.

Detail of the 3 parts of nucleotides1. Phosphate groupDNA and RNA have the same phosphate group, which links adjacent sugars together into a strand of great length.

2. Sugars

Deoxyribose sugar is found only in DNA. Ribose sugar is found only in RNA.

DNA = Deoxyribose

RNA = Ribose

3. Nitrogenous basesOne example of a nitrogenous base occurs in each nucleotide. These bases comprise the coded genetic message. Two types of bases, namely,3.1 Purines:Purine bases consist of a two-ringed structure, the longer bases.

Two examples Adenine and Guanine.

G

3.2 PyrimidinesPyrimidine bases consist of a single-ringed

structure, the shorter bases.

Two examples for DNA only Cytosine and Thymine.

Two examples for RNA only Cytosine and Uracil.

C

Nitrogenous bases for

DNA RNAAdenine AdenineThymine UracilCytosine CytosineGuanine Guanine

In RNA Thymine is replaced by Uracil

Coding for Life – DNA structure

A simplified representation of a portion of a DNA molecule

Phosphate group links two sugars together

Deoxyribose sugar

Two weak hydrogen bonds

Three weak hydrogen bonds

Complementary nitrogenous bases

Linking the nucleotides in a DNA moleculeStep one: To form a single strandThe deoxyribose sugar of one nucleotide joins with the phosphate group of another nucleotide. These nucleotides are joined by a strong sugar-phosphate bond.

Step 2: Formation of a double strandA single strand is joined to another strand with complementary nitrogenous bases. These complementary bases are linked by weak hydrogen bonds.

The two strands together form a ladder-like structure.

The phosphate group and the sugar form the frame (legs) and the base pairs form the steps of the ladder.

Phosphate group links two sugars together

Pairing of nitrogenous bases for DNA (complementary base pairs)

•A purine (long) and a pyrimidine (short) base pair with each other. Why? (Ensuring that the space/width between the two legs of the ladder stays the same)• Thymine (short) always pairs with Adenine (long). • Cytosine (short) always pairs with Guanine (long).

Pairing of nitrogenous bases

The total length of DNA in mammal cells is ± 2 meters – in your body ±10 billion km.

Thymine is pairing

with Adenine

Cytosine is pairing

with Guanine

Number of nitrogenous bases Because the nitrogenous bases are paired in a specific way the following deductions important: • The number of cytosine bases present in a specific DNA molecule is? Equal to the number of guanine bases.

• The number of Adenine in a specific DNA molecule = ? number of Thymine.

In the cell the DNA ladder is not a flat structure. It is twisted and forms a double helix. See the two presentations below.

Introduction to DNA-replication• Before cell division, it is necessary that DNA makes an exact copy of itself in order for identical genetic material to be passed on from original cell to new daughter cell. • The process of making exact copies of the original DNA molecule is called DNA replication. This process is controlled by enzymes.•The main activity during interphase (between cell divisions) is DNA replication

Process of DNA replication1. The double helix starts to unwind and

forms a flat, ladder-like structure.2. The two strands ‘unzip’ because of the

weak hydrogen bonds between the complementary bases are broken along its length by enzymes.

3. The bases of the two strands become exposed.

4. Each separate strand serves now as a template for the synthesis of a new complementary DNA strand.

Process of DNA-replication (cont.)4. Many free nucleotides in the nucleus become

bonded to the exposed bases of the template to form the new complementary DNA strands.

5. Complementary basis pairs always joins as follows; adenine of the template (exposed) with thymine of the new strand. Or thymine from the template with adenine from the nucleus by means of the weak hydrogen bonds.

6. Cytosine of the template joins with guanine from the nucleus. As well as guanine from template with cytosine of the new strand.

Process of DNA replication 7. Sugar molecules and the adjacent phosphate

groups of the new nucleotides are linked by a sugar-phosphate bond.

8. The enzyme DNA polymerase is responsible for linking an new nucleotide to the growing new complementary strand.

9. Two new identical DNA molecules are formed. They start to twist and form a double helix. (Use the diagrams on the next 3 slides to memorise the process)

Schematic representation of DNA replication

Double helix unwinds and forms a flat, ladder-like structure

Hydrogen bonds break

templatetemplate

DNA replication continuous

New bases bond with the correct pairs from each template

Free nucleotides in nucleus plasm

Two identical DNA molecules are formed.

Part of DNA molecule 1 Part of DNA molecule 2

Each DNA molecule forms a double helix.

Characteristics of a DNA molecule

• Is a very long molecule• Has a double helix structure• Sequence in which bases pairs can occur is unlimited.• There are equal cytosine bases than guanine bases and equal adenine bases than thymine bases in a specific DNA molecule.

Functions of DNA• DNA carriers the coding for all inheritance

characteristics.• The same genetic information is transferred

from one cell to the next by DNA replication and cell division.

• DNA controls the structure and functioning of a cell.

• Proteins were produced according to the sequence of the bases in the DNA molecule.

• DNA is able to make RNA (ribonucleic acid).