Ribonucleic acid (RNA) consists ofnucleotides made of a sugar,phosphate group and bases, just likeDNA, although it differs in 3 ways:

DNA RNA

Sugar present Deoxyribose Ribose

Complementary base partner of adenine

Thymine Uracil

Number of strands in one molecule

Two One

The sequence of bases in a DNA strand represent thegenetic code from which enzymes are coded for and,therefore, all information is passed on for the inheritedcharacteristics.

DNA has only four bases, however, and there are around20 types of amino acids. Three bases are required tocode for each amino acid (64 possible combinations).

Eachof the 20 amino acids is coded for by one, or more ofthese triplets of bases

Each triplet of bases is called a codon.

The information coded for by thetriplets of bases of DNA in thenucleus need to reach thestructures responsible for protein synthesisIn the cytoplasm.

This function is performed by messengerRNA (mRNA) which is formed (transcribed)from one of the DNA strands from freeRNA nucleotides in the nucleus

1. DNA unwinds2. Weak hydrogen bonds between bases separate,

separating the DNA strands3. Free RNA nucleotides find their complementary

DNA base partner4. Weak hydrogen bonds form between the base

pairs5. Strong chemical bonds formed between the

sugars and phosphates of the RNA nucleotides by RNA polymerase

6. Weak hydrogen bonds between the DNA and RNA bases break allowing the RNA strand to separate from the DNA template

7. Weak hydrogen bonds between the DNA bases form and reunite the DNA strands which then wind up, forming a double helix again.

Transcription animation - detail

A second type of RNA exists in the cytoplasm

called transfer RNA (tRNA).

Only one triplet of bases in RNA is exposed,known as an anticodon. Each anticodon corresponds to a particularamino acid.

Ribosomes are the site of translation ofmRNA into protein. Some are found free inthe cytoplasm but others are attached to

therough endoplasmic reticulum.

Ribosomes contain enzymes essential forprotein synthesis and large numbers ofribosomes are found in growing cells.

This is a system of tubules and flattened “sacs” which iscontinuous with the nucleus membrane.It is encrustedwith ribosomes on it’s outer surface.

The function of the Rough ER is to transport proteins

The Golgi Apparatus (or Golgi body) is made upof groups of fluid-filled sacs.

Vesicles containing newly made proteins become“pinched-off” from the Rough ER and fuse withthe sacs of the Golgi body. Proteins then passthrough the Golgi body via vesicles

During this time, proteins are processed by theGolgi e.g. by adding a carbohydrate component tomake it a glycoprotein

Eventually the processed protein is packaged in avesicle and excreted from the cell by exocytosis.

Golgi bodies are especially numerous in secretorycells e.g. those of gland cells that produce andexcrete hormones.

The function of the Golgi body is to process andpackage proteins for release