DNA: The Code of Life

Question 1

What are Nucleic Acids?

Answer 1

Nucleic acids have been called the molecules of life as they have the capacity to store the information that controls cellular activity and the:
- specialization of cells to form tissues
- arrangement of tissue into organs
This enables organisms to perform all the functions necessary to carry out the basic processes of life, ie movement, nutrition, respiration, excretion, growth, reproduction and responding to stimuli.
They do this by controlling the synthesis of proteins. Proteins make up the structure of the body, as enzymes are proteins, they also control chemical processes inside cells. Nucleic acids control the structure and functioning of all living organisms. Two nucleic acids found in cells are:
- DNA
- RNA

Question 2

Deoxyribonucleic Acid - DNA:

Answer 2

DNA is found in the nucleus of the cell where it forms an important part of the chromosomes that make up the chromatin network. Chromatin is the chromosomal material made up of DNA, RNA and histone proteins as found in a non-dividing cell.

Question 3

Extracellular DNA:

Answer 3

Small amounts of DNA are found outside the nucleus in mitochondria in plants and animals and in chloroplasts in plants.

Question 4

What are Chromosomes?

Answer 4

Chromosomes absorb dye very easily - called chromosomes - colored bodies. This makes them visible  under a microscope but can only be seen as individual threads when a cell is dividing. Chromosomes are long, thin, intertwined thread-like structures made up of strand DNA wound around proteins called histones. Some histones are attached to the DNA and help it coil up during cell division. The DNA molecule is coiled so that these long structures can fit inside the nucleus. 
In body (somatic) cells chromosomes occur in homologous pairs. The chromosomes are the same size and shape and have the same genes in the same place.

Question 5

What are Genes?

Answer 5

Genes are sections of DNA molecules that control hereditary characteristics, they are the basic units of hereditary in living organisms.

Question 6

Genetic Replication:

Answer 6

The pairing of the bases, ie A=T and C=G suggested that given a sequence of bases in one strand, the other strand was automatically determined. This meant that when the two strands separated, each served as a template for a complimentary new chain, ie each chain strand could replicate.
Genetic proof that a triplet code was used in reading genetic material in DNA and transferring this information from the nucleus to the cytoplasm via RNA to where proteins are made.
In DNA, form is function: The double-stranded molecule could both produce exact copies of itself and carry genetic instructions, ie that the sequence of the bases in DNA forms a code by which genetic information can be stored and transmitted.

Question 7

The Structure of DNA

Answer 7

The shape of DNA is a long, twisted ladder, forming a stable, 3-dimensional double helix.

Question 8

Nucleotides

Answer 8

The double-stranded DNA molecule was made up of repeating units, building blocks (monomers), called nucleotides, linked together to form nucleic acid chains, polymers.
Each nucleotide is made up of a:
- Sugar molecule - Deoxyribose (S)
- Phosphate molecule (P)
- Nitrogenous base which may be:
- Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C)
There are four different nitrogenous bases, there are four different nucleotides.
These four bases are the foundation of the genetic code, instructing cells on how to synthesize enzymes and other proteins.
The four nucleotides are the same in all animals and plants. An adenine nucleotide of a human is the same as that of a frog or a sunflower.

Question 9

How is the double helix made up?

Answer 9

• The outer two strands of the ladder are formed by a chain of alternating sugar/phosphate links. The bonds between the sugar and phosphate molecules are strong.
• The rungs of the ladder are formed from pairs of bases linked by weak hydrogen bonds.
• The nitrogenous base pairs are attached to the sugar molecules.

Question 10

What are base pairs?

Answer 10

The shape and size of the four bases differ so that:

• Adenine will only bond with thymine or uracil (RNA) by means of two hydrogen bonds e.g. C = G.
• The hydrogen bonds are weak.

Question 11

How are base pairs classified?

Answer 11

There are two groups of nitrogenous bases - purines and pyrimidines.
- Purines are made up of two rings of nitrogen, oxygen and hydrogen atoms. Examples are guanine and adenine.
- Pyridmidines are made up of one ring of similar atoms and are therefore much smaller and uracil.
A base pair is always made up of one purine and one pyridmidine.

Question 12

How do organisms differ?

Answer 12

Determining the order of bases in a DNA chain is known as DNA sequencing.

• It is the sequence of the four bases that determines the genetic code of an organism.
• Organisms differ because of the sequence in which the base pars are strung together.
• Example. ACCTGA represents different information than AGTCCA in the same say the word post has a sifferent meaning from stop or pots.
• The sequence in certain sections of DNA in a human is different from the same sections in every other human being (except in identical twins) which results in the differences between individuals.

Question 13

What is the role of DNA?

Answer 13

DNA Molecules:

• Carry hereditary information in each cell in the form of genes.
• Provide a blueprint for an organism’s growth and development by coding for protein synthesis.
• Can replicate, ie make copies of itself so that the genetic information is passed on to each daughter cell formed during cell division.

Question 14

What is non-coding DNA?

Answer 14

• Less than 2% of a human DNA actually codes for proteins, the rest consists of non-coding DNA.
• Protein-coding regions of a DNA molecule are called exons and are interrupted by the non-coding regions called introns.
• Complex organisms contain more non-coding DNA than less complex organisms.
• The non-coding regions were thought to be evolutionary junk by are now known to form functional RNA molecules which have regulatory functions.

Question 15

DNA Replication:

Answer 15

Replication is the process of making a new DNA molecule from an existing DNA molecule which is identical to the original molecule.
It takes place in the nucleus of a cell during interphase (in between cell divisions) in the cycle of a cell.

Question 16

Why is replication necessary?

Answer 16

DNA needs to produce another molecule to ensure that the genetic code is passed on to each daughter cell formed during cell division.

Question 17

How does replication take place?

Answer 17

The process is catalyzed by the enzyme DNA polymerase. The double helix unwinds and then the process of replication takes place in three steps:

1. Weak hydrogen bonds between the two bonds break, allowing the strands to part.
2. Free nucleotides in the cytoplasm bond to their matching, exposed base partners. The fact that the base pairing is complementary ensures that an exact duplicate of each DNA molecule can be made.
3. Two daughter DNA molecules each twist to form a double helix which then winds itself around the histones (proteins), forming a chromosome.

Question 18

DNA Profiling/Fingerprinting

Answer 18

Each person has unique DNA. The differences occur in the non-coding part of DNA.

• DNA profiling involves the extracting and identifying the highly variable regions of a person’s DNA that contain repeating sequences of base-pairs called STRs (Short Tandem Repeats).
• At the same point in the DNA of different people, the number of repeated sequences of base pairs varies considerably, so distinguishing one DNA profile from another.
• From 13-20 different sites on DNA molecules are investigated; enough to show that an individual’s profile in unique.

Question 19

How is a DNA profile made?

Answer 19

• Scientists can use these repeated sequences that vary to generate a DNA profile of an individual, using blood, bone, hair and other body tissues and products.
• These cells are treated with chemicals to extract the DNA.
• Restriction enzymes are used to cut at the beginning and end of each repeated sequence, resulting in fragments of different lengths.
• Through a complicated process known as DNA amplification, large number of these fragments are made to provide a substantial amount of DNA.
• The DNA fragments that result are then separated and detected, using different techniques such as electrophoresis.
• Gel electrophoresis = a method to separate large molecules mainly on the basis of size and electrical charge.
  In this way a pattern is obtained that reflects different numbers of base pairs repeats in different individuals; the length of a particular DNA fragment depends on the number of repeats present. These separated DNA fragments are represented as dark bands on a piece of film. This is a DNA fingerprint.
  Each of our cells carries an identical set of this unique DNA that differs from that of any other person, If two genetic profiles show identical banding patterns, its certain that they come from the same person.

Question 20

The process of Gel Electrophoresis:

Answer 20

DNA solutions - mixture of different lengths of DNA fragments are poured into each well.
Wells - holes in gel to hold DNA solution.
A gel is prepared from a jelly-like substance, agarose. It is poured into a tray and allows to set.
The gel acts asa sieve for the negatively charged DNA fragments as they move towards the positive terminal when an electrical current is applied.
Large fragments have difficulty passing through the holes of the gel and therefore lag behind.
Small fragments move easily through the pores so they move faster,
The DNA fragments are separated by their length. The size of the fragment is determined by how far it has moved through the gel.
The gel is removed from the tray. After staining the separated fragments in each lane can be seen as a series of bands spread from one end of the tray to the other. Each band will consist of thousands of fragments at the same length.

Question 21

How is DNA profiling used in Forensics?

Answer 21

Forensics is the use of different scientific technologies to investigate a crime. Identifying differences in the DNA of individuals is very useful in forensic investigations. Each individual has its own unique genetic sequence, DNA can provide a means of identification. Traces of DNA left at a crime scene can often prove to be crucial evidence. This technology has reversed convictions and set innocent people free.

Question 22

How is DNA profiling used in Diagnosing Inherited Disorders?

Answer 22

DNA profiling provides medical professionals with information needed to determine hereditary diseases. This enables parents to make decisions concerning affected pregnancies and the chance to prepare for proper treatment of an affected child.

Question 23

How is DNA profiling used in Identifying Casualties?

Answer 23

If the army kept a set of DNA fingerprints of all soldiers, could be used to identify unrecognizable casualties.

Question 24

How is DNA profiling used in Paternity Testing?

Answer 24

DNA profiling can contribute to ruling out clear non-matches in paternity cases.

Question 25

The views against DNA profiling: Violation of Privacy

Answer 25

The use of DNA profiling to store identifiable information about people is an invasion of privacy. The information regarding genetic traits could lead to health insurers denying coverage or claims.

Question 26

The views against DNA profiling: Issues of Accuracy

Answer 26

The accuracy and efficiency of DNA fingerprinting depends on the competency of equipment, laboratory personnel and experience. Possible errors could lead to incorrect information.

Question 27

The views against DNA profiling: Manipulation

Answer 27

Tampering, irresponsible tampering and manipulation of data in genetic profiling, could lead to false information.

Question 28

Ribonucleic Acid - RNA:

Answer 28

Different forms of RNA are made up in the nucleus by DNA. Together, they are essential in determining the structure and functioning of all living organisms as they are involved in protein synthesis.

Question 29

What is the Structure of RNA?

Answer 29

• RNA is a single-stranded molecule (polymer) made up of nucleotides
• Each nucleotide is made up of a sugar (ribose), phosphate and a nitrogen base
• The four nitrogen bases are adenine, uracil, cytosine and guanine

Question 30

What is the Function of RNA?

Answer 30

RNA carries instructions from DNA in the nucleus to the ribosomes in the cytoplasm if a cell where it controls the synthesis of proteins from amino acids.

Question 31

Similarities between DNA and RNA:

Answer 31

DNA and RNA are both made up of:

• Polymers
• Nucleotides, made up of a sugar (ribose), phosphate and nitrogen base
• Four nitrogenous base
• They are both responsible for the synthesis of proteins

Question 32

Protein Synthesis

Answer 32

All cell chemical processes are controlled by enzymes. The synthesis of these proteins is determined and controlled by DNA and RNA.

• Polypeptide is a chain of less than 50 amino acids.
• Protein is a chain of more than 50 amino acids.

Question 33

Different Types of RNA:

Answer 33

There are different types of RNA made in the nucleus, each has a function in the synthesis of proteins:

• Messenger RNA (mRNA) - functions on the ribosome
• Transfer RNA (tRNA) - located on the cytoplasm
• Ribosomal RNA (rRNA) - functions on the ribosome

Question 34

What happens in the Nucleus?

Answer 34

mRNA is formed in the nucleus in the same way that DNA is replicated. This process is called transcription as the coded messages in DNA is carried across (transcribed) into the new mRNA molecule, which carries it to the ribosome.

Question 35

Transcription of DNA

Answer 35

Transcription is the process by which DNA makes and codes mRNA.

• The process starts when a small piece of DNA, a gene, unwinds and two strands separate.
• New nucleotides pair up with their complimentary bases on one of the DNA strands. This is called the template as it carries the code.
• The nucleotides join up to form a strand of mRNA, The sequence of nucleotides is determined by the sequence of the template DNA nucleotides. The DNA transcribes its genetic code to the mRNA. The genetic code therefore is simply the sequence of some nucleotides in a DNA strand.
• A uracil base (not thymine base) will pair with an adenine base.
• A completed strand of mRNA breaks away from the DNA. The DNA then re-zips.
• The relatively small mRNA moves through the pores of the nulcear membrane and carries the genetic code to the ribosomes which are the sites of protein synthesis.

Question 36

What determines which protein is made?

Answer 36

A protein is a long chain (polymer) of small units (monomers) called amino acids. There are twenty different amino acids that are involved in protein synthesis. These may combine in various numbers in various sequences to form thousands of different proteins. The shortest protein has 50 amino acids.
The order in which the amino acids are linked determined what kind of protein is made. Example, the protein keratin has a different sequence from haemoglobin.

Question 37

What is the role of mRNA?

Answer 37

The sequence of amino acids is determined by the instructions from the genetic code in the DNA molecules which is passed on to the mRNA.
- The genetic code is carried as a sequence of codewords - which are transcribed to the mRNA. Each codeword is made up of any three bases and is called a codon, e.g. cytosine - adenine - guanine.
There are 64 different codons and all except 3, code for one of the 20 amino acids used to form proteins. Some amino acids are coded by more than one codon.
The three codons that don’t code for an amino acid are called stop codons. UGA, UAA and UAG are these codons. This is because there are no tRNAs that have anticodons for these three so no amino acids can be brought to the chain.
- A codon is written, using the first letter of the different bases. Example, the sequence (CCG) is the codon for the amino acid glycine and CAG is the codon for valine.
- The triplet code of bases is the basis of the genetic code as a gene is made up of a group of codons that code for the synthesis of one protein.
- The order of codons in mRNA will determine the sequence of the amino acids which will determine which protein is made.

Question 38

What happens at the ribosomes?

Answer 38

The mRNA binds to the ribosome at the start codon. The codons of the mRNA act as a template that determines the order in which the amino acids are linked.

Question 39

What is the role of tRNA?

Answer 39

• There are at least 64 tRNA molecules, made from nucleotides found in the cytoplasm of cells.
• Each tRNA has three bases at one end called an anti-codon which picks up a specific amino acid found in the cytoplasm and transfers it to a ribosome.
  The most important feature of tRNA is that it can bind to an amino acid at one end and to mRNA at the other, depositing its amino acid in the correct position to form a specific protein.

Question 40

Translation of RNA into proteins

Answer 40

One of the codons, the start signal, begins the process of making a protein from amino acids. Three of these codons act as stop signals that indicate that the message is over and the protein chain is complete. All the other codons code for specific amino acids.
- The anticodon bases link up to their complementary bases of the codon, this process is called translation, as the code on the mRNA is translated into a sequence of amino acids.
For example, if the codon on mRNA is GGA, the anti-codon of the tRNA will be CCU. This enables amino acids to link up in the correct sequence.
Why? - The tRNA molecule is released to carry more of its specific amino acid to the ribosome.
- Catalyzed by enzymes, the amino acid slink together with peptide bonds to form a polypeptide chain.
- The polypeptide chains link together to form the final functional protein.
Summary:
Translation is the process by which a specific protein is formed from a chain of amino acids due to the sequence of codons in the mRNA, which in turn, was coded by the DNA.

Question 41

What is the role of rRNA?

Answer 41

rRNA is the most common form of RNA in the cell and it, together with proteins, makes up the ribosomes. The rRNA moves from codon to codon along the mRNA, reading the code. rRNA therefore plays an important role in controlling the process of protein synthesis.

Question 42

Genetic Code:

Answer 42

• The genetic code is stored along the length of a DNA strand as a sequence of four nucleotides, adenine, cytosine, guanine and thymine.
• These nucleotides are arranged in groups of 3 in a specific order; each group acting as a unit called a codon.
• Each codon codes for a particular amino acid. Some amino acids are coded for by a single codon, while other are coded for by up to four codons.
• The order in which codons are strung together, ie the genetic code, determines the order in which the amino acids for which they code, are arranged in polypeptide chain and finally in a protein.
• The non-coding introns are spliced out during protein synthesis and therefore don’t play part in the process. The probably go on to form functional RNA which has a regulatory function.
• The genetic code is universal, all living organisms from Earth, from viruses and bacteria to plants and humans, share the same genetic code for the same proteins.