DNA

Question 1

What does DNA and RNA stand for?

Answer 1

• Deoxyribonucleic acid

- Ribonucleic acid

Question 2

What are nucleotides?

Answer 2

They are made up of:

1. A nitrogen containing base
2. A pentose sugar
3. A phosphate group
   - The base and sugar join with a glycoside bond
   - The phosphate and sugar join with an ester bond
   - Both require a condensation reaction to occur

Question 3

If DNA is a polymer what is the monomer?

Answer 3

Nucleotides and therefore RNA and DNA are polynucleotides

Question 4

What are the five different nitrogen containing bases in DNA and RNA?

Answer 4

• In a DNA molecule: adenine, thymine, guanine and cytosine
• In a RNA molecule: never contains the base thymine instead they contain uracil as well as adenine, guanine and cytosine

Question 5

What is the difference between ribose and deoxyribose?

Answer 5

• The pentose (5 carbon) sugar can either be ribose or deoxyribose
• Deoxyribose has one fewer oxygen atoms in its molecule than ribose

Question 6

How are polynucleotides formed?

Answer 6

1. Many nucleotides are linked together into a long chain and this takes place inside the nucleus during interphase of the cell cycle
2. In both DNA and RNA the polynucleotide strand is formed of alternating sugars and phosphates linked together with the bases projecting sideways
3. The covalent sugar-phosphate bonds (phosphodiester bonds) link the 5-carbon of one sugar molecule and the 3-carbon of the next
4. The polynucleotide strand is said the have 3’ and 5’ ends

Question 7

What is the triplet code?

Answer 7

1. The sequence of nucleotide bases in a DNA molecule is a code for the sequence of amino acids in a polypeptide
2. The code is a three-letter, or triplet, code and each sequence of three bases stands for one amino acid
3. The sequence is always reads in the same direction and from only one of the two strands of the DNA molecule (the so-called sense strand)
4. The complimentary stand is referred to as the anti-sense strand

Question 8

What is an example of a mutation?

Answer 8

• One mutation that has a significant effect is the one involved in the inherited blood disorder sickle cell anaemia
  1. Haemoglobin is the red pigment in red blood cells which carries oxygen around the body a haemoglobin molecule is made up of four polypeptide chains, each with one iron containing haem group at the centre, two of these chains are called alpha chains and the other two are beta chains
  2. The gene which codes for the amino acid sequence in the beta polypeptides is not the same in everyone, in most people the beta polypeptides begin with an amino acid acid sequence and this is coded from the HbsubscriptA (normal) allele of the gene
  3. But in some people the base sequence CTT is replaced by CAT and raw amino acid sequence changes and this is coded from the HbsubscriptS (sickle cell) allele of the gene
  4. This type of mutation is called substation and in this case, the small difference int he amino cid sequence results in genetic disease sickle cell anaemia in individuals with two copies of the HbsubscriptS allele

Question 9

What type of mutation is in sickle cell anaemia?

Answer 9

Substitution

Question 10

How does DNA control protein synthesis?

Answer 10

1. All chemical rections in cells, and therefore all the cells’ actives are controlled by enzymes
2. Enzymes are proteins
3. DNA is a code for proteins, controlling which proteins are made and this DNA controls the cell’s activities
4. Protein molecules are made up of strings of amino acids and the shape and behaviour of a protein molecule depends not eh exact sequence of these amino acids, that is, its primary structure
5. DNA controls protein structure by determining the exact order in which the amino acids join together when proteins are made in a cell

Question 11

What is protein synthesis?

Answer 11

The code on the DNA molecule is used to determine the sequence of amino acids in the polypeptide

Question 12

What is the first stage of protein synthesis?

Answer 12

• Transcription
  1. In the nucleus a complimentary copy of the code from a gene is made by building a molecule of a different type of nucleic acid, called messenger RNA (mRNA), using one strand (the sense stand) as a template
  2. Three RNA nucleotides are joined together by the enzyme RNA polymerase
  3. This process copies the DNA code onto an mRNA molecule
  4. Transcription of a gene begins when RNA polymerase binds to a control region of the DNA called a promoter and ends when the enzyme has reached a terminator sequence
  5. At this point, the enzyme stops addicting nucleotides to the growing mRNA. The hydrogen bonds holding the DNA and RNA together are broken and double stranded DNA reforms. The last triplet code transcribed onto mRNA is one of the DNA triplets coding for ‘stop’

Question 13

What is the second stage of protein synthesis?

Answer 13

• Translation
  1. The DNA code is translated into an amino acid sequence
  2. The mRNA leaves the nucleus and acetates to a ribosome in the cytoplasm

Question 14

What happens to the mRNA in the cytoplasm?

Answer 14

1. In the cytoplasm, there are molecules of transfer RNA (tRNA)
2. These have a triplet of bases at one end and a region where an amino acid can attach to the other
3. There are at least 20 different sorts of tRNA molecules, each with a particular triplet of bases at one end and able to attach to a specific amino acid at the other

Question 15

What to the tRNA molecules do?

Answer 15

1. They pick up their specific amino acids from the cytoplasm and bring them to the mRNA on the ribosome
2. The triplet of bases (an anticodon) of each tRNA links up with a complimentary triplet (a codon) on the mRNA molecule
3. Two tRNA molecules fit onto the ribosome at any one time
4. This brings two amino acids side by side, and a peptide bond is formed between them
5. Usually, several ribose work on the same mRNA strand at the same time, they are visible using an electron microscope as polyribosomes

Question 16

How is DNA, mRNA and tRNA related?

Answer 16

1. The base sequence ont he DNA molecule determine the base sequence in the mRNA, which determine which tRNA molecules can link up with them
2. Since each type of tRNA molecule is specific for just one amino acid, this determines the sequence in which the main acids are linked together as the polypeptide molecule is made

Question 17

What is a gene and what is a mutation?

Answer 17

1. A part of a DNA molecule, where the nucleotide codes for just one polypeptide is called a gene
2. A change in the nucleotide sequence of a gene which may then result in an altered polypeptide is called a mutation
3. Most genes have several different variants called alleles which originally arose by the process of mutation

Question 18

What is a DNA molecule made up of?

Answer 18

1. DNA molecules are made of two polynucleotide strands lying side by side, running in opposite directions
2. The strands are said to be in antiparallel
3. The two strands are held together by hydrogen bonds between the bases

Question 19

What is a purine and what is a pyrimidine?

Answer 19

A purine has a double ringed structure (adenine and guanine) and pyrimidines have a single ringed structure (thymine uracil and cytosine)

Question 20

What is the complimentary base paring?

Answer 20

A with T and C with G

Question 21

What shape is DNA?

Answer 21

1. A double helix (the 3D shape that DNA molecules form)
2. The hydrogen bonds linking the bases, and therefore holding the two strands together, can be broken relatively easily, which happens during DNA replication and protein synthesis

Question 22

What are RNA molecules like?

Answer 22

They remain as single strands of polynucleotide and can from very weak different 3D structure

Question 23

What is semi-conservative replication?

Answer 23

1. The two strands of the DNA molecule split apart
2. New nucleotides could then line up along each strand, opposite their appropriate partners and join up to form complementary strands along each half of the original molecule
3. The new DNA molecules would be just like the old ones, because each base would pair with its complimentary one
4. Each pair of strands could then wind up again into a double helix, exactly like the original one
   - Called this because half of the original molecules is kept in each of the new molecules
   - DNA replication takes place when a cell is not dividing. This is in interphase in eukaryotic cells

Question 24

How is a polypeptide formed?

Answer 24

A polypeptide is coded for by a gene

Question 25

What is a gene?

Answer 25

A gene is a sequence of nucleotides that forms part of a DNA molecule

Question 26

What is a gene mutation?

Answer 26

A gene mutation is a change in the sequence of nucleotides that may result in an altered polypeptide

Question 27

Outline the process of protein synthesis

Answer 27

1. The RNA polymerase enzyme tells the DNA to unwind breaking the hydrogen bonds between the bases, to a template strand and a coding strand
2. The mRNA molecule which is a single strand is an exact copy of the coding strand but instead of the base thymine there is the base uracil
3. The formation of this mRNA molecule is called transcription
4. The mRNA then exits the nucleus via the nuclear pores in the nuclear envelope and attaches itself to a ribosome on the rough endoplasmic reticulum
5. The mRNA feeds through the small subunit (30s) ribosome part
6. Meanwhile tRNA molecules which are attached to a specific amino acid have the anticodon and the tRNA molecules enter the large subunit of the ribosome
7. The anticodon on the tRNA is complimentary to the codon on the mRNA and a peptide bond is then formed between two adjacent amino acids each in a different tRNA molecule in the same ribosome with energy from ATP and an enzyme, peptidyl transferase
8. These amino acids join the growing polypeptide chain which in turn pushes them out of the ribosome allowing the next tRNA molecule to enter. This process is translation
   - Until STOP codon and MET amino acid is always at the start

Question 28

How are the bases linked?

Answer 28

hgfdfuhg

Question 29

What is the structure of a chromosome?

Answer 29

• DNA
• Histone proteins
• Chromatids
• Centromere
• Telomeres

Question 30

What is needed to form the polynucleotides DNA and RNA?

Answer 30

• Many nucleotides are linked together in a long chain and this takes places in the nucleus during interphase
• Formed of alternating sugars and phosphate linked together with bases projecting sideways
  1. Covalent sugar-phosphate (phophodiester bonds) link the 5 carbon of on sugar molecule to the 3 carbon of the next and the polynucleotide strand is said to have 3’ and 5’ ends

Question 31

What are DNA molecules made up of?

Answer 31

1. Two polynucleotide strands lying side by side running in opposite directions and the strands are antiparallel
2. The two strands are held together by hydrogen bonds between the bases and the way the two strands line up is very precise
3. In a DNA molecules there is just enough room between the two sugar phosphate backboned for one purine and one pyrimidine molecules so a purine in one strand must always be opposite a pyrimidine in the other