Chapter 1 - Cellular Control

Question 1

Define “gene”

Answer 1

a length of DN that codes for one or more polypeptides, including enzymes

Question 2

What are the characteristics of the genetic code?

Answer 2

• it is a triplet code (a sequence of 3 nucleotide bases code for an amino acid)
• it is a degenerate code (all amino acids except methionine have more than one code)

Question 3

What is the first step in protein synthesis?

Answer 3

Transcription

Question 4

Describe the steps in transcription

Answer 4

• first stage of protein synthesis; makes an mRNA molecule
• one strand is used as a template strand
• free DNA nucleotides in the nucleoplasm, free RNA nucleotides in nucleolus
• gene unwinds and unzips breaking hydrogen bonds
• Activated RNA nucleotides form h bonds with exposed complimentary bases on the TEMPLATE strand
• this is catalysed by RNA polymerase.
• 2 extra phosphates are released, providing energy for bonding
• mRNA produced is complementary to nucleotide base sequence on template strand
• released from the DNA and passes through the nucleus through a pore in the nuclear envelope, to a ribosome.

Question 5

What does it mean when nucleotides are activated in transcription?

Answer 5

they have 2 extra phosphoryl groups

Question 6

What is the second stage of protein synthesis?

Answer 6

Translation

Question 7

Describe the steps in translation

Answer 7

• assembles polypeptide
• a molecule of mRNA binds to a ribosome. the first mRNA codon is always AUG.
• using ATP + and enzyme, a tRNA and the anticodon UAC forms H bonds with this codon.
• a second tRNA with a different amino acid binds to the 2nd exposed codon with complimentary anti codon
• peptide bond forms between 2 adjacent amino acids, enzyme catalyses reaction
• ribosome moves along mRNA, and the polypeptide chain grows until a stop codon is reached

Question 8

Instead of ATP, what other way can a protein be activated? Explain how.

Answer 8

By a chemical, cyclic AMP, which activates proteins by changing their shape so they fit better to complimentary molecules

Question 9

where does protein synthesis occur in prokaryotes?

Answer 9

no nucleus - translation occurs as soon as the mRNA is made.

Question 10

What is a mutation?

Answer 10

a change in the amount of, or arrangement of, the genetic material in a cell

Question 11

What are the two main classes of DNA mutations?

Answer 11

Point mutations - one base pair replaces another
Insertion/deletion mutations - one or more nucleotide pairs are inserted/deleted from a length of DNA. These cause a frameshift.

Question 12

What type of mutation is more detrimental?

Answer 12

Insertion/deletion mutations are more serious as they would cause a frameshift. It alters all amino acids that are coded for after the mutation.
Point mutations(substitutions) changes only one or no amino acids.

Question 13

Explain how mutations can have beneficial, neutral or harmful effects on the way a protein functions.

Answer 13

Beneficial - the mutation changes the sequence of amino acids, and therefor the phenotype to give the organism advantageous characteristics.

Neutral - mutation in a non coding region/silent mutation - even though the base triplet has changed, it still codes for the same amino acid

Harmful - resulting characteristic is harmful

Question 14

What does cyclic AMP do?

Answer 14

activates proteins by altering their 3D structure.

Question 15

What two enzymes are needed for the lac operon? What do they do?

Answer 15

B-galactosidase (catalyses the hydrolysis of lactose to glucose + galactose)
and Lactose permease (transports lactose into the cell)

Question 16

What happens a few minutes after lactose is added to a culture medium with E. Coli?

Answer 16

E. Coli bacteria increases the rate of synthesis of B-glactosidase and lactose permease by 1000 times. Lactose is the INDUCER bc it triggers the production of the two enzymes.

Question 17

Describe the structure of the lac operon.

Answer 17

• Structural genes: Z (codes for b-galactosidase) and Y (codes for enzyme lactose permease)
• Operator gene: O (length of DNA next to structural genes that can switch them on and off)
• Promoter region: P (length of DNA where RNA polymerase binds to in order to begin the transcription of Z and Y)

the regular gene (I) is NOT PART OF THE OPERON and is some distance from it.

Question 18

How does the lac operon work when lactose is absent from the growth medium?

Answer 18

1) regulator gene is expressed and the repressor protein is synthesised. It has 2 binding sites –> to lactose and to the O region
2) repressor protein binds to the O region, covers part of the promotor region where RNA polymerase normally binds.
3) RNA polymerase cannot bind to the promotor region so structural genes cannot be transcribed
4) enzymes cannot be synthesised

Question 19

How does the lac operon work when lactose is added to the growth medium?

Answer 19

1) lactose binds to the repressor protein, changing the structure of the repressor protein
2) The repressor protein can no longer bind to the operator region
3) RNA is able to bind to promoter
4) Z and Y are transcribed and mRNA is made
5) the bacteria can now use the lactose permease enzyme

Question 20

What are homeobox genes?

Answer 20

They control the development of the body plan of an organism, including the polarity (head and tail ends) and positioning of the organs.

Question 21

What is the homeobox sequence?

Answer 21

180 base pairs in each homebox gene that form 60 amino acids.

Question 22

How are homeobox genes organised?

Answer 22

in clusters known as “hox clusters”. The homeobox genes are expressed in the same order as they are shown along the body of the organism (head to tail)

Question 23

How does retinoic acid effect a pregnant woman?

Answer 23

retinoic acid is a derivative of vitamin A. too much vitamin A taken in by pregnant women can interfere with the normal expression of homebox genes, causing birth defects.

Question 24

What is apoptosis? What is the opposite of apoptosis?

Answer 24

programmed cell death that occurs in multicellular organisms.
In contrast, cell necrosis is untidy and damaging that occurs after trauma. it realeases hydrolytic enzymes which are toxic.

Question 25

What happens in apoptosis?

Answer 25

• enzymes break down the cell cytoskeleton
• cytoplasm becomes dense, organelles tightly packed
• blebs form on the cell surface membrane
• chromatin condenses and nucleare envelope breaks
• the cell breaks into vesicles that are taken up by phagocytosis

Question 26

How is apoptosis controlled?

Answer 26

cell signals such as cytokines, hormones, growth factors and NITRIC OXIDE. nitric oxide can induce apoptosis by making the inner mitochondrial membrane more permeable to H+ ions.

Question 27

What is the importance of apoptosis?

Answer 27

• weeds out ineffective/harmful t lymphocytes
• causes the digits to separate
• not enough apoptosis leads to the formation of tumours
• too much apoptosis leads to cell degeneration