Nucleic acids

Question 1

What is a gene?

Answer 1

A segment of DNA that codes for a particular protein

Question 2

Where in the cell are chromosomes located?

Answer 2

Nucleus

Question 3

In what organelles is DNA found?

Answer 3

Mitochondria and chloroplasts

Question 4

Which bases bond together and how?

Answer 4

adenine bonds with thymine using 2 hydrogen bonds
cytosine bonds with guanine using 3 hydrogen bonds

Question 5

What is the structure of a nucleotide?

Answer 5

A pentose sugar (deoxyribose) attached to a nitrogen containing base and a phosphate group

Question 6

Which bases are purine and which bases are pyramidine?

Answer 6

Purine = adenine and guanine - 2 rings
Pyramidine = cytosine and thymine - 1 ring

Question 7

How is the structure of DNA related to its function?

Answer 7

Long molecule/coiling - compact so can store a large amount of genetic information

Sugar-phosphate backbone - bound by relatively strong phosphodiester bonds

Bases in sequence - the sequence of bases determines the mRNA molecule transcribed from it, which in turn determines the primary structure of the protein

Complementary base pairing - holds together the polynucleotide strand when not being transcribed or replicated, allows information to be replicated/transcribed as new DNA or RNA nucleotides can join to the exposed bases

Hydrogen bonds allow chain to split easily - for replication/transcription (1st stage of protein synthesis)

Helical structure - protects hydrogen bonds and helps to make the molecule stable

Question 8

How does semi-conservative replication occur?

Answer 8

• 2 strands of DNA replicate and DNA helicase breaks the hydrogen bonds between complementary base pairs breaks
• the free nucleotides are attracted to their complementary base pairs in exposed bases on the template strands
• once the new nucleotides have lined up, they are attached by the enzyme DNA polymerase in a condensation reaction which joins adjacent nucleotides
• finally, all the nucleotides are joined to form a complete polynucleotide chain using DNA polymarase. This creates 2 identical strands of DNA. As each strand retains half of the original DNA material, this is called semi-conservative

Question 9

How did meselson and stahl describe semi conservative replication through centrifugation?

Answer 9

• generation 0 has only the original polynucleotie, containing only ‘heavy’ Nitrogen 15, so each strand has a high density and therefore moves to the bottom of the centrifuge tube
• the bacteria is then transferred to a medium containing only the ‘light’ isotope of nitrogen 14. These hybrid DNA molecules are of intermediate density so move to the middle of the tube
• generation 2 has 2 hybrid DNA molecules which are of intermediate density so band in the middle of the tube . 2 DNA molecules are of low density as they only contain nitrogen 14, so form a band at the top of the tube.
• generation 3 has 2 hybrid DNA molecules which band in the middle of the tube. There is a greater proportion of low density DNA molecules, so the band of nitrogen 14 at the top of the tube is thicker

Question 10

What is the structure of a nucleotide?

Answer 10

contain a pentose sugar, a nitrogenous base and a phosphate group

Question 11

What are the bonds between nucleotides in a polynucleotide chain called?

Answer 11

phosphodiester bonds

Question 12

What bases does RNA consist of?

Answer 12

adenine, cytosene, guanine and uracil

Question 13

What is tRNA?

Answer 13

a form of RNA which has a clover-leaf structure formed when bases within an RNA molecule form complementary base pairs with other specific bases within the same molecule through the formation of hydrogen bonds. In RNA, adenine pairs with uracil, and cytosene pairs with guanine

Question 14

What are ribosomes made from?

Answer 14

RNA and proteins

Question 15

What are the roles of DNA and RNA?

Answer 15

DNA - holds genetic information
RNA- transfers genetic information from DNA to ribosomes