Chromosomes, DNA And Nucleotides

Question 1

What is the difference between a nucleoside and a nucleotide?

Answer 1

• NucleoSIDE is base+sugar

- NucleoTIDE is base+sugar+phosphate

Question 2

Why are bases synthesised as nucleosides?

Answer 2

• Synthesised as nucleosides outside the nucleus
• Can cross the nuclear membrane and enter the nucleus
• Phosphorylation prevents them from crossing the nuclear envelope
• Nucleosides phosphorylated within the nucleus to become nucleotides

Question 3

What is the different between purines and pyrimidines?

Answer 3

Purines are 2-ringed structures (A and G) and Pyrimidines are single-ringed structures (C, T and U)

Question 4

Explain the polarity of DNA and RNA chains

Answer 4

• 5’ and 3’ must run ANTIPARALLEL

- Cannot form a duplex structure when parallel

Question 5

Why would a double stranded chain of GGCACTCGC require more heat energy to separate than a chain of ATTTACGTA?

Answer 5

• Strand 1 contains more C-G base pairs than Strand 2
• C-G makes 3 H bonds whereas A-T makes 2 H bonds
• More H bonds in Strand 1 therefore requires more energy to separate (strands are bound more tightly)

Question 6

What kind of bonds are present in a double stranded polynucleotide structure?

Answer 6

• HYDROGEN BONDS between complementary bases (non-covalent)

- PHOSPHODIESTER BONDS between phosphates and ribose sugars in backbone (covalent)

Question 7

How is a phosphodiester bond formed?

Answer 7

Interaction between the 5’ phosphate group of one nucleotide and the 3’ OH group of another nucleotide

Question 8

Describe how an RNA stem loop is formed

Answer 8

• Folding over of single stranded RNA molecule
• Hydrogen bonds form between complementary antiparallel sections
• RNA duplex structure formed

Question 9

What would be the nucleotide of the nucleoside Adenosine?

Answer 9

Adenosine mono phosphate (AMP)

Question 10

Describe the features of a DNA double helix

Answer 10

• RIGHT HANDED
• Consists of two complementary strands running antiparallel to eachother
• Complementary strands linked by HYDROGEN BONDS between bases
• Sugar-phosphate backbone consisting of deoxyribose sugars and phosphate PO4(3-)
• Pitch/wavelength of 3.4nm
• 10 bases per turn (distance between 2 bases is ~0.34nm)
• Major and minor grooves

Question 11

Why do dsDNA helices have major and minor grooves?

Answer 11

Due to asymmetrical arrangement of deoxyribose sugars in the sugar-phosphate backbone

Question 12

What is the difference between euchromatin and heterochromatin?

Answer 12

• EUCHROMATIN consists of ‘beads on a string’ where the DNA is wrapped around histone proteins in a chain (GENES ARE EXPRESSED) and is light staining
• HETEROCHROMATIN consists of solenoids (condensed arrangement of beads on a string roughly 30nm in diameter) and is darker staining (GENES ARE NOT EXPRESSED)

Question 13

What is the diameter of a solenoid in heterochromatin?

Answer 13

30nm

Question 14

What is the difference between ribose and deoxyribose sugars?

Answer 14

Ribose sugars have an OH group on C2 whereas deoxyribose have a H

Question 15

Which carbon on the (deoxy)ribose sugar is the phosphate bound to in a nucleotide?

Answer 15

Carbon 5

Question 16

What is a gene?

Answer 16

Sequence of DNA that codes for a polypeptide

Question 17

What is chromatin?

Answer 17

DNA/protein complex that forms chromosomes

Question 18

Why is DNA wrapped around histones?

Answer 18

STABILITY as DNA is highly negative

Question 19

What is a nucleosome?

Answer 19

Histone core surrounded by DNA

Question 20

What stain could be used to stain the nucleus of a cell and why?

Answer 20

• Haematoxylin
• Stains acidic compounds
• High concentration of nucleic acids within nucleus (DNA)

Question 21

What is a chromosome?

Answer 21

• Tightly packed unit of DNA
• Humans contain 24 chromosomes (22 autosomes and 2 sex chromosomes)
• Genes are NOT EXPRESSED

Question 22

What is the conventional way of writing a DNA/RNA sequence?

Answer 22

5’ to 3’ left to right