DNA Structure and Replication

Question 1

Genes

Locus

Alleles

Answer 1

• segments of DNA that encode proteins by serving as template for mRNA molecules
• precise location of ea gene on a chromosome
• identical genes on pairs of chromosomes

Question 2

Centromere

Telomere

Answer 2

-where spindle fibers attach during chromosome segregation

• specialized structures that maintain integrity of chromosomes at their ends
• Define ends of all human chromosomes
• Usually contain multiple tandem repeats of TTAGGG sequence
• Important for stability and replication of chromosomes

Question 3

Nucleosome

Answer 3

• Basic unit of chromatin
• ~147 Bps and a histone octomer (octomer has 2 of ea H2A H2B H3 H4 proteins)
• Individual nucleosomes connected by linker DNA (~50 Bps bound by H1 histone)
• can be remodeled by remodeling complexes

Question 4

Nucleotide

Answer 4

• nitrogenous base + deoxyribose sugar + a phosphate

- Purines (AG) and pyrimidines (CT or U if RNA)

Question 5

Bonds in DNA molecules

Answer 5

• Glycosidic bonds- link bases to sugars
• Phosphodiester bonds- link adjacent nucleotides together; covalent; join 3’ carbon of one deoxyribose or ribose to 5’ carbon of the next (SO KEEP EA SEP STRAND INTACT)
• Hydrogen bonds- holds 2 strands of DNA together
  
  • A-T pairs held together by 2 H+ bonds
  • C-G pairs held together by 3 H+ bonds

Question 6

Watson Crick DNA Structure

Answer 6

dbl helix/R handed of ~10 bp /turn

Question 7

Sense v Antisense Strands

Answer 7

• Sense strand- DNA strand w/in gene whose sequences is same as that of mRNA copy that is translated into protein
• Antisense strand- complementary to sense strand
  
  • **Which strand is sense and which is antisense can vary in diff parts of chromosome

Question 8

DNA replication and Cell Cycle

Answer 8

Interphase (G1 S G2) and mitosis (M)
-Chromosome contains 1 DNA dbl helix from anaphase of mitosis -> DNA duplication in S phase (2C)

• • DNA duplicates in S phase
    
    • From this point until end of metaphase the chromosome consists of 2 chromatids (ea containing DNA duplex) (4C)

Question 9

DNA Helicase

Answer 9

Unwinds DNA by breaking H+ bonds

Question 10

Features of DNA Replication

Answer 10

• Origins of replication (initiator proteins bind to sequences w/in these)
• Semi-conservative
• Semi-continuous (leading strand is synthesized continuously starting from origin of replication while lagging strand is synthesized as small Okazaki fragments)
• Bi-directional
• Replication Fork (Region b/n unwound parental duplex and newly replicated daughter duplexes)
• Low frequency of error

Question 11

Replication Bubble

Answer 11

formed by replication fork initiation

Question 12

DNA Polymerases General Function

Answer 12

Enzymes that catalyze addition of deoxyribonucleoside to free 3’ hydroxyl group of growing DNA strand

Question 13

DNA Ligases

Answer 13

ligases connect the Okazaki fragments

Question 14

DNA Primase

Answer 14

• Synthesizes small RNA primers used as primers for synthesis of short initiator DNA (both leading and lagging strands)
  
  • Leading strand needs 1 RNA primer while lagging strand needs multiple RNA primers
• Associated w/ alpha DNA polymerase

Question 15

Steps of DNA Replication

Answer 15

• 1-Primase subunit of DNA polymerase alpha complex synthesizes short RNA primer (7-12 nuc long)
• 2- Then DNA polymerase portion of gamma polymerase synthesizes about 30 nuc of DNA
• 3- epsilon polymerase takes over for leading strand AND delta polymerase takes over for lagging strand
• 4- RNA primers removed by RNase H activity
• 5- Gap in strand is filled by DNA polymerase delta or epsilon
• 6- DNA sealed by DNA ligase

Question 16

Topoisomerases

Answer 16

• Bind DNA ahead of replication fork, leave phosphate backbone allowing 2 strands to unwind; then reseal DNA
• *Inhibitors of topoisomerases used as anti-bacterial and anti-cancer agents

Question 17

Telomerase

Answer 17

• Telomerase extends the TG-rich strand of telomeres by DNA synthesis using internal RNA template
• Ribonucleoprotein complex (reverse transcriptase)
• Important role in cell aging and cancer development
• Present in embryonic stem cells and germ cells but most normal somatic cells lack telomerase
• Telomerase activated in many types of cancer cells (potential target for anti tumor drugs)

Question 18

SSBs

Answer 18

Single-strand binding proteins

Hold replication fork open in DNA replication