Reviewing The Chapter Ch. 12
#4) Explain how DNA replicates semi conservatively. What role does DNA polymerase play?
What role does helicase play?
DNA replicates semi conservatively because each daughter DNA double helix contains an old strand
from the parental DNA double helix and a new strand. DNA polymerase is an enzyme complex that
carries out complementary base pairing and joining during DNA replication. Helicase unwinds the parent
DNA.
#5) List and discuss differences between prokaryotic and eukaryotic replication of DNA.
In prokaryotic DNA replication (1) occurs in two directions at once because the (2) DNA molecule is
circular and (3) there are no ends in a ring of DNA. On the other hand, Eukaryotic DNA replication occurs
at numerous replications forks 5I direction) thereby creating bubbles that spread out until they meet.
Eukaryotic ends of the chromosome (telomeres) are unable to replicate.
#6) How did investigators reason that the code must be a triplet code, and in what manner was
the code cracked? Why is it said that the code is degenerate, unambiguous, and almost
universal? Investigators reasoned that the code must be a triplet code because they used synthetic
RNA and created protein. They happened to see UUU, a three coded word, and found that it was a code
for phenylalanine. There are 20 Amino acids, and with four different types of nucleotides, they
reasoned that having 2 would result in 16 codons, which is too little, and having 4 would result in 256,
which would be too many. Having three would yield 64 different possibilities, which seem most
reasonable.
The code is degenerate because amino acids have more than one codon
The code is unambiguous because each triplet code has only one meaning
The code is universal because it is used in (almost) all species and can be transferred from one organism
to another. (Note: genetic code in mitochondria and chloroplasts differs slightly)
#11) Compare the functions of mRNAs, tRNA and rRNA during protein synthesis. What are the
specific events of translation?
During protein synthesis the role of mRNA is to get genetic information straight from the nucleus and
take message from DNA in the nucleus to the ribosomes in the cytoplasm. The role of transfer RNA is to
transfer amino acids to the ribosomes. ( one tRNA for each of the 20 amino acids; contains anti codon
complementary to the mRNA codon). The role of ribosomal RNA is that it makes up the ribosomes
where polypeptides are synthesized. The rRNA is produced from the DNA template and is packaged with
proteins in two ribosomal units. They get into cytoplasm through the nuclear pores.
The specific events of translation are:
1) initiation (Brings all translation components together (mRNA, initiator tRNA, large & small
ribosomal subunit),
2) elongation: polypeptide increases in length one amino acid at a time, brought by the tRNAs
3) Termination: Final step in protein synthesis when a stop codon is reached, a release factor binds
to the site, and the bonds are hydrolyzed by the release factor.
#12) What are the various levels of chromosomes structures?
Eukaryotic cells contain more than 2m of DNA!!
The various levels of chromosome structure includes:
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Nucleosomes – the DNA strand is wound around histones (DNA binding proteins) to form
nucleosomes
30-nm fiber – the strand is further shortened by folding It into a zigzag structure
Radial loop domains- the fiber loops back and forth into radial loops formed by zigzag structure.
Heterochromatin – additional proteins further compact radial loops
Metaphase chromosome – metaphase chromosome forms.