Chapter 7: Nucleic Acids and Proteins

At one end of the nucleotide strand, the terminal phosphate group is linked to the 5^th carbon of the deoxyribose, which is why this end is called the 5’ terminal. At the other end of the nucleotide strand, the hydroxyl group OH is linked to the 3^rd carbon of the deoxyribose, which is why this end is called the 3’ terminal.

The two nucleotide strands have their 3’ and 5’ terminals at opposite ends, which is why the strands of DNA are called anti-parallel.

Purines: Adenine and guanine are bases that contain two rings in their molecule. Pyrimidines: Cytosine and thymine are bases that contain only one ring in their molecule. Purines can only form hydrogen bonds with pyrimidines, which is the reason that complementary base paring between A and T and G and C is occurring.

1. The cell produces free nucleotides that contain three phosphate groups and are therefore called deoxyribonucleoside triphosphates. Two of the three phosphate groups are lost during replication to produce energy. 2. Helicase uncoils the DNA double helix into two template strands, forming the replication fork. 3. RNA primase adds a short length of RNA to the template strand, called a primer. This signals DNA polymerase to begin binding nucleotides in a 5’ à 3’ direction. 4. DNA polymerase III adds the free nucleotides next to the primer, forming short lengths of DNA between the RNA primers. The fragments are called Okazaki fragments. 5. DNA polymerase I removes the RNA primers and replaces them with DNA. The nick left where two nucleotides are still unconnected is sealed by DNA ligase, making a sugar-phosphate bond.

During eukaryotic DNA replication, several primers mark several initiation sites, whereas during prokaryotic DNA replication, there is only one initiation site.

Nucleosomes are globular structures that contain eight histone proteins with DNA wrapped around them and one histone protein holding together the structure. Nucleosomes help DNA to supercoil in interphase and to mark particular genes to either express them during transcription or translation or to silence them by preventing transcription.

Repetitive base sequences in DNA are called repetitive sequences (or satellite DNA). They are sequences that range from 5-300 bases and that constitute 5-45% of the eukaryote DNA. Repetitive sequences cannot be translated.

Introns are not translated, whereas exons are both transcribed and translated. After transcription, introns are removed from mRNA to form mature mRNA in a process called post-transcriptional modification.

RNA polymerase unwinds the DNA double helix in order for one strand to form the transcription template.

The sense strand is the DNA strand that contains the same sequence as the new mRNA strand only that it contains thymine instead of uracil. The antisense strand is the template strand that the mRNA uses to copy.

Two or more codons can code for the same amino acid.

All tRNA molecules contain o (Helical) double strands formed through base pairing and creating loops o An triplet of bases called the anti-codon (loop of 7 bases) o The base sequence CCA at the 3’ terminal, forming the amino acids binding site

There are 20 different activating enzymes that code for each of the 20 amino acids that a tRNA could possibly link to.

ATP is used to bind the amino acid to the tRNA. During translation, this high-energy bond is broken to release energy to link the amino acids, forming the polypeptide chain.

o Proteins and ribosomal RNA molecules (rRNA) form the structure of the ribosome o There are three binding sites for tRNA and one for mRNA. Two tRNA molecules can bind to the ribosome at the same time.