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BIOCHEM Ch21 Cheat Sheet by

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- PYRIMI­DINE: single ring with two nitrogen atoms
- PURINE: two rings each with two nitrogen atoms.

BASIC because the nitrogen atoms are H+ acceptors.


- PHOSPH­ODI­ESTER BOND: the bond between the 3' OH of a sugar (nucle­otide) and a phosphate group attached to the 5' C of another sugar.

- HYDROGEN BOND: Weak, noncov­alent linkages with favorable electr­ostatic intera­ctions.

- N-GLYC­OSIDIC BOND: In DNA, refers to the N-C linkage between the 9' N of purine bases or 1' N of pyrimidine bases and the 1' C of the sugar group.


There are 3 major types of RNA:
- messenger RNA (mRNA - 5%): carries genetic info copied from DNA to the ribosomes --> TRANSC­RIPTION phase.

- transfer RNA (tRNA - 15%): translates the genetic info in mRNA into the AA sequence for the protein --> TRANSL­ATION phase.

-ribosomal RNA (rRNA - 80%): most abundant type of RNA; combined with proteins to form ribosomes.


The three types of RNA differ by function in the cell:

- ribosomal RNA makes up most of the structure of the ribosomes,
- messenger RNA carries genetic inform­ation from the DNA to the ribosomes
-transfer RNA places the correct amino acids in a growing peptide chain.

TRANSC­RIPTION is the process by which RNA polymerase produces mRNA from one strand of DNA.
- Transc­ription factors at the promoter region bind RNA polymerase to DNA, which activates the transc­ription of a gene.
- The bases in the mRNA are comple­mentary to the DNA, except A
in DNA is paired with U in RNA.
- The production of mRNA occurs when certain proteins are needed
in the cell.

Genetic Code & Protein Synthesis

The genetic code consists of a series of codons (sequences of 3 bases that specify the order for the amino acids in a protein).
- AUG codons signals the start of transc­ription
- codons UAG, UGA, and UAA signal it to stop the transc­rip­tion.

Proteins are synthe­sized at the ribosomes in a transl­ation process that includes 3 steps: initia­tion, chain elonga­tion, and termin­ation.

- During TRANSL­ATION, tRNAs bring the approp­riate amino acids to the ribosome, and peptide bonds form to join the amino acids in a peptide chain.

- When the polype­ptide is released, it takes on its secondary and tertiary structures and becomes a functional protein in the cell.


DNA contains the bases A, G, C, and T
RNA contains A, G, C, and U.

Base Pairs

-Template Strand (ANTI-­SENSE): info that codes for genes; contains anti-c­odons.

-Coding Strand (SENSE): comple­mentary to anti-s­ense; contains codons (decoded by tRNA, which interacts with riboso­me-­bound mRNA).

-mRNA: made from template strand; has the same info as coding strand except T is replaced by U.



- POINT MUTATION: the replac­ement of one base in the template strand of DNA with another, causing a change in AA.

- SILENT MUTATION: when a point mutation does not change the amino acid.

- DELETION MUTATION: a base is deleted from codon, changing all that follow and producing a different sequence of AA.

- INSERTION MUTATION: a base is inserted into the normal order of bases, changing all the codons that follow and producing a different sequence of AA.


In reverse transc­rip­tion:
- a retrov­irus, which contains viral RNA but no viral DNA, enters a cell.
- the viral RNA uses reverse transc­riptase to produce a viral DNA strand.
- the viral DNA strand forms a comple­mentary DNA strand.
- the new DNA uses the nucleo­tides and enzymes in the host cell to synthesize new virus particles.


NucleoSide = Base + Sugar
NucleoTide = Base + Sugar + Phosphate group(s)

In nucleo­side, the nitrogen atom of the base is connected by a b-N-gl­yco­sidic bond to the C1 of the sugar.

A nucleotide forms a phosph­oester bond with the C5 OH group of the sugar.


Pentose sugars in Nucleic Acids
- RNA: ribose
- DNA: deoxyr­ibose; no hydroxyl group in C2


-helicase unwinds the parent DNA at several sections.

-DNA polymerase catalyzes the replic­ation process at each of the open DNA sections called replic­ation forks.

-polym­erase moves in the 3′–5′d­ire­ction, catalyzing the formation of new phosph­odi­ester linkages.

-lagging strand (growing in the 5′–3′ direction) is synthe­sized in short sections called Okazaki fragments.

-DNA ligase joins the Okazaki fragments.

DNA Replic­ation

- During DNA replic­ation, DNA polymerase makes new DNA strands along each of the original DNA strands that serve as templates.
- Comple­mentary base pairing ensures the correct pairing of bases to give identical copies of the original DNA.


After a retrovirus injects its viral RNA into a cell, it forms a DNA strand by reverse transc­rip­tion. The single­-st­randed DNA forms a double­-st­randed DNA called a provirus, which is incorp­orated into the host cell DNA. When the cell replic­ates, the provirus produces the viral RNA needed to produce more virus


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