Biology: DNA, RNA, & Protein Synthesis Cheat Sheet

Larger bases are called purines and have a double ringed structure: Adenine and Guanine

Smaller bases are called pyrimi­dines and have a single ringed structure: Cytosine and Thymine

For DNA
Adenine (A) <-> Thymine (T)
Cytosine (C) <-> Guanine (G)

Remember the pairs:
Apple on Trees
Car in Garage

For RNA
Adenine (A) <-> Uracil (U)
Cytosine (C) <-> Guanine (G)

Remember the pairs:
Apple are Under
Car in Garage

• The two strands of DNA that form the double helix DNA molecule are comple­mentary to each other

• The hydrogen bonds that hold the base pairs together are weak bonds and are easy to separate

Simplified Steps in DNA replic­ation
1. An enzyme called helicase unwinds the DNA. The hydrogen bonds between the base pairs are broken

2. DNA polymerase moves along each strand to unwound DNA and adds the correcr comple­mentary nucleo­tides

3. Breaks in the sugar-­pho­sphate backbone are sealed by an enzyme called DNA ligase

4. The two DNA molecules are identical to each other and to the original parent molecule
Note: DNA replic­ation is semi conser­vative

• Mistakes can occur during replic­ation. There are repair enzymes that work to fix this. Sometimes an error persists leading to a mutation, and to a genetic and phenotypic variab­ility

• The parental DNA strand is used as a template to synthesize a new daughter stand

• This happens for both parental strands

• Therefore, after DNA replic­ation you get two DNA molecules – each consisting of one parental strand and one daughter (newly synthe­sized) strand

• The inform­ation contained in DNA is stored in blocks – genes
• The genes code for mRNA, which codes for proteins
• The proteins determine how a cell functions
• The path of inform­ation is DNA -> RNA -> Protein
• When gene sequences are used by the cell to make protein, called gene expression

• mRNA is made from a DNA template
• mRNA is processed before leaving the nucleus
• mRNA moves to the ribosomes to be read
• Transc­rip­tions in both prokar­yotes and eukaryotes has 3 stages: initia­tion, elonga­tion, and termin­ation

Initiation
• RNA polymerase binds to a promoter region on DNA
• They help the polymerase locate the beginning of a gene
• Most mRNA molecules start with the codon AUG, which serves as the gene's starting point, corres­ponding to ATG on the coding strand.

Elongation
• RNA polymerase adds comple­mentary nucleo­tides to the template strand of the gene on DNA
• This produces the mRNA
• This process ensures that the mRNA sequence matches the order of nucleo­tides in the DNA coding strand, except RNA has uracil instead of thymine.
• RNA polymerase can only add nucleo­tides in the 5' to 3' direction, similar to DNA replic­ation. ATP is needed for RNA polymerase to function.

Termin­ation
• Transc­ription of a gene finishes when the polymerase enzyme encounters a terminator sequence.
• The mRNA dissoc­iates and is now free to be translated by a ribosome
• DNA remains unchanged, and the mRNA is set for transl­ation.

• The freshly formed mRNA, known as the primary mRNA or primary transc­ript, undergoes three essential steps to transform into a mature mRNA that can only be used as a template for transl­ation

• To shield the RNA from degrad­ation, a 5' cap and a 3' poly-A tail are added.

• Splicing then occurs to eliminate noncoding segments of the gene, known as introns, which don't contribute to the amino acid sequence.

• The coding parts of the gene, called exons, remain.

• Splice­osomes, along with specific proteins, remove the introns and splice together the exons, resulting in a shorter mRNA transc­ript.

• The intron sequences, consti­tuting about 90% of a typical human gene, are not transl­ated.

• During RNA proces­sing, all the exons of a gene are brought together
• By using different combin­ations of the same exons, different proteins can be created.
• So, altern­ative splicing results in the ability of one gene to produce multiple different proteins.

• Synthe­sizing a protein from an mRNA sequence on a ribosome

Ribosomes
• consist of two subunits:
• a small subunit and a large subunit
• mRNA binds to the small subunit.
• The large subunit has three binding sites, A (Amino acid), P (Polyp­eptide) and E (Exit) sites

Transl­ation Cont’d
• To correctly read a gene, a cell must translate the inform­ation encoded in the DNA into the language of proteins.
• The mRNA is “read” in three-­nuc­leotide units called codons.
• Each codon corres­ponds to a particular amino acid.
• It is the tRNA molecules that bring amino acids to the ribosome to use in making proteins.

Transfer RNA (tRNA)
• tRNA molecules each have a special three nucleotide RNA sequence called an anticodon.
• The anticodon is comple­mentary to one of the 64 codons of the genetic code
• tRNA molecules also each bind an amino acid at one end.
• There are more than 20 different tRNA molecules, so some tRNAs bind to the same amino acids.

Transl­ation Cont'd**
• After an mRNA molecule attaches to the small ribosomal subunit, the larger ribosomal subunit joins, forming a full ribosome.
• During transl­ation, the mRNA moves through the ribosome in sets of three nucleo­tides at a time.
• As this happens, a fresh tRNA carrying an amino acid to be added enters the ribosome at the A site.
• Transl­ation proceeds until a stop codon marks the end of the protein synthesis process. At this point, the ribosome disass­embles, and the newly synthe­sized protein is released into the cell.
• In eukaryotic cells, after transl­ation, proteins undergo folding into secondary and tertiary structures and may undergo additional processing within the Golgi apparatus.