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Biology: DNA, RNA, & Protein Synthesis Cheat Sheet by

Describe DNA structure.  Explain DNA replication.  Discuss DNA mutations.  Describe RNA structure and function.  Differentiate between transcription and translation.  Explain RNA processing and post-translational modifications.  Discuss regulation of transcription and translation.

Terms - Alphab­etical

Altern­ative Splicing Some exons forming part of the mature mRNA, other exons doing so at other times, to form alternate amino acid sequences and therefore different proteins
DNA (Deoxy­rib­onu­cleic acid): Genetic material of humans
Exons Sequences of DNA that translate into amino acid sequences for protein synthesis
Introns allow for altern­ative splicing, which in turn allows one gene to code for multiple transc­ripts and therefore serve multiple complex cellular functions.
Nucleic acids: long polymers composed of repeating nucleo­tides
Nucleo­tide: pentose sugar, phosphate and a nitrog­enous base
Promoter Region A region of a DNA upstream from the gene that is not transc­ribed and that RNA polymerase binds to
RNA (Ribon­ucleic acid): Used to make genes into proteins
RNA Polyme­rase: An enzyme that transc­ribes DNA into mRNA.
Semi-c­ons­erv­ative: Method of DNA replic­ation where the original strands of DNA separate and act as a template for two new strands
Splicing introns are removed from the pre-mRNA by the splice­osome and exons are spliced back together.
Splice­osome removes introns from a transc­ribed pre-mRNA,
Terminator Sequence A sequence of DNA at the end of a gene that causes mRNA molecule to form a hairpin loop, causing the polymerase to dissociate from DNA
Transc­rip­tion: A Messenger RNA (mRNA) is made from a gene within DNA
Transl­ation Using the mRNA to direct the production of a protein

DNA Structure

Double helix which is composed of 2 strands of nucleo­tides that are antipa­rallel

One strand runs 5’ to 3’ and the other strand runs in the opposite direction 3’ to 5’

The sugar and phosphate make up the backbone while the bases make up the "­run­gs" of the ladder.


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

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

Remember the pairs:
Apple on Trees
Car in Garage

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

Remember the pairs:
Apple are Under
Car in Garage

DNA Replic­ation

• 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
DNA replic­ation is semi conser­vative

DNA Replic­ation is Semi-C­ons­erv­ative

• 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

DNA Mutations

How do cells deal with mutations?
Proofr­eading – Polymerase is able to recognize some mistakes that occur during replic­ation
Repair enzyme – Enzymes that correct DNA mutations
What if mutations still occur?
Apoptosis – programmed cell death
Immune cells – kill cancer cells
What is mutations still occur?
There are different types of DNA mutations
Substi­tution – The wrong base or bases are matched
Insertion – An extra base or bases are added in
Deletion – A base or bases are removed
Insertion and Deletion are the most harmful – results in frame-­shift mutations (a change in multiple codons)
Mistakes during replic­ation
Inherited mutations
Mutagens and Carcin­ogens
Cancer – usually two or more mutations in genes that code for repair enzymes, or genese that affect cell cycle


Single stranded nucleic acid molecule transc­ribed from a DNA gene sequence that codes for synthesis of a protein

Sugar-­pho­sphate backbone

Types of RNA

Ribosomal (rRNA) Joins with proteins to form ribosomes
Messenger (mRNA) carries genetic inform­ation from DNA to the ribosomes (made in transc­rip­tion)
Transfer (tRNA) transfers amino acids to a ribosome where they are added to a forming protein (used in transl­ation)


End-pr­oducts of gene expression – take a gene and make it a protein
Composed of subunits called amino acids
20 different amino acids in proteins (that are synthe­sized on ribosomes)

Central Dogma of Gene Expression

• 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
Replic­ation (DNA -> DNA)
Transc­ription (DNA -> RNA)
Transl­ation (RNA -> protein)

Transc­ription Overview

• 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


• The comple­mentary RNA nucleotide for each DNA nucleotide is as follows:


• 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.

• 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.

• 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.



RNA Processing

• 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.

RNA Processing

Altern­ative Splicing

• 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.
In humans, genes may be spliced together in different ways.


• Synthe­sizing a protein from an mRNA sequence on a ribosome

• 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.

Genetic Code (need to understand for mRNA)

• Made of 3 bases (nucle­otides)
• Every 3 bases on the mRNA is called a codon that codes for a particular amino acid in transl­ation
• There are 64 possible codons
• Also called the triplet code
‘Start’ refers to the first amino acid in a protein. (It is almost always a methionine with codon AUG).
‘Stop’ refers to the signal that indicates that transl­ation is over.
• Does not code for an amino acid.


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