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All You Need to Know About DNA and Protein Synthesis
DNA is a double helix made out of nucleotides
(a sugar, a phosphate, a base).
The sides are called the sugar-phosphate backbone.
The rungs are made of the nitrogenous base: A, T, G, C
A pairs with T, C pairs with G.
The bases are held together by weak hydrogen bonds.
DNA has to copy itself when new cells are made. DNA copies itself during the S stage of Interphase during the cell cycle. DNA
replication starts when helicase unzips the DNA by breaking the hydrogen bonds. Then, DNA polymerase adds the appropriate bases,
a new backbone is built, and the molecule is proofread. Replication results in two identical strands of DNA. Replication is said to be
semi-conservative because each half of the old molecule is used as a template to create the new molecule.
A pairs with T, G pairs with C. If one half of a DNA strand is GCC CTA, the complementary side would be CGG GAT.
There is another type of nucleic acid, RNA. The table below summarizes the differences between DNA and RNA.
Nucleic Acid
Location
Size
Sugar Present
Bases Present
DNA
Stays in the nucleus
Double stranded, long
Deoxyribose
ATCG
RNA
Can be in nucleus or
cytoplasm depending
on RNA type
Single stranded, shorter
Ribose
AUC G
The central dogma of DNA is that DNA codes for mRNA and then mRNA is used to link amino acids together building a protein.
The protein product then results in a trait about the organism. You can think about it this way DNA → mRNA → Protein → Trait.
To make a trait apparent in an organism, DNA is involved in a process called protein synthesis.
Protein Synthesis occurs when DNA in the nucleus sends instructions to the ribosomes out in the cytoplasm to make a certain protein.
Transcription (in the nucleus) is when a piece of DNA is used to make mRNA. mRNA then leaves the nucleus.
During translation (in the cytoplasm), rRNA reads and decodes mRNA and calls out for the amino acids. Amino acids are brought
by tRNA and are connected together with peptide bonds, making the protein. (When going from DNA to RNA, remember that A
pairs with U, G pairs with C). So if a piece of DNA reads ACT GCA, the mRNA would read UGA CGU.
Do you remember how to use the codon chart?
- A codon is 3 bases of mRNA
What does mRNA UUU codes for?
Phe.
What does DNA sequence TTC code for?
(mRNA AAG) codes for LYS.
** Remember that if you are given mRNA (codon), you can
look it up right away. If you are given DNA or tRNA (the
anti-codon) you have to flip it to mRNA and then look it up.
Gene Regulation – Cells can turn genes on and off depending on if they need the gene’s product (protein that it codes for). Often, the
environmental conditions signal a gene to turn on or off. For example, bacteria turn on a gene to digest lactose only if lactose is
present. If lactose isn’t there, the cells don’t waste energy making the lactose digesting product.
Look at the diagram on the right showing protein synthesis.
Highlight the word transcription.
Where in the cell does this occur? Nucleus
Highlight that word translation.
Where in the cell does this occur? Cytoplasm
Talk yourself through the diagram starting with DNA and then
following the arrows.
Protein type is determined by the order and number of amino acids hooked together. If the sequence of amino acids is wrong, the
protein won’t function in the same way. Some mutations may be inherited from your parents (like sickle-cell). Others may be a result
of exposure to a mutagen like radiation or chemicals such as those in tobacco products.
Mutations that effect DNA include:
Point – a single base is incorrect. If the amino acid coded for is still the same, the mutation is silent.
Insertion – there are extra bases or genes added in. Anything other than a multiple of 3 is really bad and results in a frame shift.
Deletion – there are bases or genes missing.
Inversion – bases or genes are flipped in order.
Translocation – a section of DNA switches places with another chromosome of a different pair.
1. A segment of a DNA strand has the following bases: TAC GAT. What is the complimentary strand of DNA?
A. UAG CAU
C. ATG CTA
B. TAG CAT
D. AUG CUA
2. A sugar, a phosphate group, and a nitrogen base form the building blocks of which organic compound?
A. carbohydrates
C. nucleic acids
B. lipids
D. proteins
3. Which of the following is a characteristic of RNA, but not DNA?
A. single-stranded
B. double-stranded
C. thymine
D. double helix
4. What type of RNA is responsible for bringing amino acids to the ribosome for protein synthesis?
A. messenger RNA
C. ribosomal RNA
B. transfer RNA
D. mitochondrial RNA
5. If a portion of a DNA strand has the sequence TACGCA, what will be the sequence of the mRNA strand transcribed?
A. TACGCA
C. AUGCGU
B. UACGCA
D. ATGCGT
6. The messenger RNA will carry the DNA’s instructions out of the nucleus to which of the following?
A. vacuole
C. chloroplast
B. mitochondria
D. ribosome
7. To determine the molecular sequence of a gene of a protein, which molecule should be analyzed?
A. tRNA
C. DNA
B. ATP
D. rRNA
8. A strand of DNA with sequence AAC AAG CCC undergoes a mutation and the first A is changed to a C. How would this mutation
affect the amino acid sequence?
A. One amino acid will change.
C. Two amino acids will change
B. All the amino acids will change.
D. The amino acids will remain the same.