DNA Structure, DNA Replication, and Protein Synthesis Notes
DNA – Deoxyribonucleic acid – the genetic material – directs cell activities
DNA Structure – model developed by James Watson and Francis Crick
Shape – double helix – twisted ladder
Sides of the ladder – alternating phosphates and deoxyribose sugars
Rungs of the ladder – nitrogen bases
DNA is a polymer its monomers are nucleotides (deoxyribonucleotides)
DNA nucleotides consist of a phosphate, a sugar (deoxyribose), and a nitrogen base
Nucleotides are named for the nitrogen base.
P P
S – T – A – S
P P
S – A – T – S
P P
S – C – G – S
P P
S – C – G – S
P P
S – G – C – S
P P
S – A – T – S
P P
S – A – T – S
P P
S – C – G – S
P P
S – A – T – S
Phosphates are bonded to sugars by carbon-oxygen bonds.
Sugars are bonded to nitrogen bases by carbon-nitrogen bonds.
Nitrogen bases are bonded together by hydrogen bonds.
The hydrogen bond between the nitrogen bases is the weakest point in DNA.
Obligatory Base Pairings of Nitrogen Bases in DNA
There are two broad categories of nitrogen bases.
Purines are double ring structures.
Pyrimidines are single ring structures.
A purine always pairs with a pyrimidine.
Purines Pyrimidines
Adenine Thymine
Guanine Cytosine
Adenine always pairs with thymine.
Guanine always pairs with cytosine.
DNA Replication
DNA replication occurs in the nucleus.
DNA replication occurs during the S phase of interphase.
Steps
- DNA uncoils
- DNA separates between the nitrogen bases because the hydrogen bonds are the weakest point (unzips)
- The enzyme DNA – helicase and energy from ATP are necessary to break the hydrogen bonds.
- Each side of the ladder acts as a template for making DNA
- Each exposed nitrogen base attracts its complementary base nucleotide. The nucleotides are in the nucleoplasm.
- The nucleotides are attached with the help of the enzyme DNA – polymerase.
- The 2 new DNA molecules recoil.
P P P P
S – T – A – S S – T A – S
P P P P
S – A – T – S S – A T – S
P P P P
S – C – G – S S – C G – S
P P P P
S – C – G – S S – C G – S
P P P P
S – G – C – S S – G C – S
P P P P
S – A – T – S S – A T – S
P P P P
S – A – T – S S – A T – S
P P P P
S – C – G – S S – C G – S
P P P P
S – A – T – S S – A T – S
P P P P
S – A – T – S S – A T – S
P P P P
S – T – A – S S – T A – S
P P P P
S – T – A – S S – T A – S
Protein Synthesis
Protein synthesis – a formation of proteins using information coded on DNA and carried
by RNA
Protein synthesis occurs in the ribosomes.
DNA directs protein synthesis from the nucleus with the help of three types of RNA – ribonucleic acid.
3 Types of RNA
Messenger RNA (mRNA) – carries the directions for making proteins from the DNA in
the nucleus to the ribosomes
Transfer RNA (tRNA) – brings the amino acids coded by mRNA to the ribosomes
Ribosomal RNA (rRNA) – function not sure; maybe a double check that the mRNA code
is correct
Differences between DNA and RNA
DNA RNA
Deoxyribose sugar Ribose sugar
Double strand of nucleotides Single strand of nucleotides
Contains the pyrimidine thymine Contains the pyrimidine uracil
Can’t leave the nucleus Can leave the nucleus
Steps of Protein Synthesis
A. Transcription – process in which RNA is made from DNA
occurs in the nucleus (making mRNA from DNA)
- DNA uncoils.
- DNA separates between the nitrogen bases because the hydrogen bond is the weakest point
- Each exposed nitrogen base attracts its complementary base ribonucleotide
- Ribonucleotides are combined with the aid of the enzyme RNA – polymerase
- mRNA separates from DNA
- DNA goes back together and recoils.
- mRNA takes the code for making proteins to the ribosome. The code is contained in the sequence of nitrogen bases on mRNA. 3 consecutive nitrogen bases code for an amino acid. Codon – the three nitrogen bases on mRNA, that code for an amino acid.
P
S – T –
P
S – A –
P
S – C –
P
S – C –
P
S – G –
P
S – A –
P
S – A –
P
S – C –
P
S – A –
P
S – A –
P
S – T –
P
S – T –
B. Translation – the process of converting the genetic code in RNA into the amino acid
sequence that makes up a protein. (The actual making of the protein.)
occurs in the ribosomes
- tRNA brings in the amino acids coded by the mRNA codon to the ribosomes the amino acids are in the cytoplasm
tRNA molecules are amino acid specific
two important parts of a tRNA molecule:
-
- amino acid attachment site
- anticodon – three exposed nitrogen bases on tRNA that are the complement of the mRNA codon The amino acids are bonded together with peptide bonds. The tRNA molecules are free to go pick up another amino acid.
- The chain of amino acids held together by many peptide bonds forms a polypeptide. Polypeptides form proteins.
- ribosomes move across mRNA making a protein
- 100 ribosomes may move across mRNA, each making a protein
A U G G C U U G U U A A
P-S-P-S-P-S-P-S-P-S-P-S-P-S-P-S-P-S-P-S-P-S-P-S-P
*COMPARE AND CONTRAST STRUCTURE AND FUNCTION OF DNA AND RNA*
COMPARE STRUCTURE
Both are polymers with nucleotides as the monomers
Both contain phosphate, sugar, and nitrogen bases
Both contain the nitrogen bases adenine & guanine (purines) and the pyrimidine cytosine
CONTRAST STRUCTURE
DNA RNA
Double strand of nucleotides single strand of nucleotides
Pyrimidine thymine pyrimidine uracil
Deoxyribose sugar ribose sugar
One form three forms
COMPARE FUNCTION
Both are necessary for protein synthesis
Both code for amino acids in the sequence of nitrogen bases
CONTRAST FUNCTION
DNA directs protein synthesis
mRNA carries the directions to the ribosomes
tRNA brings in the amino acids coded in the directions
rRNA double checks mRNA directions from DNA
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