dna deoxyribonucleic acid structure and function
TRANSCRIPT
DNADeoxyribonucleic Acid
Structure and Function
Remember from the Genetics Unit…
• Chromosomes carry hereditary information (information that is passed to offspring) that is present in distinct units called genes arranged along chromosomes like beads.
• Those beads are called alleles.
• It is the combination of the alleles that create the gene, which will eventually create the expression in the organism.
Make Up of Chromosomes • Chromosomes are made of chromatin, which describes
the combination of DNA and proteins wound tightly together.
• They must be wound up tightly to fit inside of the nucleus.
Nucleic acids
• 2 kinds:• DNA = Deoxyribonucleic acid• RNA = Ribonucleic acid
Both found in the nucleus of the cell
Let’s start with DNA…
• DNA is the hereditary material that is passed on from one generation to the next through reproduction
• No 2 people in the world have 100% the same DNA.
• The Human Genome Project mapped the genes of a single person and found that the human genome contains more than 3.4 billion base pairs and between 20 000 and 25 000 genes.
• It took 13 years to map the human genome and was complete in 2003.
DNA has three components
1. 5 carbon sugar (deoxyribose)
2. Phosphate group (PO4)
3. Nitrogenous base
There are 4 nitrogenous bases in DNA:• Adenine (A) Thymine (T)• Guanine (G) Cytosine (C)
• When the 3 components are combined together, they create a nucleotide.
• This unit repeats and composes the majority of the DNA structure.
• DNA nucleotides are composed of:
• 1 molecule deoxyribose sugar
• 1 molecule phosphate group
• & 1 of the 4 nitrogenous bases (A,T,C or G)
• Nucleotides are arranged in a ladder structure.
• Alternating phosphate and sugar molecules make up the sides of the ladder
• Pairs of nitrogenous bases make up the rungs of the ladder
The shape of DNA is called a Double Helix
Complementary base pairing rule
• In any pair of bases: • Adenine will bond only to Thymine• Guanine will bond only to Cytosine
• Nitrogenous bases are attached
to each other with hydrogen bonds:
• There are 2 hydrogen bonds between Adenine and Thymine
• There are 3 hydrogen bonds between Guanine and Cytosine
• Each pairing must be with a ‘Purine’ and ‘Pyrimidine’.
• Adenine and Guanine are purines.
• Thymine and Cytosine are pyrimidines.
• So, keeping the complementary base pairing rule in mind: If the order of Nitrogenous bases along one arm of DNA was…
A-A-A-C-G-A-T-A-T-T-G-C-C-G-AT T T CG AT T TA A G GC C
• Question: If 12% of the nitrogenous bases in a persons DNA was Adenine, then what percent of their nitrogenous bases would be Guanine?
CYTOSINE 38 GUANINE 38
ADENINE 12 THYMINE 12
•The sequence of nitrogenous bases acts as a code.•The code determines what proteins will be made by the cell.
•Proteins in turn determine the nature and activities of the cell.
•This is why the amount of DNA is not important.
•Sequence determines outcome.
Replication of DNA• Due to the fact that cells divide so rapidly, DNA must be
replicated often and efficiently in order to keep up with demand.
• DNA is replicated during interphase of either mitosis or meiosis depending on the cell.
• Human cells take about 20 hours to replicate all 46 chromosomes.
• However, some can take only minutes. E.Coli bacteria can replicate DNA and divide a cell in only 20 minutes.
Steps in Replication
1: The hydrogen bonds between the base pairs (nitrogenous bases) break and the two strands of the DNA molecule pull apart.• this exposes the base pairs along each strand
• 2: Bases of free nucleotides (floating around in the nucleus) fasten on to complementary bases of the now exposed DNA chain.
• Result: complementary strand is created that is identical to the one that split off earlier.
• Remember that this process is done for each of the two original DNA strands.
• Think back to mitosis: when the chromosomes doubled
• Replication does not actually begin at one end of the DNA strand and end at the other.
• To speed up the process, Replication begins at many points on the DNA strand that connect later.
• These are called Replication bubbles.
RNA• Similar to in composition to DNA…
• The molecule consists of only 1 chain (instead of 2).
• Half of the ‘ladder’ is present.
• The sugar is Ribose (not deoxyribose)
• Thymine is replaced with Uracil • (which eventually bonds with
adenine)
• RNA is involved in the synthesis of proteins
A-A-A-C-G-A-T-A-T-T-G-C-C-G-AU U U CG AU U A A G GC UC
RNA nucleotide
• How many different DNA & RNA nucleotides are there total?
8