DNA Deoxyribonucleic 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 and 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 (PO 4 ) 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-A TTTCGATTT AA GG C 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 38GUANINE 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.

Amino acids: the building blocks of proteins simple atomic structure: Central carbon atom bound to: 1 carboxyl group (COOH) 1 amino group (NH 2 ) 1 hydrogen atom 1 side chain represented by “R” (meaning it is different in every amino acid)

Amino acids: the building blocks of proteins The sequence of nitrogenous bases in DNA is the “code” that tells cells what amino acids to produce There are 20 different amino acids that are commonly found in proteins Every 3 nitrogenous bases code for an amino acid

Example: CAC is a nitrogenous base sequence that codes for the amino acid histidine

Amino acids bond together by dehydration synthesis. Peptide bond forms between the amino group of one molecule and the carboxyl group of another. Resulting molecule is called a dipeptide

RNA Similar to in composition to DNA…...A little too similar… The molecule consists of only 1 chain (instead of 2) 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-A UUUCGAUU AA GGCUC

RNA nucleotide

How many different DNA & RNA nucleotides are there total? 8

A C G C C T T G A T C T G T C G C A T T T A G C

Polypeptide: multiple dipeptides bonded together Proteins: multiple polypeptides bonded together smallest protein = 50 amino acids Largest protein = amino acids Each different sequence of amino acids codes for a different protein Sections of folded chain will bond together to form cross-links Different shapes can therefore code for different proteins

A-A-A-C-G-C-G-A…. Amino acid + Amino acid + Amino acid… Polypeptide (1 or more) Proteins Dipeptide

One gene-one enzyme hypothesis: Genes control the production of enzymes. Genes produce their effects by controlling the synthesis of a single enzyme: Genes actually control the production of all proteins. Some of which are enzymes, some are hormones, some are structures of the cell. Proteins are made of polypeptides which are long chains of amino acids. One gene-one polypeptide hypothesis: each gene directs the synthesis of a particular polypeptide chain.

The DNA code: The order of the nitrogenous bases in the DNA molecule is actually a “code” for the order of amino acids. Each string of 3 nitrogenous bases codes for an amino acid.

If there are 4 nitrogenous bases (Adenine, Thymine, Cytosine, and Guanine) then there are 64 different 3 base sequences. However, there are only 20 different amino acids in proteins. Therefore some amino acids have more than one code. Today scientists know that genes are made of many nucleotides. The order of the bases codes for the amino acids that will be added to the polypeptide chain.

Protein synthesis: 1) TRANSCRIPTION: takes place in the nucleus. The nucleus will copy the DNA code for a polypeptide into a molecule of RNA. The DNA strand separates for a short time to serve as a template (or pattern) for RNA. Complimentary RNA nucleotides take their place on the DNA strand (just like when DNA replicates, with one difference) A-A-A-C-G-A-T-A-T-T-G-C-C-G-A UUUCGAUU AA GGCUC

When the RNA reaches a specific “STOP” code (a code of Nucleic acids e.g. UAC) it leaves the DNA strand. The RNA strand is now a separate molecule that carries the complete code for one Polypeptide The involved RNA in this step is known as messenger RNA or mRNA Codon: group of 3 bases on RNA that codes for an amino acid

TRANSLATION: code amino acids Once mRNA has copied the code, the next step is to actually create the polypeptides. Transfer RNA ---- tRNA There are 20 different kinds of tRNA, one for each amino acid The have a short tail on one end that can “pick up an amino acid” Each tRNA can carry only one specific amino acid

tRNA At the base is an Anticodon. that complements the mRNA code This is how the tRNA knows where to put its amino acid.

Ribosomal RNA -- rRNA Formed in the nuclei of the cell. A protein from the cytoplasm migrates in to the nucleus rRNA joins with a protein to create a ribosome rRNA + Protein = ribosome The ribosome is where a polypeptide is assembled during protein synthesis

Nucleus Cytoplasm DNA strand mRNA Amino Acid tRNA rRNA ProteinrRNARibosome