Molecular Genetics DNA Structure DNA Deoxyribonucleic acid DNA

Molecular Genetics

DNA Structure DNA – Deoxyribonucleic acid DNA molecules are long polymers made up of nucleotides Each nucleotide has 3 parts: 1. Phosphate group 2. Deoxyribose (sugar) 3. Nitrogen base

Each DNA molecule has billions of nucleotides BUT there are only 4 types of nucleotides (they are each repeated many times) The 4 types of nucleotides differ only in their nitrogen bases 4 bases in DNA are: 1. Adenine (A) 2. Guanine (G) 3. Cytosine (C) 4. Thymine (T)

Adenine and Guanine – 2 rings sugars Cytosine and Thymine – 1 ring sugars

DNA is a double helix 2 strands of DNA wind around each other like a twisted ladder The double strands are complementary A–T G–C

Nucleotides always pair up the same way Base-pairing rules: Adenine always pairs with Thymine Cytosine always pairs with Guanine Base pairing example: AGCTATGGC TCGATACCG

Universal Genetic code It is the sequence of the 4 bases that make us unique Every living organism has DNA that is composed of the same 4 bases The sequence determines which proteins will be made

DNA Replication – when DNA is copied during the cell cycle Replication is an exact process that ensures that every cell has identical DNA Proteins help DNA to replicate (DNA polymerase) This process happens very quickly Each of our 46 chromosomes replicate before a cell divides through mitosis

How replication works: Enzymes “unzip” the double helix (hydrogen bonds that connect the nitrogen bases are broken) The molecule now separates into 2 strands with the bases exposed Nucleotides floating in the cell attach one by one to the bases on each exposed strand Remember A – T and C – G

As each complementary strand is formed it reforms a double helix – now there are 2 identical double stranded DNA molecules DNA polymerase enzyme connects the 2 strands together Each new DNA molecule has 1 original (template) strand one newly formed strand

DNA replication is fast and accurate Replication conserves genetic information because it ensures exact copies of DNA are made DNA replication can also “proofread” for mistakes If a mistake is found (an incorrect base) – DNA enzymes can cut it out and replace it with the correct base

What if a mistake is not fixed? Mutation – change in the DNA sequence Mutations that affect an entire chromosome happen during meiosis (down syndrome) Mutations that affect a single gene happen during replication Point mutation – when one nucleotide base is substituted for another Frameshift mutation – when a nucleotide is either inserted or deleted from the DNA sequence

Point Mutation Example: Correct sequence: TTACGCA Mutated sequence: TTACCCA Frameshift Mutation Example: Correct sequence: TAC CAT TAG Mutated sequence: TCC ATT AG

Point Mutation

Frameshift Mutation

Mutagens – agents in the environment that can cause changes in DNA sequence Ex. UV radiation, chemicals

Turning DNA to Protein The DNA code must somehow be converted into proteins that make our body function There are 3 steps: DNA RNA Protein Replication copies DNA Transcription converts a DNA message into RNA Translation interprets an RNA message into a string of amino acids (protein)

DNA is in the nucleus (this is where replication occurs) Protein synthesis (making proteins) occurs in the cytoplasm RNA is the link between DNA and proteins synthesis RNA is like a temporary copy of a portion of DNA that is used and then destroyed

RNA – ribonucleic acid Single stranded One base differs from DNA – A, C, G, and U Contains URACIL instead of Thymine “A” now base pairs to “U” (not T like in DNA)

Transcription – process of copying a sequence of DNA to produce a complementary (base-paired) strand of RNA The code for A SINGLE GENE is transferred into an RNA message Enzymes called RNA polymerases help in transcription

3 types of RNA Messenger RNA (m. RNA) – message that is made off of DNA and will be translated into a protein Ribosomal RNA (r. RNA) – makes up ribosomes (remember: ribosomes are where proteins are made!) Transfer RNA (t. RNA) – bring amino acids from the cytoplasm to a ribosome to help make a growing protein

Transcription is similar to replication When a cell needs to make a particular protein, the gene that contains the sequence on DNA is unwound m. RNA code is then made off of the DNA using base pairing Hundreds or thousands of copies of the m. RNA code are made Each code will direct the synthesis of one protein molecule

Translation Process that, converts an m. RNA message into a polypeptide Proteins are made up of one or more polypeptide chains DNA codes in A, T G, C RNA codes in A, U, G, C Proteins code in amino acids (20)

Genetic code is read in units of 3 bases Codon – 3 nucleotide sequence that codes for an amino acid Certain codons signal for translation to start or stop; these are called start and stop codons Ex: codon: AUG codes for amino acid named methionine

The genetic code is shared by almost all living things This means that the same codons code for the same amino acids Ex: UUU codes for amino acid phenylalinine in humans, cacti, armadillo

Here’s how translation works m. RNA sequence is transcribed off of DNA in the nucleus m. RNA travels out of nucleus m. RNA attaches to a ribosome in the cytoplasm with the help of r. RNA ……

Ribosome reads the m. RNA message 1 codon at a time As a codon on the m. RNA is read, a t. RNA molecule with the complementary anticodon sequence attaches to the m. RNA The t. RNA also holds the amino acid linked to the anticodon Each amino acid is linked to the one next to it making a protein