Molecular Genetics

DNA: The Genetic Material MAIN IDEA: The discovery that DNA is the genetic code involved many experiments.

Rosalind Franklin Used X-ray crystallography to study DNA (pattern made when x-rays bombard them) Franklin concluded: DNA is a double helix

Watson and Crick Heard of Franklin’s work before she published it Used her X-ray diffraction pictures and other mathematical data Designed the 3-D model of DNA

DNA Structure Double helix (twisted ladder) Outside: sugar (deoxyribose) and phosphate Rungs: nitrogenous bases  Adenine, guanine, cytosine, and thymine  A-T, G-C

Do Now One strand of DNA has the following sequence of nitrogen bases: ATTCGTAGCTAGCTAAC What is the sequence of nitrogenous bases on the complementary strand of DNA? How did scientists depend on each other to discover DNA?

Answer ATTCGTAGCTAGCTAAC The complementary strand is: TAAGCATCGATCGATTG

12.2: Replication of DNA MAIN IDEA: DNA replicates by making a strand that is complementary to each original strand.

DNA Replication Occurs during S phase of mitosis/meiosis DNA must make an exact duplicate of itself “mistakes” (changes in the genetic code) = mutations

Semiconservative Replication Process proposed by Watson and Crick Original strands of DNA separate, serve as templates (patterns), and produce new DNA with one old strand and one new strand

Step 1: Unwinding DNA helix must first untwist and “unzip” (H-bonds break between nitrogen bases)

Step 2: Base Pairing Add new, complementary nucleotides to either side

Step 3: Joining DNA replication may start at many different places on one chromosome These sections must then be joined together when complete

Result of Replication 2 identical DNA molecules Each molecule: one old strand and one complementary new strand Semi-conservative

12.3: DNA, RNA, and Protein MAIN IDEA: DNA codes for RNA, which guides protein synthesis.

Protein Synthesis DNA codes for RNA, which codes for building proteins One gene directs the synthesis of one protein

DNA and RNA DNA Has two strands Has thymine, not uracil Cannot leave nucleus Bigger than RNA Only 1 type Has sugar deoxyribose RNA Has one strand Has uracil, not thymine Can leave nucleus Smaller than DNA 3 types Has sugar ribose

BOTH DNA and RNA Both… Are made of nucleotides Have adenine, cytosine, and guanine Carry genetic code Are nucleic acids

Types of RNA 3 types:  Messenger RNA (mRNA) – takes DNA’s message from nucleus to ribosome  Ribosomal RNA (rRNA) – makes up the ribosome and helps make protein  Transfer RNA (tRNA) – brings amino acids (protein parts) to ribosome

Transcription DNA cannot leave nucleus, but RNA can DNA is first transcribed (copied) into mRNA in the nucleus DNA unzips, and complementary mRNA strand is made RNA nucleotides attached according to base-pairs

Transcription

Practice transcription DNA code: TTTAGGCATCCG What’s the complementary RNA code?

Translation mRNA leaves nucleus through nuclear pores goes to cytoplasm mRNA joins ribosomes, and is translated into a protein tRNA brings over the appropriate amino acid Each amino acid joins to make a chain  protein!

Reading the Code Codon – 3-base code of DNA Each codon = 1 amino acid of protein chain 64 codons, but only 20 amino acids Some codons are repetitive and code for the same amino acids Some start and end the protein

Practice mRNA code: AUGCGGAUUUGA 1. Separate into codons 2. Use the chart to translate 3. Write the amino acids in the chain

Summary One gene codes for one protein Transcription (in nucleus)– DNA copied to mRNA Translation (in cytoplasm)– ribosomes translate mRNA, and tRNA attaches amino acids to make proteins

Checkpoint What are the 3 types of RNA and what do they do? How many proteins does one gene make? What is transcription? What is translation? Draw a diagram, concept map, etc to explain how DNA and RNA work together to make proteins.

Gene Regulation and Mutations Chapter 12.4

Gene Regulation Gene expression is regulated by the cell; mutations can affect this expression

Mutations Change in DNA sequence, either a single base pair (point mutation) or a large segment of DNA Can cause alternate phenotypes, diseases/disorders, non-functioning proteins Caused by mutagens

Point Mutations Substitution – one base pair exchanged for a different one Can cause missense – wrong amino acid is used Can cause nonsense – codes for a stop codon and ends protein synthesis early

Examples NORMAL: THE BIG FAT CAT ATE THE WET RAT MISSENSE SUBSTITUTION: THE BIZ FAT CAT ATE THE WET RAT NONSENSE SUBSTITUTION: THE BIG RAT

Point Mutations cont’d Deletion – one base pair eliminated Insertion – one base pair added Both result in frameshift, which is a change in the groups of 3 bases making codons

Examples NORMAL: THE BIG FAT CAT ATE THE WET RAT DELETION (FRAMESHIFT): THB IGF ATC ATA TET HEW ETR AT INSERTION (FRAMESHIFT): THE BIG ZFA TCA TAT ETH EWE TRA

Large Section Mutations Larger sections of DNA can be:  Deleted  Inserted  Inverted (made backwards)  Translocated (moved to a different section or chromosome)  Duplicated (copied again)  Tandem repeats (copied many times)

Think… What would be the result of a missense substitution mutation to an intron? What would most likely be the result of a mutation in the last base position of a codon?