Chapter 12 Molecular Genetics
The structure of DNA and DNA replication
DNA - Deoxyribonucleic Acid To understand genetics, scientists had to learn the chemical makeup of the gene. Scientists discovered that genes are made of DNA Scientists also found that DNA stores and transmits the genetic information from one generation of an organism to the next. Scientists began studying DNA structure to find out how it carries info, decides traits, and replicates itself.
DNA – The structure DNA is a long molecule made up of units called nucleotides. Each nucleotide contains a 5-carbon sugar, a phosphate group, and a nitrogen-containing base. There are four kinds of bases in DNA: Adenine (A) Guanine (G) Cytosine (C) Thymine (T)
The Nucleotides Green = 5-C sugar Peach = phosphate
Watson and Crick Watson and Crick made a 3-D model of DNA. Their model was a double helix, in which two strands are wound around each other. A double helix is like a twisted ladder Sugar and phosphates make up the side of the ladder Hydrogen bonds between the bases hold the strands together (the rungs)
DNA
A always pairs with T G always pairs with C Chargaff’s rule Aka the “base pairing rule” Chargaff’s rule states that bonds only form between certain base pairs A always pairs with T G always pairs with C You must memorize this!!!!
Base-pairing
Where can you find dna? Most prokaryotes have one large DNA molecule floating in the cytoplasm. Eukaryotes have DNA in the nucleus. Amazing fact: The nucleus of a human cell contains more than 1 meter of DNA!
Chromosomes Contain both DNA and protein, which are tightly packed together to form chromatin. Chromatin consists of DNA that is tightly coiled around proteins called histones. This forms a beadlike structure called a nucleosome. Nucleosomes pack with one another to form a thick fiber, which is shortened by a system of loops and coils. Nucleosomes allow enormous amounts of DNA to fit into such a small region.
DNA Replication Before a cell divides, it copies its DNA in a process called replication. During replication: The DNA molecule separates into 2 strands – each strand of the DNA molecule serves as a model for the new strand. Following the rules for base pairing, new bases are added to each strand. The end result is two identical strands.
How does replication occur? Its carried out by a series of enzymes Some enzymes “unzip” the molecule of DNA – the hydrogen bonds between bases are broken The two strands unwind and serve as templates to create two new strands The key enzyme is called DNA polymerase – joins individual nucleotides to produce a DNA molecule DNA polymerase is also involved in “proofreading” each of the new DNA strands to minimize errors.
DNA Replication
Let’s Replicate! Original Strand = T A C G T T New Strand = Original Strand = ATCGGCAATCACGAT New Stand = A T G C A A TAGCCGTTAGTGCTA
RNA and Protein Synthesis
Cookbook Story
RNA – The basics For a gene to work, the genetic instructions in the DNA molecule must be decoded. The first step is to copy the DNA sequence into RNA. RNA is a molecule which contains instructions for making proteins.
RNA – The structure RNA is similar to DNA, except for: RNA contains the sugar ribose DNA has deoxyribose RNA is single-stranded DNA is double stranded RNA has uracil in place of thymine – A, U, C, G DNA has A, T, C, G
RNA Molecules – what do they do? There are 3 kinds of RNA – Messenger RNA (mRNA) What is this in the story? Has the instructions for joining amino acids to protein Ribosomal RNA (rRNA) Assembles the proteins Transfer RNA (tRNA) Carries each amino acid to the ribosome according to the coded message in mRNA Contains an anticodon to pair up with mRNA Cookbook Cook Translator
Transcription Process of making (or copying) RNA from DNA Occurs in the nucleus During transcription: DNA strands are separated RNA strand is built using one DNA strand as a template DNA is transcribed (written) into RNA following base-pairing rules except that U binds to A AU, TA DNA Strand = TACGCTACGCCTAATACT New mRNA Strand = AUGCGAUGCGGAUUAUGA
Transcription to translation The directions for making proteins are in the order of the four nitrogenous bases This code is read 3 letters at a time Each grouping of 3 letters is called a codon AUG/CGA/UGC/GGA/UUA/UGA Each codon stands for one amino acid There is one “start” codon and 3 “stop” codons Start = AUG Stop = UGA, UAA, UAG
Translation Process in which the cell uses info from mRNA to make proteins Takes place in the ribosome How does it work? mRNA moves into the cytoplasm and attaches to ribosome As each codon of the mRNA moves through the ribosome, the proper amino acid is brought into the ribosome by tRNA. The ribosome joins together each amino acid and the protein chain grows. When the ribosome reaches a stop codon, the polypeptide chain is released.
How do we figure out the amino acid? Use a Codon Chart
Let’s do an example! DNA Strand = TACGCTACGCCTAATACT mRNA Strand = AUGCGAUGCGGAUUAUGA Split into codons = AUG/CGA/UGC/GGA/UUA/UGA Create Protein = Methionine - Arginine - Cysteine - Gylcine - Leucine
Let’s Review! http://www.teachersdomain.org/resource/tdc02.sci.life.gen.proteinsynth/
Mutations Mutations are mistakes made when cells copy their own DNA. Mutations are changes in the genetic material of a cell. There are two types of mutations: Gene mutations – changes in a single gene Chromosomal mutations – change in the number of structure of chromosomes and affect multiple genes
Gene Mutations Point mutations (substitution)– Occur at a single point in the DNA sequence Causes one base to replace another, which only affects one amino acid Frameshift mutations (insertion, deletion) – A base is added or removed All the codons are affected/changed resulting in a completely different protein
Chromosome mutations Change in structure of chromosomes Four types: Deletions = ABC-DEF AC-DEF Duplications = ABC-DEF ABBC-DEF Inversions = ABC-DEF AED-CBF Translocations = ABC-DEF ABC-JKL GH-IJKL GH-IDEF