ch. 12 lecture notes - knox central high school...
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Mr. Hoyle
Biology Lecture Notes
Ch. 12: DNA and RNA
12.1 DNA
A. To understand genetics, biologists had to learn the chemical makeup of the gene
� Genes are made of DNA
� DNA stores and transmits the genetic information from one generation of an organism to the next
B. How does DNA carry information, decide traits, and replicates itself?
� DNA is a long molecule made up of units called nucleotides
� Each nucleotide is made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base
� There are four kinds of bases: adenine (A), guanine (G), cytosine (C), and thymine (T)
C. Watson and Crick made a 3D model of DNA
� It is a double helix, in which two strands are wrapped around each other
� It is like a twisted ladder
� Sugar and phosphates make up the sides of the ladder
� Hydrogen bonds between the bases hold the strands together
� In base pairing, bonds form only between certain base pairs
� Adenine and thymine; A-T
� Guanine and cytosine; G-C
12.2 Chromosomes and DNA Replication
A. Most prokaryotes have one large DNA molecule in their cytoplasm
� Called a DNA plasmid
B. Eukaryotic cells have 1000 times more DNA than prokaryotic cells
� DNA is in the nucleus and packed into chromosomes
� DNA coils tightly around proteins called histones
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Mr. Hoyle
Biology Lecture Notes
� It coils more and more to form a thick fiber called chromatin, which makes up the chromosome
C. Before a cell divides, it copies its DNA in a process called replication
D. During DNA replication
� The DNA molecule separates by an enzyme that “unzips” it
� The base pairs are separated and the strands unwind
� Each strand of the DNA molecule serves as a pattern for the new complementary strand
� Following the rules for base pairing, new bases are added to each strand
� A with T, G with C
� An enzyme, DNA polymerase, add new nucleotides to the strands and “proofreads” it
� The end result is two identical strands – exact copies of each other
� Replication may happen at hundreds of sites along the DNA strand
� DNA replication
� DNA replication activity
12.3 RNA and Protein Synthesis
A. A gene is a segment of DNA that has the code for a certain protein
� Many proteins are enzymes, which catalyze and regulate chemical reactions.
� Proteins are each specifically designed to build or operate a component of a living cell.
B. For a gene to work, the genetic instructions in that segment of DNA must be decoded
� The first step is to copy the DNA sequence into RNA
� RNA is a molecule which contains instructions for making proteins
C. RNA is similar to DNA, except for three differences:
� The sugar in RNA is ribose instead of deoxyribose
� RNA is single-stranded
� RNA has uracil in place of thymine
D. RNA is copied from DNA in a process called transcription where:
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Mr. Hoyle
Biology Lecture Notes
� The enzyme RNA Polymerase binds to DNA and separates the two DNA strands
� RNA Polymerase builds a complementary strand of RNA using one strand of DNA as the template
� The DNA is transcribed into RNA
following base-pairing rules except that uracil binds to adenine
� Transcription video
E. There are three main kinds of RNA:
� Messenger RNA (mRNA) has the instructions for joining amino acids into a chain to make a protein
� Ribosomal RNA (rRNA) - Proteins are assembled on ribosomes, which are made up of proteins and rRNA
� Transfer RNA (tRNA) – during protein construction it transfers each amino acid to the ribosome
� carries each amino acid to the ribosome according to the coded message in mRNA
� each tRNA carries only one kind of amino acid
� the three bases are called the anticodon, and match up with a codon
F. The directions for making proteins are found in the order of the four bases – A, U, C, G
� The genetic code is read three letters at a time, called a codon
� Each codon, or group of three nucleotides, stands for a particular amino acid
� Some amino acids are specified by more than one codon
� One codon is a start signal for translation
� There are three codons that can signal the end of a protein
G. Translation is the process in which the cell uses information from mRNA to make proteins
� Translation takes place on ribosomes
� mRNA is transcribed from DNA in the nucleus
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Mr. Hoyle
Biology Lecture Notes
� The mRNA moves into the cytoplasm and attaches to a 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; in this way the chain grows
� When the ribosome reaches a stop codon, it releases the newly formed polypeptide and the process of translation is complete
� Translation video � Protein synthesis active art
12.4 Mutations
A. Mutations are mistakes made when cells copy their own DNA
B. Mutations are changes in the genetic material of the cell
� Gene mutations are changes in a single gene
� A point mutation occurs at a single point in the DNA sequence of a gene
• Substitution - when one base replaces another
♦ only one amino acid is affected
• Point mutation video
� If a nucleotide is added or removed, it causes a frameshift mutation
• This changes the whole sequence of codons
♦ May change every amino acid after that point
♦ can cause the gene to make a completely different protein
♦ can alter a protein so much that it is unable to function
• Insertion – a base is added to the strand
• Deletion – a base is deleted from the strand
� In a chromosome mutation, there is a change in the number or the structure of chromosomes
� There are four kinds of chromosomal mutations:
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Mr. Hoyle
Biology Lecture Notes
• Deletions – involve the loss of all or part of a chromosome
• Duplications – produces extra copies of parts of the chromosome
♦ Duplication & deletion video
• Inversions – reverse the direction of part of the chromosome
• Translocations – occurs when part of one chromosome breaks off and attaches to another
• Translocation & Inversion video
C. While most mutations are harmful, some mutations have little or no effect on gene expression
� Some mutations are the cause of genetic disorders
� Beneficial mutations may produce proteins with new or altered activities that can be useful
12.5 Gene Regulation
A. Genes can be turned on and off as different proteins are needed
� In prokaryotes, some genes are turned on and off by a chromosome section called an operon
� An operon is a group of genes that work, or operate, together
� In bacteria, one operon controls whether the organism can use the sugar lactose as food
� It is called the lac operon
� The lac genes are turned off by repressors and turned on by the presence of lactose
� Operators and promoters are DNA sequences in the operon that control when genes are turned on and off
� When the cell needs a certain protein, RNA polymerase attached to the promoter and makes a messenger RNA that is translated into the needed protein
� When the cell no longer needs the protein, it makes another protein called the repressor
• The repressor attaches to the operator
• This blocks the promoter so RNA polymerase cannot attach to it
• This turns the genes of the operon off
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Mr. Hoyle
Biology Lecture Notes
B. Most eukaryotic genes are controlled individually and have regulatory sequences that are much more complex than those of the lac operon
� In eukaryotes, genes are regulated by enhancer sequences located before the point at which transcription begins
� Some proteins can bind directly to these DNA sequences
� Ways in which these proteins affect transcription include:
� Increasing the transcription of certain genes
� Attracting RNA polymerase
� Blocking access to genes
Chapter 12 Resources