Download Ch 12 Lecture Notes - PPT

Chapter 12 DNA and RNA
Class Lecture Notes
12-1 First Discoveries
• 1950’s- British scientist, Rosalind Franklin used x-ray
diffraction to study DNA
•Aimed x-rays at DNA samples – recorded scattering
pattern of x-rays on film
•Showed the shape being twisted like a coil spring or a
helix – angle of “x” suggested 2 strands
•1953 – American scientist, Watson and British scientist,
Crick (both biochemists) proposed a model for structure
of DNA
•Model was double helix – 2 strands wound around each
other
12-1 Structure of DNA
• Nucleic acid is one of the four major organic molecules that make up
all living things.
•DNA (deoxyribonucleic acid) consists of 2 long nucleotide chains that
twist around each other to form a double helix (spiral staircase shape)
•Weak bonds (hydrogen bonds) between chains hold nucleotides to each
other
•Nucleotide = 3 parts (see fig.12-5 pg. 291 & fig.12-7 pg. 294)
•1.) nitrogen base, 2.) phospate group, 3.) molecule of deoxyribose (a
5-carbon sugar)
•Nitrogen bases are of 2 types:
•1 = purines: adenine (A), guanine (G)
•2 = pyrimidines: cytosine (C), thymine (T)
12-2 DNA Replication
•Before a cell divides it replicates its DNA
•When does this occur in cell cycle? Why?
•DNA separates into 2 strands (zipper unzips) (fig 12-9
p.296)
•Bonds between nitrogen bases are broken
•Each strand serves as a template to create the new strand
from
•Each replicated DNA strand will have one original strand
and one new
•Enzyme called DNA polymerase assists in creating the new
nucleotides, also “proof reads” each new strand
•In the double helix, the nitrogen bases in the first strand pair
up and form bonds with the bases in the second strand holding
the two strands together
•The pairing of bases is not arbitrary. The base adenine(A)
always pairs up with thymine (T) and cytosine (C) always pairs
up with guanine (G).
•Test yourself, let’s see how well you remember:
•Type answers and save file in student drive under “DNA Unit”
folder.
•What 3 parts make up a DNA nucleotide?
•What type of bonds hold each strand of DNA together?
12-2 DNA and Chromosomes
•Nucleus of human cell contains more than 1 meter of
DNA (long chains of nucleotides make up DNA)
•Smallest human chromosome has 30 million base pairs
of DNA
•How is so much DNA folded into tiny chromosomes?
•Chromosomes contain both DNA and protein tightly
packed together to dorm chromatin
•Chromatin is DNA tightly coiled around proteins
called histones. (fig 12-12 p.299)
•They form a system of loops and coils with a beadlike
structures = nucleosomes
•Nucleosomes pack with each other to form a thick fiber
or chromosome
12-3 RNA and Protein Synthesis
•To transcribe something means to write a copy of it
•DNA is transcribed to produce a molecule of RNA
•How RNA is different from DNA:
•Sugar in RNA is ribose not deoxyribose
•RNA is single-stranded
•RNA uses the base uracil (U) instead of thymine (T)
•RNA helps create amino acids which are then formed into
proteins
•Proteins are structural components that make up your body
and form enzymes that direct every chemical reaction in
your body
•RNA = a blueprint of DNA in a 4-letter language
•There are 3 types of RNA:
•Messenger RNA, Ribosomal RNA, Transfer RNA
•mRNA (messenger) is found in the nucleus and
cytoplasm and carries the instructions (messages) for
creating amino acids into proteins
•rRNA (ribosomal) is found in the nucleolus and the
ribosomes, and is involved in construction of ribosomes
•tRNA (transfer) helps to transfer amino acids, produced
by mRNA instructions, to the ribosome
•Transcription is the process in which mRNA is
manufactured, it occurs in the nucleus
•Enzyme, RNA polymerase binds to a strand of DNA,
causing bonds between nitrogen bases to break and separates
strands
•Individual mRNA nucleotides floating around in the nucleus
bind with one unraveled DNA strand
•Uses DNA strand as a template to create the RNA strand
•Starts transcription at a spot on DNA called the promoter,
which is a signal for where the RNA should start making its
strand
•After RNA is created, it is proof read and edited (like the
rough draft of a paper). Pieces called introns are cut out and
remaining portions called exons are spliced back together.(fig
12-15 p. 302)
•mRNA genetic code is read 3 letters at a time (like a word).
This is called a codon.
•The four letters in the RNA alphabet, A, U, C, and G are
combined in various combinations, each unique codon
representing a specific amino acid. (64 possible
combinations which represent 20 amino acids in total)
Some amino acids are specified by more than one codon
•A string of several amino acids creates a protein (aka:
polypeptide)
•See fig 12-17 p. 303
•Special codons: start = AUG, stop = UGA, UAA, UAG
•Translation – to express something in another language
•Genetic code language is translated into amino
acid/protein sequence
•A ribosome, out in the cytoplasm, reads the instructions
and assembles the proteins
•mRNA,leaves the nucleus through a pore in the nuclear
membrane and finds its way to a ribosome
•The ribosome binds to the start codon of the mRNA
•tRNA then binds to the mRNA with complementary base
pairs called anticodons. These carry the correct amino
acids to make the protein strand.
•
Practice Questions:
Type answers and save file in student drive under “DNA
Unit” folder.
1. The nucleotide sequence of an unraveled section of
DNA is ATGCCCTAAGTG. What is the
corresponding mRNA sequence?
2. The nucleotide sequence of a strand of mRNA is
AUGUGCAAUUUUUAA. What is the amino acid
sequence of the protein that this mRNA codes for?
Use wheel chart on p. 303. Note that amino acid
names are abbreviated using the first three letters of
the amino acid.
3. The nucleotide sequence of an unwound strand of
DNA is TACATGCTAGCCGCTATC. What is the
amino acid sequence that the DNA codes for?
12-3 Mutations
•Sometimes, even with the proof reading, mistakes are
made by inserting the wrong base or skipping a base
•These changes in DNA or RNA sequence affect genetic
information and are called mutations .
•Mutations that affect a single gene = gene mutations
•Mutations that affect a single base and therefore a single
amino acid = point mutations
•If a base is deleted or an extra is inserted it can throw
off the whole chain since they are read in groups of three.
•Frame shift mutation is when a bas is deleted or extra
is inserted so that the reading frame is shifted, therefore
affecting every amino acid following the mutation.
•Mutations that affect an entire chromosome =
chromosomal mutations
•4 types of chromosomal mutations:
•1.) deletion – loss of all or part of a chromosome
•2.) duplication – segment is repeated
•3.) inversion – part of a chromosome gets oriented in the reverse of its
normal direction
•4.) translocation – part of a chromosome breaks off and attaches to
another non-homologous chromosome
•Chromosomal mutations occur by failure to separate properly during
meiosis. Some may even have an extra chromosome. This is called
polyploidy.
•Down Syndrome – when person has 3 copies of chromosome # 21.
•Cystic Fibrosis – caused by a deletion of 3 bases in the middle of a
sequence for a protein on chromosome #7. (Causes digestive problems,
breathing problems, mucus clogs lungs)