cell structure review and introduction to dna
Post on 07-Jan-2016
15 Views
Preview:
DESCRIPTION
TRANSCRIPT
Cell Structure Review and Introduction to DNA
Did you know? 100 years ago we did not know why some
children had brown eyes and some blue 75 years ago we did know the structure of
dextrose 50 years ago we did not know the correct number
of chromosomes 25 years ago we did not know any of the genes
linked to cancer What is one you know of?
BRCA breast cancer gene
Differences Between Eukaryote and Prokaryote Cell
Eukaryotes Nucleus Membrane-bound
organelles Microtubules are the
building blocks for a flagella
Cell membranes can contain cholesterol
Cell size is usually bigger Introns in the DNA
Prokaryotes No nucleus No membrane-bound
organelles Flagella is the building
block No cholesterol in cell
membranes Cell size is usually smaller Usually no introns
Eukaryotes
Sources of DNA How are Eukaryotes and Prokaryotes different?
Eukaryotes DNA is in the nucleus
Prokaryotes have no nucleus, so where is the DNA? Floating in the cytoplasm, which is usually attached to the cell
membrane. Bacteria contain 1 long circular DNA molecule, super coiled. E. coli contains 1 chromosome w/4000 genes and 4.6 million base pairs
(bp) R Plasmids: bacteria with small ring of DNA floating in cytoplasm and
these contain the antibiotic resistance genes Genes are turned “on” or “off” easily
Eukaryotic DNA DNA packaged into chromosomes Each single DNA may contain several million
nucleotides and many thousands of genes Humans have 46 chromosomes per cell with
about 3 billion base pairs making up about 40,000 genes
Historical Figures in Molecular Biology (visit DNAi.org)
Miescher Griffith Avery, McCarty and MacLeod Chargaff Wilkins, Franklin, Watson & Crick
Key people in genetics and DNA Gregor Mendel: heredity passed down from parents;
relationship between phenotype and genotype Schleiden and Schwann’s “cell theory” explained fertilization
of sperm and egg to make zygote 1905 discovered sex chromosomes existed; years later realized
there are more chromosomes that are responsible for traits Watson and Crick: 3D structure of DNA in 1953
DNA actually discovered in 1869, by 1900 understood that it was composed of 5C sugar, phosphate, and 5 types of nitrogen rich bases (ATCGU), 1920s we understood that RNA and DNA were different
Key Historical People Erwin Chargaff
determined percents of purines and pyrimidines present
Rosalind Franklin X-ray diffraction technique was key to
understanding the helix structure
C
C
C
C
N
N
OO
N
CC
CC
N
N
N
N
N
C
Role of X-Ray Crystallography X-rays diffracted by the regularly arranged
atoms of a simple crystal (Max von Laue)
Pauling, Franklin, Wilkins, Watson and Crick were all working diligently to discover the structure of DNA
What Nucleotides are involved? What are the 4 nucleotides of DNA?
Adenine, thymine, guanine, cytosine
What pairs with what? Adenine and Thymine Cytosine and Guanine
The three parts of the nucleotide building block of DNA are the sugar, the base and the phosphate. The complex of the sugar with the base is called a nucleoside.
Sugar The sugar is the 5-carbon sugar deoxyribose. By convention the carbons on this sugar are labeled 1' through 5'.
Phosphate The phosphate is attached to the 5' carbon of the deoxyribose sugar.
Base The base is attached to the 1' carbon of the deoxyribose sugar. There are four different bases found in DNA. Because each base contains at least two nitrogen atoms, they are called nitrogenous bases. There are two classes of bases, the pyrimidines (cytosine (C) and thymine (T)), and the purines (adenine (A) and guanine (G)).
Complementary Base PairingDNA consists of two polynucleotide chains wound around each other to form a double helix. The two chains are held together by complementary base pairing; that is, specific bonding between A and T bases and between G and C bases on the two strands
Two antiparallel DNA polynucleotide chains held together by complementary base pairing.
To make a stable double helix, the two strands of DNA are antiparallel; that is, the 5’ - 3' direction of one strand runs opposite to the other strand.
The two DNA chains are held together by complementary base pairing between A and T bases and between G and C bases.
The helix has a right hand twist.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
In a DNA polynucleotide chain, nucleotides are joined by phosphodiester bonds formed between the 5' carbon of one sugar and the 3' carbon of the next sugar. A free phosphate defines the 5' end of the chain and a free hydroxyl group defines the 3' end of the chain.
1. 3’ end2. phosphate3. 5’ end4. thymine5. 3’ end6. phosphodiester bond
7. cytosine8. deoxyribose9. guanine10. 5’ end
11. adenine
Basics to DNA Structure Rails of ladder: run in opposite
directions (anti-parallel) Contains alternating units of
deoxyribose sugar and phosphate Each rung composed of a base pair
held together by weak hydrogen bonds 10 base pairs per turn 34 A total so 3.4 A between pairs
DNA Replication DNA helicase Single-strand binding proteins Primase DNA polymerase DNA ligase Okazaki fragments
So why is DNA replication so important to us? DNA is the carrier of genetic information for
all living organisms Through the process of replication, the entire
genome is copied and passed down to each new cell made in the body.
Replication is also the way genetic information is passed from parents to offspring.
Replication DNA polymerase can only directly synthesize
new DNA in the 5' to 3' direction Chargaff’s Rule for determining how many
nucleotides are present: In double-stranded DNA, G = C, and A = T. If C = 21, then G = 21 and G + C is 42. Therefore A + T = 100 - 42 =
58, and T = 58/2 = 29 percent. A/T, G/C, and (A+G)/(C+T) are all equal to 1
Semiconservative Replication: making 2 daughter stands from a single parent strand Therefore DNA replication takes place prior to cell
division
DNA Helicase and SSB DNA Helicase is an enzyme which begins the unzipping process. Also prevents DNA from rebinding. Problem is that it creates a knotted up mess of DNA Topoisomerase cuts one strand of unwound and allows
it to unwind and then reseals it. It prevents damage to the DNA by allowing it to swivel.
Once DNA is unzipped the base pairs of each single stand will begin forming helix structure SSB (single strand binding proteins) are formed to
block this action; prevent recombining
Getting Replication Started
Replication Bubble
The DNA begins to split from many points along the strands and separate from that point, creating a bubble-like area.
Replication Fork
When referring to the replication of DNA in a singular direction, the original DNA splits in two, forming two prongs, which resemble a fork.
Primase and DNA Polymerase Primase is an RNA polymerase which does not
need a primer to initiate synthesis RNase H comes into remove the RNA primer made
by primase before DNA is replicated DNA polymerase III can only add nucleotides
onto the 3’ end of an existing DNA fragment so if this is the case then where does the first piece of DNA come from?
Okazaki fragments and DNA Ligase DNA synthesis is always 5’ to 3’ Leading strand is synthesized and the lagging strand has
small fragments formed which are later joined together. Fragments are called Okazaki fragments after the
scientist who discovered this process. Polymerase I removes RNA primer and replaces it with
DNA nucleotides in Okazaki fragments DNA ligase is the enzyme which joins the
Okazaki fragments together
DNA Replication
Checking your knowledge What are the two strands of DNA called after they
unzip? Leading and lagging
What enzyme is used for unzipping? DNA helicase
What direction does DNA replicate? Actually both, 5’ to 3’ easy, continuous and self
correcting; 3’to 5’ takes longer, more chance of error and requires DNA polymerase and DNA ligase
Checking your knowledge What are the fragments formed during
replication called and what strand are they formed on? Okazawki fragments; lagging 3’ to 5’ strand
What 3 enzymes are required for DNA replication? DNA helicase, DNA polymerase, topoisomerase
RNA: how is it different from DNA? Pentose sugar is ribose instead of deoxyribose Uracil replaces Thymine RNA is single stranded
What is transcription? Where does it occur? Transcription: the process of deciphering a
DNA nucleotide code and converting into into an RNA nucleotide code; RNA carries genetic message to a ribosome for translation into a protein.
Proteins do the work of cells and give cells and organisms their unique characteristics.
Note: single strand of RNA
Transcription occurs in 5’ to 3’ direction
Transcription Initiation
Transcription factors bind to the TATA box which guide the RNA Polymerase to the starting point of the gene.
Transcription Elongation
RNA Polymerase continues to assemble RNA nucleotides in a complementary fashion to the 3’5’ template strand
Transcription Termination
Once the RNA Polymerase hits the termination sequence, it releases from the template and the RNAS transcript floats away.
RNA Processing
Slicesome cuts out the intron sequences and joins the exons to make the final mRNA
Poly A tails are added to the 3’ end
Methylated G cap added to the 5’ end
More essential terms Intron: region on a gene that is transcribed
into a mRNA molecule but not expressed in a protein; spacer DNA
Exon: region of a gene that directly codes for a protein, it is the region of the gene that is expressed
Eukaryote
Some essential terms
Operon: section of prokaryotic DNA consisting of one or more genes and their controlling elements.
Promoter: the region at the beginning of a gene where RNA polymerase binds; the promoter promotes the recruitment of RNA polymerase and other factors required for transformation
Operator: region on an operon that can either turn on or off expression of a set of genes depending on the binding of a regulatory molecule
Genetic engineers use promoter and operator regions to turn on / off the production of certain genes
Lac Operon
Codon Chart
Translation
Translation Initiation
Initiator tRNA binds to the AUG codon of the mRNA. This tRNA has an anticodon of UAC and carries Met amino acid so all translated products start with the Met amino acid
Translation Elongation
A site – tRNA enters with its new amino acid
P site – growing amino acid chain is linked to newly arriving amino acid by a peptide bond
E site – tRNA leaves without its amino acid
Translation Termination
When stop codon appears in A site, there is no tRNA to bind so a release factor binds instead
This causes the polypeptide chain to release
Finally, it also causes the ribosome subunits to disassociate and translation is terminated
Amino Acids and Proteins
Mutations
• Occur when DNA Polymerase makes a mistake or environmental factors cause an alteration in the DNA sequence
• Mutations can be harmful, beneficial, or neutral
top related