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Cell Unit III: Cell Division, Cell Cycle, Transcription and Translation. Chapters 12, 13, 16, 17. Limits to Cell Growth. The larger a cell becomes, the more demands a cell places on its DNA If extra copies of DNA are not made, an “information crisis” would occur
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Chapters 12, 13, 16, 17
p53 is a protein that functions to block the cell cycle if the DNA is damaged. If the damage is severe, this protein can cause apoptosis (cell death).
Meiosis - cell division that reduces the diploid # to the haploid # in the formation of sex cells (gametes).
Example (Humans) - 46 chromosomes is reduced to 23.
MOST IMPORTANT - the cells produced at the end of meiosis contain one chromosome of each homologous (matching) pair.
GENE - HEREDITARY INFORMATION, IN A SECTION OF A DNA MOLECULE ON A CHROMOSOME.
LOCUS (LOCI) - A GENE’S SPECIFIC LOCATION ON A CHROMOSOME.
CLONE - A GROUP OF GENETICALLY IDENTICAL INDIVIDUALS ( WHAT MITOSIS PRODUCES)
ASEXUAL REPRODUCTION - REPRODUCTION W/O SEX (NO MALE/FEMALE; 1 PARENT; OFFSPRING IS A CLONE OF PARENT.
HOMOLOGOUS CHROMOSOMES - A MATCHING PAIR ALWAYS ONE FROM EACH PARENT.(one paternal/ one maternal.)
AUTOSOMES - CHROMOSOMES NOT DIRECTLY INVOLVED IN DETERMINING SEX. (IN HUMANS: 22 HOMOLOGOUS PAIR).
SEX CHROMOSOMES - THE CHROMOSOMES DIRECTLY INVOLVED IN DETERMINING SEX (IN HUMANS THE LAST HOMOLOGOUS PAIR).
(a) CALLED (X) & (Y) CHROMOSOMES.
(b) XX = FEMALE & XY = MALE.
(c) In other organisms:
(1) Insects (Grasshoppers, Roaches): X-O sex chromosomes. O represents no sex chromosome = Male
(2) Birds, Butterflies and some fish: Z-W sex chromosomes. Female gamete determines sex. Males are ZZ, Females are ZW
(3) Parthenogenesis – wasps, bees and ants. If the egg is fertilized it becomes a female and is diploid. If the egg is unfertilized it is male and haploid.
FERTILIZATION (or SYNGAMY) - UNION OF GAMETES.
KARYOTYPE: DISPLAY OF AN INDIVIDUAL’S CHROMOSOMES. CHROMOSOMES ARE COLLECTED DURING METAPHASE. THIS IS DONE BY NUMBER, SIZE & TYPE CHROMOSOME.
(characteristic of most animals)
Gametes are the only haploid cells.
The diploid zygote divides by mitosis producing a diploid organism.
(a) Each chromosome replicates. (This shows 1 homologous pair). Remember - sister chromatids & centromere.
(b) Meiosis I segregates the homologous pair into 2 different cells (each new daughter cell is in HAPLOID).
(c ) Meiosis II separates sister chromatids into chromosomes. No chromosome duplication)
Synapsis - ( in prophase I ) - the duplicated chromosomes pair with their Homologues). This is a PROCESS.Homologous chromosomes made of two sister chromatids come together as pairs.
Homologue - one of a homologous pair.
Tetrad - the four closely associated chromatids of a homologous pair together. This happens during synapsis.
Crossing over - (a process) reciprocal exchange of genetic material between nonsisterchromatids.
MEIOSIS - Prophase I with -(a) Tetrad & synapsis making a synaptonemal complex (b) Crossing over with the chiasma.
MITOSIS- No tetrads, synapsis, or crossing over.
DAUGHTER CELL DIFFERENCE - Mitosis has produced 2 identical cells. Meiosis produced daughter cells with one of each homologous pair.
FIG. 13.8- This shows the mostimportant concept of meiosis (how it produces genetic variation in organisms).
INDEPENDENT ASSORTMENT: At the end of meiosis chromosome pairs distribute themselves independently of one another. This causes 4 different combinations of chromosomes with 2 homologous pair.
1st MEIOTIC DIVISION RESULTS IN INDEPENDENT ASSORTMENT OF MATERNAL & PATERNAL CHROMOSOMES IN DAUGHTER CELLS.
FORMULA: The number of combinations possible when chromosomes assort independently into gametes during meiosis is 2n, where (n) is the haploid # in the organism.
EXAMPLE - Human haploid (n) is 23. 223 is over 8 million. A male can produce 8 million genetically different combinations of sperm & a female 8 million combinations of eggs.
RANDOM FERTILIZATION then would produce 8 million x 8 million(over 64 Trillion) possibly different genetic combinations in the offspring.
Crossing Over -produces individual chromosomes that combine genes inherited from our two parents.
Independent Assortment, Random Fertilization, & Crossing Over - result ways that genetic variation can be produced.
Prophase I & Anaphase I produce the most variation in the 4 new daughter cells.
If clones were genetically different, this would be due to mutation (change in the code of DNA).
Remember these !!!!
Which might be a daughter cell of meiosis I ?
Which might be a daughter cell of meiosis II?
DNA - most celebrated molecule of all time. It is made of nucleic acids that have the unique ability to direct their own replication.
PROBLEM: Since a chromosome is made of protein & DNA which one is carrying the genetic material?
There can be an infinite # of proteins so it would be a prime candidate to carry genetic material.
Watson & Crick working on the DNA structure model. (April 1953)
The captions under the picture is all that is needed to explain this experiment.
TRANSFORMATION - the change in genotype & phenotype due to the assimilation of external DNA by a cell.
Virus is made of a protein coat & DNA core. Virus injects DNA into a Bacteriophage. DNA coat has radioactive protein coat (S35) while DNA is radiated with (P32).
That the bacteria are called T2 Phages.
Erwin Chargaff - Said that the bases of DNA (A, T, C, G) vary from one species to another.
He also found a regular ratio of bases. (A approximately = T; and G approx. = C). This was known as Chargaff’s Rules.
NOTE: All these discoveries were before Watson & Crick discovered the double helix structure of DNA.
DNA is composed of nucleotides ( 5 carbon sugar, phosphate & a nitrogenous base (A,T,C,G). Phosphate of one nucleotide is attached to the sugar of the next nucleotide.
Adenine (A) is always paired with Thymine (T) & Guanine (G) is always paired with Cytosine (C).
The nitrogenous bases are held together with Hydrogen bonds (weak).
We even know the distances between steps of the DNA rungs. What’s a nm?
This entire structure was worked out by Watson & Crick in 1953 with help from Rosalind Franklin’s x- ray diffraction photo of DNA
A & G are double ring compounds called Purines.
T & C are single ring compounds called Pyrimidines.
Each rung of DNA is made of a Purine attached to a Pyrimidine. Held together by H bonds.
Elongation of DNA at a replication fork is catalyzed by a enzyme called DNA polymerase.
Rate of elongation in humans is approx.50/sec.
A similar molecule to ATP (NTP) is used to link the new nucleotide to the proper position.
The enzyme that catalyzes the reaction is DNA POLYMERASE.
Know: Where the 5’ & 3’ end are.
PROBLEM: DNA polymerase can ONLY add nucleotides to the free 3’ end of a growing DNA strand.
So..A new DNA strand can only elongate in the 5’ to 3’ direction.
SYNTHESIS OF LEADING & LAGGING STRANDS DURING DNA REPLICATION.
Okazaki fragments are about 100 -200 nucleotides long in eukaryotes.
DNA polymerase is adding new DNA fragments in a 5’ to 3’ direction continuously along a replication fork, adding to the 3’ end.
Lagging strand is synthesized in segments called Okazaki fragments. DNA ligase joins the fragments into a single DNA strand.
DNA polymerase cannot initiate a polynucleotide strand; it can only add to the 3’ end of an already-started strand.
The primer is a short segment of RNA synthesized by the enzyme primase. Each primer is eventually replaced by DNA.
DNA must also be able to form complementary base pairs with both DNA & RNA nucleotides. The sequence of nucleotides will be decoded into a sequence to make amino acids into proteins.
Replication -> Transcription -> Translation
Enzymes must proofread DNA during its Replication and repair damage in existing DNA.
Mismatch Repair fixes mistakes made in DNA. DNA polymerase itself carries out the mismatch repair.
Telomeres- special sequences of DNA nucleotides found at the end of the DNA molecule. They do not contain genes. They protect the organism’s genes from being eroded through successive rounds of DNA replication.
Secret to aging? http://www.youtube.com/watch?v=J9QApCHsrJk&feature=related
Transcription - the synthesis of mRNA (messenger RNA) under the direction of DNA. This is a code to make a polypeptide (protein). This is also the synthesis of any RNA from DNA.
Translation - the actual synthesis of a polypeptide (which occurs at the ribosomes.)
Gene to RNA to Protein.
The difference in Eukaryotic & Prokaryotic cells.
1. There is a total of 20 amino acids possible in any protein.
2. 3 Nucleotides on mRNA code for an amino acid. This is called the triplet code.
3. Only one strand of DNA is transcribed into mRNA. This strand is called the TEMPLATE strand. The other strand is called the complementary strand.
4. All Translation & Transcription occur in a 3’ to 5’ direction.
5. The mRNA is in triplet bases called CODONS.
mRNA is only a single helix & that Uracil (U) is a substitute for Thymine (T).
The number of nucleotides making up a genetic message must be 3 times the number of amino acids making up the protein.
EXAMPLE- 4 amino acids = 12 nucleotides.
Amino Acids are connected by polypeptide bonds.
AUG codon is a start codon & the amino acid Methionine (Met).
Start Codon begins the sentence & UAA,UAG & UGA = no amino acid but stops the amino acid chain (read in a 5’ to 3’ direction)= STOP CODON, like the period at the end of a sentence
1. RNA binds to the promoter region of DNA (several dozen nucleotides “upstream” from the transcription startpoint).
2. RNA moves “downstream” from promoter, unwinding DNA & elongating RNA at the 3” end (5’ to 3’ direction).
3. RNA polymerase transcribes a terminator (this sequence of nucleotides along DNA signals the end of transcription unit.)
Progresses at about 60 nucleotides/sec in Eukaryotes.
4. Eventually RNA is released & the polymerase moves from DNA.
5. Prokaryotes - RNA transcript immediately used to make protein.
6. Eukaryotes - mRNA will undergo additional processing.
Enzymes modify 2 ends of a eukaryote pre-mRNA molecule.
Cap made of modified guanosine triphosphate added to the 5’ end of RNA.
A Poly(A) tail consiting of 200 adenine nucleotides attached to 3’ end. ( may helps export mRNA from the nucleus.)
***Role of Cap and Tail - protect RNA from degradation****
The leader, trailer & termination signal.
Leader & trailer are not translated.
Pre-mRNA - Exons (Expressed sequence) are keep & the Introns(Intervening sequence) are removed (both by enzymes).
Exonsare then spliced together. We now have the processed RNA ready to leave the nucleus & go to the ribosome for translation.
1) tRNApicks up amino acids & transport them to the ribosome
2) Each tRNA has an anticodon (3 letters) that pick up one of the twenty amino acids.
3) When the tRNA’s deliver their amino acid, they add them to a growing polypeptide chain. tRNA’s are now available to pick up another amino acid to repeat the process.
4) New Polypeptide chain added in the 5’ to 3’ direction.
Ribosomes are made of 2 subunits each made of many molecules or rRNA (ribosomal RNA) and proteins.
The Anatomy of a Ribosome:
The sites on the ribosome:
(1) P site - holds the tRNA attached to the growing polypeptide
(2) A site - holds the tRNA carrying the amino acid to be added to the polypeptide chain
(3) Discharged tRNA leaves via the E site.
Peptide bonding between amino acids maintains the shape of tRNA.
1. Small ribosomal subunit binds to molecule of mRNA.
2. Initiator tRNA with anticodon UAC base-pairs with the start codon, AUG carrying the amino acid Met.
3. A large ribosomal unit arrives & completes the initiation complex.
4. Initiator tRNA is in the P site. A site is available to tRNA carrying the next amino acid.
5. Proteins called initiation factors bring translation components together. GTP provides the energy for all this.
GTP & proteins called elongation factors needed to drive this process.
1. When ribosome reaches a termination codon on mRNA, the (A) site of ribosome accepts a protein called a release factor instead of tRNA.
2. Release factor hydrolyzes the bond between tRNA in the P site & the last amino acid of the chain. This frees the polypeptide from the ribosome.
3. The 2 ribosomal subunits dissociate
A. An mRNA molecule is generally translated together with several ribosomes in clusters called polyribosomes.
B. This enables a single mRNA to make many copies of a polypeptide simultaneously.
Proteins can be chemically modified by attachment of sugars, lipids, phosphate groups etc.
Example: Enzymes may remove leading amino acids from a chain. Sometimes several proteins will join together to allow them to function or one protein may split into several proteins.
Proteins formed here are only the primary structure & must develop a secondary, tertiary, or even Quaternary structure.
Bacteria (Prokaroytes) have no nucleus, so mRNA does not need to move through the membrane to the ribosome.
Streamlined operations here - Transcription & Translation can be occurring at the same time.
3. There is not RNA processing in bacteria. (All exons).
Point Mutations: Chemical changes in just one or a few base pairs in a single gene.
If a point mutation occurs in a gamete or cells giving rise to them, it could be transmitted to offspring & future generations.
TYPES OF MUTATIONS:
1. Base-pair substitution - replacement of one nucleotide & its partner in the complementary DNA strand with another pair of nucleotides.
Some substitutions are silent mutations since genetic code is redundant, there may be no change in the amino acid coded for.
EXAMPLE: CCG mutated to CCA would make mRNA GGC become GGU which is still glycine.
2. MissenseMutation - altered codon still codes for an amino acid & makes sense although not necessarily the RIGHT sense. (Make a protein, just not the correct one)
3. Nonsense mutation - Alterations that change an amino acid code to a stop codon. Almost always leads to a nonfunctional protein.
4. Insertions & deletions are additions or losses of one or more nucleotide pairs in a gene.
a. Note this can cause missense or nonsense. Where the amino acid is incorrect in a chain can be important or not.
Frameshiftmutation - alters “reading frame” of message (# of nucleotides inserted or deleted is not a multiple of 3.
a. ( the big cat) remove the h = tebigc at_.) This will make all amino acids downstream from this incorrect.
What can cause Mutations?:
Mutagens- Physical & chemical agents that cause mutations or increase the mutation rate.
Examples - X-rays, Radiation, UV light, chemicals (pesticides, radon), Viruses & Bacteria