Download Unit 08 Notes - Pierce College

Pierce College
Putman/Biol 160
Unit 08 Notes: Chromosomes, Cell Cycle, Meiosis, Transcription & Translation
SEXUAL & ASEXUAL REPRODUCTION
1) Changing environment: Sexual reproduction results in offspring genetically different than either parent
a. Strategy: Offspring may have traits better suited for survival in changing environment.
b. Involves meiosis
2) Stable environment: Asexual reproduction results in offspring genetically identical to parents.
a. Strategy: Environment’s not changing, so why risk having offspring more poorly adapted?
b. Involves mitosis (or binary fission, in bacteria)
BINARY FISSION
=Prokaryote cell division
1) Process:
a. Circular chromosome divides
b. Copies of circular chromosome migrate to either end of cell as cell lengthens
c. Cell membrane divides cell into two, followed by cell wall
2) Daughter cells have single, circular chromosomes like parent
EUKARYOTIC CELL CYCLE
There are two stages in the life of a somatic cell: Interphase + Mitosis
Interphase
Approximately 90% of the cell cycle is spent in interphase. Under the microscope, interphase may be identified by
the cell having an apparent nucleus and nucleolus, but NO chromosomes. There is a high metabolic rate during
interphase, supporting polypeptide synthesis which can occur because the chromatin is still loose. Interphase is
divided into three phases:
1) G1 phase. The cell has just divided and it is actively growing, meaning it is synthesizing polypeptides. During
the G1 phase, the cell decides whether it will divide or not, as influenced by growth factors secreted from
tissues adjacent to the cell. If cell is triggered to divide, it will enter the S phase. If it is not receive the
appropriate growth factors, it can enter the G0 phase, where it neither grows nor divides, or if it receives other
growth factors, it may differentiate into a specific cell type. Undifferentiated cells lack a specific structure and
function, and can undergo mitosis; differentiated cells have a specific structure and function, and can no longer
undergo mitosis. Skin cells, muscle cells and nerve cells—all of these cells are differentiated cells. Also, the
paired centrioles within the centrosomes replicate so that there are now four centrioles.
2) S phase. DNA replicates. After a cell divides, it has single chromatid chromosomes consisting of a single DNA
molecule. During the S phase (S is for synthesis), DNA replicates, meaning that chromosomes now have double
chromatids, each chromatid being a duplicate DNA molecule, joined at the middle into one structure.
3) G2 phase. The cell grows more, synthesizing polypeptides, some of which are processed into the proteins
needed for cell division. Centrosomes divide. V/SA ratio triggers cell division at the end of G2.
Mitosis:
Mitosis, or cell division, consists of two stages: karyokinesis, or division of the nucleus, and cytokinesis, or division
of the cytoplasm. Karyokinesis occurs during prophase, metaphase and anaphase; cytokinesis occurs during
telophase. The function of mitosis is growth of the organism, the repair of tissues and asexual reproduction.
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Prophase. Chromatin supercoils into visible chromosomes made of two chromatids each—each DNA molecule has
supercoiled into a chromatid. The chromatids are held together at regions called centromeres by structures called
kinetochores. The mitotic apparatus forms as the centrioles migrate to the poles with spindle fibers between the
centrioles and aster fibers forming from the centrioles toward the edge of the cell. The nuclear envelope atrophies,
along with the nucleolus.
Metaphase. Chromosomes migrate to the equatorial plate. Mitotic apparatus is centered at the poles of the cell with
centrioles. Spindles attach to kinetochores at region of centromeres on chromosomes.
Anaphase. Cell elongates. Kinetochores release chromosomes, each two-chromatid chromosome becoming two,
one-chromatid chromosomes. Chromosomes begin to migrate toward poles. Cleavage furrow forms.
Telophase. Chromosomes have migrated to the poles; nuclear envelope reforms, nucleolus reforms. Cleavage furrow
completely divides cell into two, completing cytokinesis. Cytokinesis is the division of the cytoplasm; karyokinesis
is the division of the nucleus.
Control of Mitosis
1) Density-dependent inhibition & growth factors control cell division.
a. In density-dependent inhibition, cells grow until they contact other cells. This causes them to stop growing.
b. Growth factors secreted by other cells, cause cells to undergo mitosis or differentiate.
2) Mutations that cause a cell not to recognize density-dependent inhibition is one cause of cancer. The cell divides
into non-functional cells that crowd out healthy cells.
Products of Mitosis
Mitosis produces two cells with the same number of chromosomes as the parent cell—two identical daughter cells,
except smaller. In humans, each somatic (body) cell has 46 chromosomes, 23 pairs. At the beginning of mitosis,
these 46 chromosomes consist of doubled (replicated) DNA molecules, so there are 92 DNA molecules total. Each
DNA molecule has supercoiled into chromatids, so there are 92 chromatids, held together by kinetochores as 46
chromosomes. The daughter cells also contain 46 chromosomes, only these chromosomes consist of single
chromatids, single DNA molecules. Thus, each daughter cell has 46 DNA molecules.
All humans have 22 pairs of homologous autosome chromosomes and a 23rd pair of sex chromosomes that are
heterologous (XY) in the male, homologous (XX) in the female. (So, females actually have 23 pairs of homologous
chromosomes!)
GERM CELL CYCLE
Gametes are haploid (1n), meaning they have only one set of chromosomes. Somatic cells are diploid (2n), meaning
they have two sets of chromosomes. Meiosis reduces the ploidy of gametes (eggs & sperm) to 1n each so that when
they unite, the resulting offspring will be 2n.
Germ cell cycle consists of interphase + meiosis. Meiosis is divided into Meiosis I and Meiosis II.
Interphase: Same as interphase in mitosis.
Meiosis I: Homologous chromosomes pair up then separate
Prophase I: Is the most complex meiotic phase & is about 90% of meiosis.
1) Chromatin supercoils into chromosomes
2) Chromosomes undergo synapsis = homologs pair up into tetrads
a. Crossing over may occur; homologous portions of sister chromosomes exchange forming chiasma
1. This is genetic recombination and is a source of genetic variability as the end result is
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chromosomes with allele combinations different than if crossing over hadn’t occurred
Alleles are genes with slightly different DNA sequences that code for variations of that gene; for
instance, brown, black and pink would be alleles of the gene that codes for eye color in mice.
3) Nuclear envelope & nucleolus atrophy
4) Centrosomes migrate to poles as centrioles form mitotic apparatus
5) Spindle fibers attach to kinetochores
2.
Metaphase I: Tetrads line up on equatorial plate
1) Orientation of homologs is random leads to genetic variability
Anaphase I: Tetrads divide, chromosomes migrate toward poles; cell elongates
Telophase I: Tetrads are at poles; cytokinesis occurs
Meiosis II: Chromosomes line up, divide, 4 haploid cells form
Interphase? Occurs in some species, where chromosomes unravel back into chromatin & nucleolus and nuclear
envelope reform.
Prophase II: Centrosomes divide, migrate to poles; new mitotic apparatus forms, spindles attach to kinetochores
Metaphase II: Chromosomes line up on equatorial plate
Anaphase II: Chromosomes divide, migrate toward poles; cell elongates
Telophase II: Chromosomes are at poles; cytokinesis occurs
1) Product: 4 cells with one set of chromosomes each (haploid (1n))
FERTILIZATION (= syngamy)
Randomly brings together sperm & egg chromosomes
Considering number of possible chromosome combinations from metaphase I random orientation of homologs only:
(2n)(2n) = (8 x 106)(8 x 106) = 64 x 1012
CHROMOSOME PROBLEMS
1) Diagnosed using karyotypes
2) Chromosomes number problems
a. Caused by nondisjunctons during meiosis I or II
1. Tetrads or chromosomes don’t separate, resulting in too many or too few chromosomes
b. Most common in children of older women
c. Autosomal number problems usually result in spontaneous abortion
d. Too many sex chromosomes usually result in sterility, but otherwise healthy offspring
e. Eg: Trisomy 21 (Down syndrome)
1. Symptoms include round, flattened face, epicanthal folds, short, stubby digits, short stature, heart &
respiratory problems, mild to severe mental retardation.
2. Most common in children born to older mothers.
3) Chromosome structure problems
a. Breakage of chromosomes leads to various rearrangements.
1. In somatic cells, may lead to cancers
2. In gametes, leads to birth defects
NUCLEIC ACID STRUCTURE
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1) DNA is a double helix made of DNA-nucleotides
a. Phosphate + deoxyribose form outer backbone
b. Bases hydrogen bond together on the inside, purines w/ pyrimidines:
1. Adenine (purine) forms two hydrogen bonds with thymine (pyrimidine)
2. Guanine (purine) forms three hydrogen bonds with cytosine (pyrimidine)
a. Molecule is antiparallel, running 3’ to 5’ in one direction, 5’ to 3’ in other direction
1. “3’” indicates the third carbon atom of deoxyribose, which has a hydroxyl attached to it
2. “5’” indicates the fifth carbon atom of deoxyribose, which has a phosphate attached to it
2) RNA is a single helix made of RNA-nucleotides
a. Phosphate + ribose form outer backbone
b. RNA bases include the purines, adenine and guanine, and the pyrimidines, uracil and cytosine.
DNA REPLICATION
During the S phase of interphase, DNA replicates.
Overview:
1) DNA is separated at many origins of replication along its length, forming replication bubbles.
2) Replication occurs in both directions.
3) Two DNA replicates result, each with ½ of the original strand of DNA.
Steps
1) In the nucleus, Helicase separates DNA strands at origins of replication.
2) Primase adds RNA primer to DNA.
3) DNA polymerase III attaches complementary DNA nucleotide to primer and attaches complementary DNA
nucleotides to base attached to primer and to subsequent bases, forming hydrogen bonds between bases and
covalent bonds between phosphate and deoxyriboses. Complementary bases are: adenine with thymine (each
form two hydrogen bonds), guanine with cytosine (each form three hydrogen bonds
4) Helicase continues to open DNA molecule.
5) DNA polymerase III continues to add complementary DNA nucleotides to leading strand.
6) Primase adds RNA primer to lagging strand.
7) DNA polymerase III adds complementary DNA nucleotides to primer, then to subsequent bases attached to
lagging strand.
8) Helicase continues to open DNA molecule (in one direction only!).
9) DNA polymerase III continues to add complementary DNA nucleotides in both directions.
10) DNA polymerase I replaces primers with DNA nucleotides
11) Ligase covalently links fragments
12) Replication bubbles meet, 2 daughter DNA molecules result, each with ½ of original.
PROTEIN SYNTHESIS
Protein synthesis involves three processes: transcription, translation and polypeptide processing. Transcription
occurs in the nucleus, translation occurs on ribosomes in the cytoplasm or on the rough ER, and polypeptide
processing occurs in the rough ER.
Transcription
Transcription occurs in the nucleoplasm during interphase and is the process of RNA synthesis.
Steps:
1) RNA polymerase bonds to the DNA molecule at a region called the promotor and opens or “unzips” DNA
molecules at the relatively weak hydrogen bonds between the nitrogenous bases.
2) RNA polymerase then attaches complementary RNA nucleotides, one at a time, to only one side of the open
DNA strand—the sense strand. If an adenine is on the sense strand, it attracts the RNA nucleotide uracil. If
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3)
4)
5)
6)
thymine is on the sense strand, it attracts the RNA nucleotide adenine. If cytosine is on the sense strand, it
attracts the RNA nucleotide guanine, and if guanine is on the DNA sense strand, it attracts the RNA nucleotide
cytosine. The first three DNA nucleotides to be “read” are TAC.
As this process occurs, a single-strand of RNA is produced, AUG being the first three RNA nucleotides of the
RNA strand.
RNA polymerase ends transcription when it reaches a DNA sequence called the terminator region, sequence
AAUAAA. This triggers enzymes to cut the RNA molecule free.
Once this RNA molecule is produced, other enzymes clip out pieces of the RNA that are nonsense, called
introns, while joining the useable pieces of RNA together, called exons. The exons, joined together, make up
the RNA molecule. If the particular RNA in question carries information to make a polypeptide, it is termed a
messenger RNA (mRNA). Fully processed mRNA is exported through a nuclear pore into the cytoplasm.
Before mRNA is released, more processing is required. In the cytoplasm, there are nucleases—enzymes that
attack and degrade lost RNA molecules! So, on the leading end of the mRNA, a 5’ (five prime) cap is added,
and on the end of the mRNA molecule, a series of adenine nucleotides are added, called a “poly-A tail.” The 5’
cap helps ribosomes recognize the beginning of the mRNA strand; the poly-A tail gives the shark-like nucleases
something to attack and “feed” on while the mRNA is making its way to the ribosomes!
Translation
Translation occurs at ribosomes, either free in the cytoplasm or on rough ER.
Steps:
1) In translation, the mRNA goes into the cytoplasm where a small subunit of a ribosome recognizes the 5’ cap
and bonds to it. The small subunit holds the mRNA so that its unbonded bases are exposed. The first three
exposed base, which are always the same (AUG), attract a second kind of RNA called a tRNA (transfer RNA).
Transfer RNA has a complementary group of RNA bases on one end and a specific amino acid on the other end.
The AUG of the mRNA, also called a triplet code or a codon, is complementary to the triplet code of the tRNA,
called an anticodon, which would be UAC. The UAC (anticodon) hydrogen bonds to the AUG (codon).
2) A large subunit of a ribosome then attaches to the small subunit over the mRNA strand and around the tRNA.
The large subunit has three sites of attachment: E, P and A. Initial attachment occurs in the middle, P site.
3) A second tRNA with its specific amino acid (= peptide) enters the A site and hydrogen bonds its anticodon to
the second mRNA codon. If the second mRNA codon is CGA, then the matching tRNA anticodon would be
GCU.
4) The large subunit covalently bonds the two amino acids together forming a peptide bond.
5) The P site amino acid is released from its tRNA.
6) The ribosome moves down one codon so that the first tRNA is now in the E site, the second is in the P site with
a vacant A site.
7) The intial tRNA detaches and leaves, without its amino acid.
8) A third tRNA with specific amino acid and a complementary anticodon hydrogen bonds to the codon exposed
in the A site.
9) The large subunit covalently bonds the amino acid chain to the new amino acid.
10) The P site amino acid is released.
11) The ribosomes moves down one codon so that the A site is now vacant.
12) The tRNA in the E site detaches and leaves. A fourth tRNA with specific amino acid and a complementary
anticodon hydrogen bonds to the codon exposed in the A site…and so on, forming a chain of amino acids called
a polypeptide.
13) This process continues until the mRNA shows a triplet code called a stop codon in the A site. There are no
tRNA anticodons that correspond to stop codons; rather, a release factor protein enters the A site. Release factor
proteins add an H-OH to the terminal amino acid then releases the polypeptide chain.
There are 20 amino acids, each carried by a tRNA with a specific tRNA anticodon that matches a specific mRNA
codon. Since mRNA codons are transcribed from DNA, it is DNA that carries the genetic code determining the
amino acid sequence of polypeptides.
Transfer RNA molecules that have left their amino acids behind need to have new amino acids attached. An enzyme
called aminoacyl-t-RNAsynthetase performs this function.
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The Genetic Code
Codon dictionaries give the amino acids corresponding to the mRNA codons.
Mutations, changes in the DNA sequence, may or may not result in changes in the amino acid sequence of a
polypeptide.
Polypeptide Processing
Polypeptides are variously processed into final proteins. For instance, most have a signal sequence of 20 amino acids
that specifies where the polypeptide needs to go in the cell—this needs to be clipped off before the polypeptide is
useable. Enzymes are produced in their inactive forms and must be activated by clipping off a part of the molecule;
cofactors (vitamins or minerals) must usually be added to the active site for enzymes to work. Most proteins consist
of two or more polypeptides, so polypeptides must be combined before they are functioning proteins.
THE GENE
A section of DNA that codes for one polypeptide is called a gene. Genes exist at locations on chromosomes called
loci. Genes with slight differences in DNA sequence that allow it to code for variations of a trait are called alleles.
One gene codes for one polypeptide. Proteins are made from one or more polypeptides. Thus it usually takes more
than one gene to code for a protein.
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