CET - MATHEMATICS - 20

Culle004

Description

• exposé

CET – MATHEMATICS – 20 10 VERSION CODE: B – 2 EXPERT IS NOT HELD RESPONSIBLE IF ANY CHANGE IN THE ANSWER PROVIDED BY THE CET CELL 1. If A =    11 23 , then A2 + xA + yI = 0 for (x, y) = …………. a) (-4, 1) b) (-1, 3) c) (4, -1) d) (1, 3) Ans (1) Characteristic equation is 0142 =+λ−λ , using Cayley-Hamilton theorem 0IA4A2 =+− therefore x = -4 and y = 1 2.

• commutative law
• space vector
• vector with the positive direction of z-axis
• equation of the bisector of the angle pqr
• constant term
• area of triangle
• area of the triangle
• area of a triangle
• cos
• positive integer
• angle

Sujets

• Exposé
• Positive
• Vector
• Triangle
• Équation
• Direction

Informations

• Publié par: lukam
• Nombre de lectures: 25
• Langue: English
• Poids de l'ouvrage: 1 Mo

Extrait

GENE STRUCTURE


Previous lectures have detailed the chemistry of the DNA molecule, the genetic material,
as well as the mechanisms for replicating and maintaining the integrity of the DNA. We now
want to understand the functional aspects of DNA as the genetic material. How does this DNA
molecule function as a gene, what is a gene, and how can we consider mutation in the context of
the gene?

The Genetic Code
The DNA molecule is a linear array of nucleotides. In most cases, we think of the purpose of
the genetic information as storing the information as a code for a linear array of amino acids that
constitute a protein. It is also true, that some genes encode RNA molecules that function as
RNAs and do not encode proteins. Examples include the ribosomal RNAs that form the
ribosome structure as well as playing a catalytic role in protein synthesis, the transfer RNAs
(tRNAs) that carry specific amino acids to the translation machinery, and small nuclear RNAs
(snRNAs) that play a critical role in the processing of mRNA precursors by splicing.
Most genes, however, function to encode proteins. Twenty amino acids are utilized in the
synthesis of proteins. Therefore, since there are only four possible nucleotides, a single
nucleotide cannot code for an amino acid. Pairs of two would also be insufficient. However, a
group of three nucleotides would give the potential for 64 specific codons (4 x 4 x 4), thus
sufficient to code for all amino acids. This is in fact the code - a triplet code such that every
three nucleotides encodes one amino acid.

Properties of the genetic code:
- Read in groups of three
- Unambiguous (a triplet codon specifies a unique amino acid)
- Degenerate (more than one codon specifies an amino acid)
- Stop codons







99 The genetic code is universal (same code used in all organisms, both prokaryotic and
eukaryotic) with one exception - there are a few differences in the code used in mitochondria.

trp? UGA is not a stop signal but codes for trypophan. The mit tRNA recognizes both UGG and
UGA, obeying traditional wobble rules.

? Internal methionine is encoded by both AUG and AUA; initiating methionines are specified
by AUG, AUA, AUU, and AUC.

? AGA and AGG are not arginine codons but are stop codons. Thus, there are four stop codons
(UAA, UAG, AGA, and AGG) in the mitochondrial code.


A mutation is any heritable change in the genetic material, resulting in an alteration of the DNA
sequence. Mutations are usually considered in the context of a change that alters gene function
and thus the phenotype of the organism. Understanding the molecular mechanisms responsible
for mutations, either simple changes in DNA sequence or more drastic deletions, insertions, or
rearrangements of DNA material, as well as the mechanisms responsible for recognizing and

100 correcting these alterations, is of central importance to the understanding of disease
mechanisms.

The Nature of Mutations –Mutations, which can alter the coding properties of a DNA segment,
are of several types:

A. Substitution mutations convert one type of base pair into another. G-C to A-T and A-T to
G-C changes are referred to as transition mutations (replacement of a purine to pyrimidine
base pair by a purine to pyrimidebase pair). G-C to C-G, G-C to T-A, A-T to T-A, and A-T to
C-G are called transversions (replacement of a purine-pyrimidine base pair by a pyrimidine-
purine base pair). Although transitions are more common than transversions, both kinds of
mutations occur as a consequence of replication errors, both can result from chemical
damage to DNA, and both have been implicated as causative factors in inherited genetic
disease and cancer. Single nucleotide changes can change the codon to that of another amino
acid, thus altering the protein. In addition, such changes can also create a stop codon.

B. Small insertions/deletions comprise a second relatively common class of mutation.
Genetic changes of this sort involve insertion or loss of a small number of contiguous base
pairs (one to several hundred). Repetitive runs of a mono, di-, or trinucleotide sequence are
extremely prone to insertion/deletion mutation, an effect that has been attributed to slippage
of template and primer strands during replication:

101
Repeat elements like (CA) shown above or the (A) element, which contains a run of n n
adenine residues on one strand paired with a run of thymine bases on the other, are very
common in human chromosomes. For example, about 50,000 (CA) repeats are distributed n
throughout the human genome, with each repeat element typically containing 10 to 60
copies of the (CA) dinucleotide (eg., n = 10 to 60). Due to their propensity to slip during
DNA biosynthesis, repetitive sequences are particularly prone to mutation. As described
below, a high incidence of mutations in (CA) microsatellite sequences is a valuable n
diagnostic for certain human malignancies.
Deletions or insertions will result in a frameshift if it is not a multiple of three base pairs.

DNA Sequence Protein Sequence Type of Mutation
ATG AAA TTT TGT CGT AAA MET LYS PHE CYS ARG LYS Wild type
ATG AAc TTT TGT CGT AAA MET asn PHE CYS ARG LYS Missense
ATG AAC TTT TGa CGT AAA MET ASN PHE stop Nonsense
ATG AAA T TGT CGT AAA MET LYS leu ser stop Deletion/Frameshift


Definition of a Gene
Traditionally, a gene has been defined as either the unit of heredity or defined as that
portion of a chromosome encoding a functional RNA or protein. Although these two views
generally coincide in the case of prokaryotic genes, the situation is much more complex in the
case of a eukaryotic gene. A prokaryotic gene is relatively simple in structure, including the
coding sequence to specify the synthesis of a protein and a minimal amount of regulatory
sequence to control the expression of the gene. In contrast, a eukaryotic gene can be vastly more
complex and can occupy large regions of chromosomes. This is due to the fact that most
eukaryotic genes, particularly those in mammalian cells, are discontinuous. That is, coding
regions are often separated by non-coding sequence. More importantly, the regulatory sequences
that are responsible for the expression of the gene can be complex and separated by large
distances from the actual gene sequence. Since a gene must ultimately be defined in a phenotypic
sense, then the expression of the gene is critical - phenotype is determined not only by the
sequence of a particular protein but also by the ability of a given cell to express that protein. In
consideration of all of these issues, the definition of a functional gene would be those DNA
sequences necessary to achieve the normal expression of the gene product.

102 Obviously, the ability to precisely define a gene is critical for an understanding of the
basis of gene function, including the nature of sequences important for the normal, regulated
expression of the gene. In addition, a knowledge of the gene structure and sequence is critical
for evaluating and understanding the molecular basis for gene mutation that underlies a disease
state.
In addition, we will see later that a knowledge of the characteristics of a gene, including
those sequences that define open reading frames, splice site signals that define exon/intron
junctions, and the sequences that constitute transcription regulatory signals, is critical in the
search for an unknown gene

Isolation and Study of a Eukaryotic Gene:
How does one go about studying the structure and characteristics of a particular gene.
9
Since there are approximately 3 x 10 base pairs in the human genome, and any given gene may be
4no more than 10 base pairs, analysis of the total population of human DNA is impossible.
Clearly, a gene must be isolated apart from the total DNA and amplified to allow a detailed study.
Early studies of gene structure and function in eukaryotic cells made use of animal viruses,
particularly the so-called DNA tumor viruses including adenovirus and polyomaviruses, as a
mechanism to isolate and study individual genes. Viruses simply represent a relatively small set
of genes packaged in a protein coat. The ability to isolate and purify viruses thus provided a
mechanism to isolate pure populations of specific genes. Since these viruses make use of
cellular activities for the expression of the viral genes, the basic aspects of viral gene structure
and function generally reflect that found for cellular genes. Thus, the initial studies of these
viruses laid much of the groundwork for subsequent analysis of cellular genes.
With the advent of molecular cloning through recombinant DNA procedures, it then
became feasible to isolate individual eukaryotic genes. Cloning allows both the purification
(isolation) of the gene, away from all others, as well as the amplification of the gene to provide
sufficient material to carry out biochemical analyses.
We have already discussed the procedures involved in the generation of a library of
clones and the methods for detection of a particular clone by hybridization with