12th Class Biology Chapter 6 Chromosomes and DNA Short Questions Answer

The German embryologist Walther Fleming first observed them in 1882.When he was examining the rapidly dividing cells of salamander larvae.

The protion of the chromosome except heterochromatin is called euchromatin.It is condensed only during cell division.when compact packaging facilitates the movement of the chromosome.At all other time euchromatin is present in an open configuration and its genes can be expressed.

Transformation is the transfer of genetic material from one cell to another and can alter the gentic make up of the recipient cell.Recipient cell will express some the characteristics of the donar cell.

Miescher extracted a white substance from the nuclei of human cells and fish sperm.He called substance Nuclein because it seemed to be specifically associated with the nucleus.

In this form of DNA replication the daughter DNAs have one of the original strands and the other strand is newly synthesized from complementary nucleotides.

DNA polymerase can initiate synthesis of DNA only if another enzyme primase constructs an RNA primer.DNA polymerase III recognizes the primer and adds DNA nucleotides to it to construct the DNA strands.The RNA nucleotides in the primers are then replaced by DNA nucleotides.

The lagging strand which elongates away from the replication fork is synthesized discontinuously as a series of short segments that are later connected.These segments are called Okazaki fragments.

They are about 100-200 nucleotide3s long in eukaryotes and 1000-2000 nucleotides long in prokaryotes.

Many enzymes contain multiple protein or polypeptide subunits,each encoded by a separate gene this relationship is referred to as one gene/one polypeptide.

The first step of central dogma is the transfer of information from DNA to RNA, which occurs when an mRNA copy of gene is produced.The process is called transcription.

Out of 64 codons UAA,UAG and UAG do not code for any amino acid and hence are known as nonsense codon.These codons are usually present at the end of the gene and hence are also called stop codons.

Every gene starts with initiation codon AUG which encodes the amino acid methionine.

If the DNA in all of the cells of an adult human were lined up end to end it would stretch nearly 100 billion kilometer-60 times the distance from Earth to Jupiter.

The primer is a sequence of about 10 RNA nucleotides complementary to the parent DNA template.It is synthesized by enzyme primase.

mRNA,tRNA and rRNA.

Transcription is initiated when the enzyme RNA polymerase binds to a particular binding site called a promoter located at the beginning of the gene.

The genetic code for specifying amino acids does consist of 3-bases.This is called the triplet code.

A sequence of three nucleotides in tRNA that is complementary to mRNA is called anticodon.

Chromosomes are thread like structures that appear inside the nucleus at the time of cell division.They were first observed by Walther Fleming in 1882, when he was examining the repidly dividing cells of salamander larvae.

Penicillin a fungus has only one pair of chromosomes while some ferns have more than 500 pairs.

Typically a chromosome is made of chromatids centromere and secondary constriction.

The particular array of chromosomes that an individual possesses is called its Karyotype.They vary in size staining properties the location of centromere the relative length of the two arms neither side of centromere and the position of constricted regions along the arms.

The Chromosomes are called.Telocentric acrocentric sub metacentric and metacentric depending upon the location of centromere between the middle and tip of the chromosomes.

The usual shapes of chromosomes are i,j and v.

Chromosomes are comosomes of DNA and protein.Most are about 40% DNA and 60% protein.

A typical human chromosomes contains about 140 million nucleotides in its DNA.

The amount of information one chromosome contains would fill about 280 printed books of 1000 pages each if each nucleotide corresponds to a word and each page had about 500 words on it.

If the strand of DNA from a single chromosome were laid out in a straight line it would be about 5 centimetre long.

Every 200 nucleotides the DNA duplex is coiled around a core of eight histone proteins forming a complex known as a nucleosomes.

Histones are positively charged due to an abundance of the basic aminon acids arginine and lysine.They are thus strongly attracted to the negatively charged phosphate groups of the DNA.

The histone cores act as magnetic forms that promote and guide the coiling of the DNA.Further coiling occurs when the string of nucleosomes wraps up into higher order coils called supercoils.

Highly condensed portions of the chromatin are called heterochromatin.Their DNA is never expressed.

According to this theory the genes are physical units located on the chromosomes.It means that one member of gene pair is located on one homologous chromosome and the other member of a gene pair is located on the other homologous chromosome.

A trait determined by a gene on the X chromosome is said to be sex linken.

In 1944 in a classic series of experiments Oswald Avery along with colin Macleod and Maclyn McCarty repeated Griffith’s experiments and characterized what they referred to as the Transforming principle.

In 1952 by Alfred Hershey and Martha Chase experimented with bectriophages T₂ and provided additional evidence supporting Avery’s conclusion.

1) Phosphate (PO₄) groups.
2) Five carbon sugars and
3) Nitrogen containing bases called purines and pyrimidines.

In a typical nucleotide nitrogen base is attached to carbon number 1 of a pentose suger and phosphate group is attached to carbon number 5 of the suger.In addition a free hydroxyl (-OH) group is attached to the 3 carbon atom.

In a polynucleotide chain the linkage between two groups is called a phosphodiester bond because the phosphate group is now linked to the two sugars by means of a pair of ester bonds.

Erwin Chargaff showed that the amount of adenine in DNA always equals the amount of thymine and the amount of guanine always equals the amount of cytosine.

In this analysis a molecule is bombardede with a beam of X-rays.When individual rays encounter atoms their path is bent or diffracted and the diffraction pattern is recorded on the photographic film.When carefully analyzed this pattern gives three dimensional structure of a molecule.

Rosalind Franklin prepared this X-ray diffraction pattern fo DNA in the laboratory of Birtish Biochemiest Maurice Wilkins who propared DNA fibres.

The diffraction pattern prepared suggested that the DNA molecule had a shape of a helix with a diameter of 2 nm and a complete helical turn every 3.4 nm.

In 1953 james Watson and Francis Crick proposed structure of the DNA molecule.

The process by which DNA of a living organism gives rise to a copy of itself is called DNA replication.

In semi conservative replication the two strands of the duplex separate out each acting as a model or mold along which new nucleotides are arranged thus giving rise to two new duplexes.

The conservative model stated that the parental double helix would remain intact and henerate DNA copies consisting of entirely new molecules.

The dispersive model predicted that parental DNA would become dispersed throughout the new copy so that each strand of all the daughter molecules would be amixture of old and new DNA.

The three hypothesis of DNA replication were evaluated by Mathew Meselson and Franklin Stahl of the California Institute of Technology in 1958.

DNA polymerase I is a relatively small enzyme that plays a supporting role in DNA replication.

The chromosomes were first seen by Walther Flemming in 1882 in the dividing cells of salamander larvae.

The number of chromosomes in 6, 32, 20, 26, 40 and 46, respectively.

A chromosome is typically made of chromatids, centromere and a secondary constriction.

The chromosomes differ in size staining properties, the location of centromere; the relative length of the two arms and the position of the constricted regions along the arms.

A chromosome having the centromere located at its one end is called telocentric chromosome.

The centromere of the acrocentric chromosome lies very near to one end with the result that one arm of the chromosome is very small or even imperceptible. In sub-metacentric chromosome the centromere lies at some distance from one end so that the two arms of the chromosome are unequal.

In metacentric chromosome the centromere lies at or near the center and both the arms of the chromosome are equal or almost equal.

The total chromosome complement of a cell is called karyotype.

These are the histones.

The chromosomes are chemically composed of 40% DNA and 60% protein.

It is about 140 million.

After every 200 nucleotides, the DNA duplex (double strand) is coiled around a core of eight histone proteins forming a complex known as a nucleosome.

It is so because histones have an abundance of the basic amino acids arginine and lysine.

The portions of the chromatin which are highly condensed are called heterochromatin while the rest of the chromatin which is condensed only during cell division is called euchromatin.

Karl Correns, a German geneticist first suggested central role of chromosomes in heredity in 1900.

Walter Suttan proposed the chromosomal theory of inheritance in 1902.

The normal eye colour of Drosophila is red while the mutant one is white.

The gene for eye colour in Drosophila is located only on the X-chromosome and not on the Y-chromosome.

A trait, the gene for which is situated on the sex chromosome (X- chromosome in Drosophila) is called, a sex-linked trait.

The first evidence that DNA is the hereditary material was provided by a British microbiologist, Frederick Griffith.

The bacteria with polysaccharide coat are virulent which are commonly referred to as the s-type (smooth type).

Transformation is the transfer of genetic material from one bacterium to another which can alter the genetic makeup of the recipient bacterium.

Friedrick Miescher, a biochemist isolated a material in 1869 from the nuclei of human cells and fish sperms that he called nuclei which was later named as nucleic acid.

These are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

These units are known as the nucleotides.

The nitrogenous bases of the nucleotides of DNA are the adenine, guanine, thymine and cytosine.

The components of a nucleotide of DNA are the deoxyribose sugar, a phosphate group and a nitrogenous base, according to P.A. Levene.

James Watson and Francis Crick presented the model of the structure of DNA in 1953.

An anticodon is a group of three nucleotides (nucleotide triplet) on transfer RNA which is complimentary to the codon of messenger RNA.

It is the sequence of bases along the DNA molecule that serves as code for the amino acid.