name the genetic disorder occurs if the base sequence ctt changes to cat
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Answer:
1. DNA and RNA are polynucleotides, made up of long chains of nucleotides.
2. A nucleotide contains a pentose sugar, a phosphate group and a nitrogen-containing base. In RNA the sugar is ribose, and in DNA it is deoxyribose.
3. A DNA molecule consists of two polynucleotide chains, linked by hydrogen bonds between bases.
There are four bases – adenine always pairs with thymine, and cytosine with guanine. RNA, which
comes in several diff erent forms, has only one polynucleotide chain, although this may be twisted
back on itself, as in tRNA. In RNA, the base thymine is replaced by uracil.
4. DNA molecules replicate during interphase by semi-conservative replication. Th e hydrogen bonds
between the bases break, allowing free nucleotides to fall into position opposite their complementary
ones on each strand of the original DNA molecule. Adjacent nucleotides are then linked, through their phosphates and sugars, to form new strands. Two complete new molecules are thus formed from one old one, each new molecule containing one old strand and one new.
5. The sequence of nucleotide bases on a DNA molecule codes for the sequence of amino acids in a polypeptide. Each amino acid is coded for by three bases. A length of DNA coding for just one polypeptide is a gene.
6. A change in the nucleotide sequence of DNA is a mutation, producing a new allele of the gene.
7. The DNA sequences for the HbA (normal) and HbS (sickle cell) alleles of the gene for the β-globin
polypeptide diff er by only one base. Th e triplet CTT in HbA is replaced by CAT in HbS, changing
the amino acid glutamic acid to valine. This single diff erence in the polypeptide results in sickle cell
anaemia in individuals with two HbS alleles.
8. During protein synthesis, a complementary copy of the base sequence on a gene is made, by building a molecule of messenger RNA (mRNA) against one DNA strand. Th is stage is called transcription.
9. After transcription, the next stage is called translation. In this stage the mRNA moves to a ribosome in the cytoplasm. Transfer RNA (tRNA) molecules with complementary triplets of bases temporarily pair with base triplets on the mRNA, bringing appropriate amino acids. As two amino acids are held side by side, a peptide bond forms between them. The ribosome moves along the mRNA molecule, so that appropriate amino acids are gradually linked together, following the sequence laid down by the base sequence on the mRNA.