IB Biology: DNA Structure

IB Biology: DNA Structure DNA Structure Assumed Knowledge ● ● ● Hydrogen bonds between the bases hold the two polynucleotide strands that make up the DNA double helix together. Two polynucleotide chains that make up a DNA double helix are antiparallel of each other. There are 23 pairs of chromosomes in every cell in the body with the exception of gamete cells (sperm and egg cell). Complementary nucleotides are paired Adenine Thymine 2 hydrogen bonds Guanine Cytosine 3 hydrogen bonds DNA Structure ● The DNA strand consists of a sugar-phosphate backbone and Complementary base pairings (nucleotides) The structure of a single nucleotide consists of one… ● ● ● Phosphate Group Deoxyribose sugar Nitrogen Base (Adenine, Thymine, Guanine, Cytosine) ● The phosphodiester bond (bond joining two nucleotides) must start from a 3 carbon to attach to a 5 carbon of another nucleotide. It is because of this that the DNA polynucleotide is put together through a series of 3-5 linkages (3 prime to 5 prime linkages). The DNA strand is described as having antiparallel strands as the two polynucleotide strands are going in opposite directions. The DNA strand terminates at either the 3 prime or 5 prime linkages. The bond linking one nucleotide to another in a polynucleotide chain is called a phosphodiester bond (covalent). ● ● ● Nitrogen Bases ● ● ● ● Adenine and Guanine are purine bases. Thymine and Cytosine are pyrimidine bases. Pyrimidine contains 1 ring. Purine contains 2 rings. THEREFORE… Only a purine paired with a pyrimidine can fit the space between the polynucleotide backbone. Nucleosome Structure ● ● A nucleosome is a group of 8 histone proteins grouped together to form an octomer. The chemical complex of DNA and protein (histone) is known as chromatin. Process of Supercoiling DNA and the function of Histone Proteins ● ● DNA must become densely packed during nuclear division for the chromosomes to move and divide. During interphase DNA needs to extend to replicate and allow for the transcription of gene. The function of histones are… ● ● To supercoil DNA To regulate gene expression. (DNA not exposed to enzyme helicase and DNA polymerase. Supercoiling DNA allows DNA strands to be more compact and fit into the nucleus. Supercoiling also controls transcription as unraveled DNA is exposed to helicase. Gene Types Type of Gene Known as… Function Structural Genes Regulatory proteins Codes for functional proteins. E.g. enzymes, hormones, antibodies. Single copy genes Highly repetitive sequences Controls transcription of other genes. Introns and Exons The entire sequence of nucleotides in a cell of an organism is known as a genome. ● ● ● An exon is a segment of a gene that contains instructions for making part of a polypeptide. Exons are often separated by intervening strand of DNA known as introns. Introns are strands of DNA which do not code for proteins.