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FOCUS BBC MAGAZINE Collection THE ULTIMATE GUIDE TO YOUR GENES Understand what DNA is







DNA The composition Deoxyribonucleic acid of (English DeoxyriboNucleic DNA Acid, abbreviated as DNA) DNA and protein, and is a biological macromolecule DNA is a long polymer composed of Each nucleotide consists of a side chain is a kind of nucleic acid, one of the four essential for the development and normal backbone that can be covalently bonded biological macromolecules contained operation of organisms. repeating nucleotide units, with a chain to adjacent nucleotides and a nitrogen-co in biological cells.DNA carries the width of 2.2 to 2.6 nanometers, and a ntaining base, and the nitrogen-containing genetic information necessary for the length of each nucleotide monomer is bases on the two chains are connected synthesis of RNA 0.33 nanometers. Although each mono by hydrogen bonding through base compl mer occupies a relatively small space, ementation. Sugars and nitrogenous bas the length of a DNA polymer can be very es form nucleosides, and nucleosides long because each strand can have mill combine with one or more phosphate gro ions of nucleotides. For example, the lar ups to form nucleotides. gest human chromosome (chromosome The DNA backbone structure is formed by 1) contains nearly 250 million base the interaction of phosphoric acid and pairs. carbohydrate groups. The carbohydrate DNA in organisms almost never exists as molecules that make up deoxyribonucleic a single strand, but as a pair of closely acid are cyclic 2-deoxyribose, which is a related double strands, intertwined with kind of five-carbon sugar. each other to form a structure called a double helix. 05 06

DNA is a long polymer composed of five-carbon sugar. The two oxygen atoms on repeating nucleotide units, with a chain the phosphate group are respectively conn width of 2.2 to 2.6 nanometers, and a l ected to the carbon atoms 3 and 5 of the fi ength of each nucleotide monomer is 0 ve-carbon sugar to form a phosphodiester b .33 nanometers. Although each monom ond. This asymmetric covalent bond positio er occupies a relatively small space, the n on both sides makes each long DNA chain length of a DNA polymer can be very lo have directionality. The two strands of nucle ng because each strand can have millio otides in the double helix are arranged in op ns of nucleotides. posite directions to each other. For example, the largest human chromo This arrangement is called antiparallel. One some (chromosome 1) contains nearly side of the asymmetric ends of the DNA cha 250 million base pairs.DNA in organism Each nucleotide consists of a side in is called the 5'end,and the other side is c s almost never exists as a single strand, chain backbone that can be coval alled the 3'end.One of the mostimportant d but as a pair of closely related double st ently bonded to adjacent nucleoti ifferences betweendeoxyribonucleic acid an rands, intertwined with each other to fo des and a nitrogen-containing bas d RNAis thedifference in the composition of e, and the nitrogen-containing bas sugar molecules. DNA is 2-deoxyribose and es on the two chains are connected by hydrogen bondi RNA is ribose. ng through base complementation. Sugars and nitrogenous bases form nucleosides, and n A genetic chromosome ucleosides combine with one or more phosphate group is along polymer s to form nucleotides.The double helix of DNA is stabiliz ed by hydrogen bonds established between the nitroge composed of repeating no us bases present on the two strands. The four base nucleotide units. s that make up DNA are adenine (A), cytosine (C), guan ine (G), and thymine (T). 08 All four bases have a heterocyclic structure, but structu rally, adenine and guanine are derivatives of purine an d are called purine bases, while cytosine and thymine a re related to pyrimidines and are called pyrimidine bas es.The DNA backbone structure is formed by the intera ction of phosphoric acid and carbohydrate groups. The carbohydrate molecules that make up deoxyribonuclei c acid are cyclic 2-deoxyribose, which is a kind of 07

SHOULD A BABY HAVE ITS The two nucleotide chains are entwined GENOMES SEQUENCED in opposite directions along the central axis, much like a spiral staircase. The h Olly and Lily are expecting their andrails on both sides are the skeletons first child. The baby, currently kn of the sugar-phosphorus genes of the tw own as ‘Bump’, is scheduled to b o polynucleotide chains alternately com e born by caesarean section on t bined, and the pedal is the base. base. he 1st of March. Angie, Lily’s mid Angie explains that along The DAN double helix is a right-handed wife, is running through the pape with beingweghed, meas helix. The grooves between the differen rwork in preparation for the birth ured and having a heel p t phosphate groups are still exposed. Th and asks the couple if they would rictest, a simple swab tak e main groove is 2.2 nanometers wide, like Bump’s genome sequenced. enat birth will reveal Bum and the minor groove is 1.2 nanometers p’s genetic background, i wide. The different widths of the two gro ndicating the various conditions he or oves determine the accessibility of the she may or may not be at risk of devel protein to different bases, depending on oping. The results will be presented on whether the base is in the main groove an ID cardand will form part of Bump’s or the minor groove. Proteins that intera medical records. Olly thinks it’sagreat ct with DNA, such as transcription factor idea but Lily is concerned that the info s, usually make contact with bases in th rmation revealed could lead to discrim ination and inequality later in life. DeoxyriboNucleice major groove. Acid 09 10

Great breakthrough The history and progress of Friedrich Miescher Phoebus Levene William Astbury deoxyribonucleic acid research Erwin Schrödinger Martha Chase DNA was first isolated from the In 1937, William Astbury showed the results of the fir Francis Harry Alfred Hershey pus of surgical bandages by Swis st X-ray diffraction study, showing that DNA has an e James Dewey Watson s biochemist Friedrich Miescher xtremely regular structure. Meselson in 1869. Because this microscopic In 1928, British scientist Frederick Griffith (1877-1941 substance is located in the nucleu ) discovered in experiments that smooth pneumococ 12 s, it was called nuclein (nuclein). cal bacteria can be transformed into coarser bacteria of the same species. While the system did not provid In 1919, Phoebus Levene determi e any evidence of changes caused by substances, it s ned that DNA is composed of nu howed that certain substances could pass genetic inf cleotides composed of nitrogeno ormation from the remains of dead bacteria to organ us bases, sugars and phosphates. isms. The experiment by Oswald Avery and others in Levene proposed that DNA consi 1943 proved that DNA was the reason behind this tr sts of a single nucleotide held to ansformation phenomenon. gether by phosphate. He is convi In 1944, Erwin Schrödinger, in view of the chaotic be nced that the long DNA chain is havior theory of the system of a few atoms in quant shorter and the bases in it are re m physics, asserted that genetic material must be co peated in a fixed order. mposed of large non-repetitive molecules in order to maintain the stability of genetic information. 11

Main categories of DNA Single-stranded DNA (single-stranded The plasmid or viral DNA Junk DNA (Junk DNA) refers to DNA that is not translated into protein in organisms. In the past, it was considered useless, so it is called junk DNA . Later, scientists DNA) Most DNA exists in a double helix extracted from the cell co discovered that junk DNA contains important regulatory mechanisms that can structure, but it becomes single-strand ntains two molecules of c control basic biochemical reactions and developmental processes, which will help ed after heat or alkali treatment. Single losed loop and open loop. organisms evolve more complex organisms. The more complex the organism, -stranded DNA refers to DNA that exists The two can be separate the more important junk DNA seems to be. in this state. Single-stranded DNA is di d according to their ability fferent from double-stranded DNA in m to bind to pigments. olecular hydrodynamic properties, abs orption spectrum, and base reaction p roperties. Certain phage particles cont ain single-stranded circular DNA, and s uch phage DNA forms double-stranded DNA when proliferating in cells. Closed circular DNA (closed circular DN A) double-stranded circular DNA withou t fracture, also known as supercoiled D NA. Since the double strands with the h elical structure are closed individually, the entire DNA molecule is further twis ted to form a tertiary structure. In addit ion, if there is a break in different parts of one or two strands, it will become an open-loop DNA molecule without twist. 13 14

The Genome biological structure function of DNA The genomic DNA of eukaryotes is located in the nucleus, and there is also DNA in mitocho In the genome, genetic information is ndria and chloroplasts. Prokaryotic DNA is en not encode proteins can also be transcribe stored in DNA sequences called genes, capsulated in the nucleus of irregular organell d into non-coding RNA to participate in the es without cell membrane in the cytoplasm. regulation of gene expression. Some non- and the transmission of this genetic Genetic information is contained in genes, wh information is guaranteed by the presence ich are hereditary units that can affect the ph enotype of organisms. of complementary nitrogen-containing base sequences. Each gene contains an open reading frame (a coding sequences are structural compone region that can be transcribed into RNA) and a nts of chromosomes. Telomere and centro regulatory region composed of a promoter an mere regions usually contain very few gen d an enhancer. In many species, only a small es, but are essential for the function and part of the genome sequence can be transcrib stability of chromosomes. ed and translated. For example, only 1.5% of the human genome contains protein-coding e xons, and more than 50% of the human geno me consists of repetitive non-coding DNA seq 15 16uences. In any case, DNA sequences that do