
DNA、遺伝子、ゲノムという言葉を同じ意味で使っている記事やメディアがたくさんありますが、それぞれには正確な意味があります。テレビや新聞などでも間違って使用されていることが多いため、混同されるのも無理はありません。
しかし、生物学者は明確にそれぞれの意味を使い分けています。中学生や高校生の生物学の勉強においても、「DNA」「遺伝子」「ゲノム」の違いは知っておきたいものです。また、この記事ではもう一歩踏み込んで「染色体」についても解説しています!
DNA(デオキシリボ核酸)とは何か

DNAはdeoxyribonucleic acidの略で、日本語での正式名称はデオキシリボ核酸です。
DNAは細胞の核に存在する物質名です。つまり、水素、炭素、ナトリウム、グルコース——と物質名を述べるときと同じ化学物質の名称なのです。

なので「DNA=遺伝子」ではありません!
遺伝子はたしかにDNA(またはRNA)でできています。しかし、わたしたちの体の中にあるDNAの大部分は遺伝子ではありません。なので、この2つの言葉は正確に区別しなければなりません。
DNAはとても小さな物質なのですが、それを一列に並べてみるとヒトで約2メートル、長いものでは90メートルに及ぶものもあります。1
遺伝子は有名な「二重らせん構造」をしており、自己を複製する能力があります。DNAに複製する能力があることは、生物を生物たらしめている特徴のひとつです。

DNAの外側のリン酸とデオキシリボースの部分は同じ構造の繰り返しですが、中央の部分は異なります。この中央の部分のことを「塩基」といいますが、塩基は四種類の分子(アデニン・チミン・グアニン・シトシン)からできていて、これらの分子の並び方が独自の遺伝情報となるのです。
遺伝子とは何か

遺伝子は英語のgeneの訳語で、「遺伝する単位」という意味です。専門書では「遺伝単位」と書かれる場合もあります。
ご存じのように、ヒトをはじめとするほとんどの生物の遺伝子はDNAなのですが、一部のウイルスではRNAという名前のDNAに似た物質が遺伝子です。2
前のDNAの項目で、DNAを伸ばしてみると、とても長くなると述べましたが、DNAのすべてが遺伝子であるわけではありません。DNAには遺伝子である部分とそうでない部分があります。なので、生物の「遺伝子がDNAという物質か」というとイエスですが、DNAがすべて遺伝子かというとノーなのです。また、一部のウイルスはRNAが遺伝物質です。
生物はタンパク質からできており、遺伝子は「ここがこのタンパク質になる」という情報をもっています。
タンパク質をコードしないDNAのことは「ジャンクDNA」と呼ばれます。ジャンクDNAは「ATATATATAT」というように塩基が繰り返しをしています。なので「がらくた」だと思われて、このように命名されたのですが、近年、ジャンクDNAにも機能が備わっていることがわかってきました。
染色体とは何か

ゲノムとは何かについて語るまえに、染色体についてお話しします!
染色体についてわかっていれば、ゲノムについて理解するのが簡単になるからです。
染色体とはDNAが巻き付いてそれが束になったものです。細胞分裂が行われるときにだけ、染色体は出現します。普段はDNAはばらばらに細胞の核の中を漂っています。しかし、細胞分裂を行うときには、まとまって2つに分かれないといけないので、束になるのです。

染色体の本数は生物によって決まっています。ショウジョウバエは8本、ネズミは40本、ヒトは46本です。しかし、ヒトよりジャガイモのほうが48本と多くの染色体をもっています。複雑な生物ほど多くの染色体をもつわけではありません。

染色体を区別するために、番号が振られていて、大きいものから1番、2番…と決定されています。メスとオスに分かれている生物は、同じ大きさの染色体を2本ずつもっています。ヒトでは22番染色体まであるので、合計で44本です。そして、これとは別に、性別を決定する性染色体という特殊な染色体があり、男性ではXY、女性ではXXという組み合わせの染色体をもちます。これで合計46本になります。
ゲノムとは何か

上記で染色体について学びましたが、生物が染色体を2組ずつもつのは、母方から1組、父方から1組とそれぞれ染色体を受け継ぐので、合計で2組の染色体をもつことになるからです。
しかし、生物として1個体が形成されるには一組分の染色体があればよいので、この染色体1組分のことをゲノムといいます。または、1組分の染色体に含まれるDNAの総体をゲノムということもあります。
なので、ヒトゲノムといったら、22本の常染色体とXXまたはXYの性染色体のこと、またはその染色体がもつDNAのことを指します。
まとめ – 遺伝情報を「本」に例えてみると…
DNAは「紙」です。つまり文字が書かれている素材、または物質といえます。そして、その紙で「本」が作られています。その本はA・T・C・Gの4文字で書かれています。アルファベットは26文字、日本語はひらがな・カタカナ・漢字とかなりたくさんの文字を使っていますから、4文字は少ないですね。
そして、その「本」にはいろいろなタイトルがあります。「ライオン」というタイトルだったり、「ハエ」というタイトルだったり「サクラ」だったり「大腸菌」だったり。わたしたちは「ヒト」というタイトルの本を持っています。そして、それぞれの本に書かれている内容は、その生物の設計図です。この部分はこういう物質で作る、という情報が書かれています。
しかし、設計図は複雑なので、1巻に収めることはできません。「イヌ」という本は78巻あります。「ネコ」という本は38巻あります。「ヒト」という本は46巻です。
このたとえは、
・ 紙 = DNA
・ 文字 = 塩基(アデニン・グアニン・チミン・シトシン)
・ タイトル = 種
・ 巻 = 染色体
・ 設計図 = DNA配列
と置き換えられます。

この記事では、この転写と翻訳の過程について詳しく見ていきます!
ちなみに、同じタイトルの本が100冊あれば、その100冊にはほとんど同じことが書かれています。しかし、ところどころが少しずつ違います。なので「夏目漱石」だったり「チャールズ・ダーウィン」だったりという「サブタイトル」もあります。
あなたの遺伝子は「ヒト」という本の「あなたの名前」というサブタイトルが書かれた本です。そして、全部で46巻あるその本がすべて、あなたの60兆個の細胞それぞれに1つずつ入っているのです。なので、あなたの体には60,000,000,000,000個の本が入っており、それぞれ46巻ずつあるので、2,760,000,000,000,000冊があなたの体におさめられているのです。
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