The instructions for making protein sit in DNA, but protein is built on the ribosomes in the cytoplasm. Who covers the gap between the two? RNA. And one molecule type comes in three working forms, each with its own job.
1. What RNA is and where it sits
A nucleic acid whose polynucleotide units hold ribose sugar, and uracil as one of the bases, is ribonucleic acid (RNA).
Every living cell has RNA. Of a cell's RNA, 90% is in the cytoplasm and 10% in the nucleus. It also occurs in ribosomes, chromosomes, mitochondria and plastids, and some viruses carry it too.
RNA is a single-stranded chain, coiled in places. It carries several U-shaped hairpin loops. Chemically it has three ingredients: ribose sugar, four bases (adenine, guanine, uracil, cytosine) and phosphate.
2. Five kinds of RNA
By structure and function RNA is of five kinds: tRNA, rRNA, mRNA, gRNA and minor RNA. Take them one at a time.
Transfer RNA (tRNA) carries each amino acid to the mRNA according to the genetic code and so helps make protein. A cell has about 31-42 kinds of tRNA, made inside the nucleus. Each tRNA has about 70 to 90 nucleotides.
tRNA folds and pairs its bases to form several loops. The key loop is the anticodon loop, which faces the codon of mRNA. Three bases make one anticodon.
The 3' end of tRNA is single-stranded and always ends in the CCA sequence. The amino acid attaches right there, so the spot is called the amino acid site. The anticodon loop and this site lie at opposite ends.
The clover leaf model of tRNA has five labels: the amino acid attachment site, the D loop, the T loop, the anticodon, and the 3' and 5' ends. The board often asks you to draw it. Tap all five points.
Fig 1.30Tap a point
Shape of tRNA
Amino acid attachment site. At the 3' end, in the CCA sequence. At the end opposite the anticodon loop.
D loop. One of the loops of tRNA.
T loop. Another loop of tRNA.
Anticodon. Three bases that pair with the mRNA codon. It sits in the anticodon loop.
3' and 5' ends. The two ends of tRNA. CCA is at the 3' end.
All five seen. The anticodon and the amino acid site lie at opposite ends.
Drawn by hand after the book's figure, not to scale.
Ribosomal RNA (rRNA) is the main structural part of the ribosome. It makes up 80-90% of all the RNA in a cell.
Messenger RNA (mRNA) is the template for protein synthesis, following the gene's signal. It is made from DNA by transcription and carries the message from the nucleus to the cytoplasm.
The non-coding stretch at the 5' end of mRNA is the 5'-leader, and at the 3' end the 3'-trailer. The middle is the coding region. Three successive bases make one codon.
Some viruses have no DNA, and there RNA itself is the hereditary material. This is genetic RNA (gRNA); an example is TMV.
Small cytoplasmic and nuclear RNAs join proteins to give enzymes their structure, and also act as enzymes themselves. These are minor RNA.
sort themTap a chip, then pick a bin
Which RNA?
Pick up a chip, then drop it in a bin.
Right. Next one.
Not yet. Ask yourself: is this an end, a share, or a job?
All in place. The CCA end and the anticodon belong to tRNA: one is the amino acid site, the other lies in a loop. 80-90% and the main part of the ribosome are rRNA. The 5'-leader and the template are mRNA. TMV has no DNA, so its material is gRNA. Giving enzymes their structure is minor RNA.
RNA's main work is protein synthesis. tRNA moves amino acids, rRNA forms ribonucleoprotein, and mRNA carries the message from DNA to the ribosome. Now match jobs to the RNAs.
match themchip, then bin
Whose job?
Pick up a job, then drop it in the RNA that does it.
Right. Next one.
Not yet. Recall the hooks for t, r, m and g.
All matched. tRNA moves amino acids, rRNA joins the ribosome as ribonucleoprotein, mRNA delivers the message, and gRNA is a virus's hereditary material.
3. DNA against RNA
DNA is double-stranded, like a twisted ladder; RNA is single-stranded, like a chain. DNA has deoxyribose sugar and thymine; RNA has ribose and uracil.
DNA has no kinds, while RNA has five. New DNA arises by replication; RNA is made afresh and does not replicate.
DNA sits mainly in chromosomes, sometimes in mitochondria and chloroplasts. RNA sits in chromosomes, cytoplasm, ribosomes and the nucleolus. DNA holds, carries and controls heredity; RNA makes protein.
DNA carries hereditary traits, RNA usually does not. DNA has many more nucleotides, RNA far fewer. DNA weighs from a million to many crore daltons; RNA not more than a few lakh.
Take one run of bases from DNA to RNA and see for yourself what changes and what stays.
DNA to RNApress RNA
One strand, two molecules
Now cover one cell of the table, recall it, then tap to check.
gridCover a cell and recall it
DNA versus RNA
point
DNA
RNA
structure
double-stranded ladder
single-stranded chain
sugar
deoxyribose
ribose
pyrimidine
thymine, cytosine
uracil, cytosine
kinds
none
five
origin
by replication
made afresh, no replication
location
chromosome (also mitochondria and chloroplasts)
chromosome, cytoplasm, ribosome, nucleolus
main work
carries heredity
protein synthesis
heredity
carries it
usually does not
size
more nucleotides
fewer
DNA is a double-stranded twisted ladder; RNA is a single-stranded chain.
DNA has deoxyribose, RNA has ribose.
Cytosine is in both. Where DNA has thymine, RNA has uracil.
DNA has no kinds. RNA has five: tRNA, rRNA, mRNA, gRNA, minor RNA.
DNA makes new DNA by replication. RNA is made afresh and does not replicate.
DNA is mainly in chromosomes. RNA is spread out: chromosome, cytoplasm, ribosome and nucleolus.
DNA holds, carries and controls heredity. RNA makes protein.
DNA carries hereditary traits, RNA usually does not.
DNA has far more nucleotides than RNA.
4. Check yourself
Three questions, starting with the first.
check yourselfpick one answer
Question 1
Read the stimulus: In class the teacher spoke of a chemical molecule that is formed by transcription. The molecule in the stimulus:
i. Is formed from DNA ii. Contains deoxyribose sugar iii. Plays a role in making polypeptide
Not i alone, since iii is also right: the RNA takes part in making polypeptide.
i and ii: ii is wrong, since RNA has no deoxyribose.
ii and iii: ii is wrong.
Right. RNA made from DNA in transcription contains ribose and takes part in protein (polypeptide) synthesis.
The second is about the five kinds of RNA.
check yourselfpick one answer
Question 2
Which is the smallest RNA molecule?
mRNA is long, with hundreds to thousands of nucleotides.
Right. tRNA has only about 70 to 90 nucleotides, so it is the smallest RNA.
rRNA is a large molecule, the main component of the ribosome.
gRNA is larger than tRNA.
Think before you mark the last one.
check yourselfpick one answer
Question 3
Which of the following has a CCA tail at the 3' end?
Right. The 3' end of tRNA is always CCA, where the amino acid attaches.
gRNA has no CCA tail at its 3' end.
rRNA builds the ribosome and has no CCA tail.
The 3' end of mRNA carries a non-coding untranslated part, not CCA.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionHow many kinds of RNA are there and what?
AnswerFive: tRNA, rRNA, mRNA, gRNA and minor RNA.
QuestionWhat is tRNA? How many kinds per cell?
AnswerThe RNA that carries amino acids to the mRNA according to the genetic code. About 31-42 kinds per cell; made in the nucleus.
QuestionWhich sequence is at the 3' end of tRNA? What attaches there?
AnswerAlways CCA; the amino acid attaches there (the amino acid site).
QuestionWhat is mRNA? How is it made?
AnswerThe RNA that acts as template for protein synthesis and carries the message from nucleus to cytoplasm. Made from DNA by transcription.
QuestionWhat is the main work of RNA? What do tRNA, rRNA and mRNA each do?
AnswerProtein synthesis. tRNA: moves amino acids. rRNA: forms ribonucleoprotein. mRNA: carries DNA's message to the ribosome.
QuestionState differences in structure, sugar and base between DNA and RNA.