Biology by Bradford
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The subtopic in the middle and eleven tasks round it: label four diagrams, complete two tables, put a process in order and fill 64 gaps. Every standard level statement has a box; the five boxes for higher level only are dashed and badged HL, and drop off for an SL class. Type into the blanks and check them, or print it on A4, Letter or A3 with an answer key.
Each amino acid is brought by a carrier molecule whose pairs with a codon. The ribosome joins each new amino acid to the chain with a bond, then moves on by bases, so the chain grows one amino acid at a time.
This is part of a table of mRNA codons, the ones that start with G. The mRNA GGU GCA GAU GUG codes for , , and . Four different codons code for alanine: the code is .
In sickle cell disease, a in the gene for the β chain of haemoglobin changes the mRNA codon GAG to . The sixth amino acid becomes instead of glutamic acid. At low oxygen the altered molecules stick together and distort red blood cells, which can then block .
12345The enzyme separates the two DNA strands by breaking the between their bases. Each incoming base pairs with an exposed base on one strand, adenine in the DNA with in the RNA, and the enzyme links them into a chain to that strand.
| Amino acid | DNA template | mRNA codon | tRNA anticodon |
|---|---|---|---|
| Methionine | TAC | ||
| Phenylalanine | UUU | ||
| Glycine | CCG | ||
| Histidine | GTA |
Read in threes, four bases give codons, more than enough for 20 amino acids. Almost all organisms use the same codons: the genetic code is .
| Sequence | What it does |
|---|---|
| Promoter | Where bind, so that RNA polymerase can attach and begin transcription |
| Introns | Transcribed, then before the mRNA leaves the nucleus |
| Telomeres | Repeated sequences that the ends of chromosomes |
| Genes for tRNA and rRNA | Transcribed into RNA that is used without being |
Transcription runs in the direction: each new nucleotide joins the growing end of the RNA.
A DNA strand can be transcribed again and again without any change to its base sequence. In cells that will never again, such as most nerve cells, that sequence must be for the whole life of the cell. Only some of a cell's genes are at any one time. Transcription is the stage of gene expression, so it is a key point at which genes are switched on and off.
In cells the first RNA copy of a gene is modified before it is translated. The structures added at its two ends the mRNA. The bottom mRNA joins the coding sections in another combination, , so one gene can code for more than one .
Number the steps in order.
As the chain grows, a tRNA bringing a new amino acid enters the site, the tRNA holding the chain sits in the site, and the empty tRNA leaves from the site.
Many polypeptides must be modified before they can work. Insulin is made as pre-proinsulin: cutting off the gives proinsulin, and cutting out a middle section, the , leaves two chains held together by . Cells also break down proteins all the time in and reuse their , keeping the proteome working.
The enzyme separates the two DNA strands by breaking the hydrogen bonds between their bases. Each incoming base pairs with an exposed base on one strand, adenine in the DNA with uracil in the RNA, and the enzyme links them into a chain complementary to that strand.
A DNA strand can be transcribed again and again without any change to its base sequence. In cells that will never divide again, such as most nerve cells, that sequence must be conserved for the whole life of the cell. Only some of a cell's genes are expressed at any one time. Transcription is the first stage of gene expression, so it is a key point at which genes are switched on and off.
Each amino acid is brought by a carrier molecule whose anticodon pairs with a codon. The ribosome joins each new amino acid to the chain with a peptide bond, then moves on by three bases, so the chain grows one amino acid at a time.
| Amino acid | DNA template | mRNA codon | tRNA anticodon |
|---|---|---|---|
| Methionine | TAC | AUG | UAC |
| Phenylalanine | AAA | UUU | AAA |
| Glycine | CCG | GGC | CCG |
| Histidine | GTA | CAU | GUA |
Read in threes, four bases give 64 codons, more than enough for 20 amino acids. Almost all organisms use the same codons: the genetic code is universal.
This is part of a table of mRNA codons, the ones that start with G. The mRNA GGU GCA GAU GUG codes for Gly, Ala, Asp and Val. Four different codons code for alanine: the code is degenerate.
In sickle cell disease, a point mutation in the gene for the β chain of haemoglobin changes the mRNA codon GAG to GUG. The sixth amino acid becomes valine instead of glutamic acid. At low oxygen the altered molecules stick together and distort red blood cells, which can then block capillaries.
| Sequence | What it does |
|---|---|
| Promoter | Where transcription factors bind, so that RNA polymerase can attach and begin transcription |
| Introns | Transcribed, then removed before the mRNA leaves the nucleus |
| Telomeres | Repeated sequences that protect the ends of chromosomes |
| Genes for tRNA and rRNA | Transcribed into RNA that is used without being translated |
Transcription runs in the 5′ to 3′ direction: each new nucleotide joins the growing end of the RNA.
In eukaryotic cells the first RNA copy of a gene is modified before it is translated. The structures added at its two ends stabilize the mRNA. The bottom mRNA joins the coding sections in another combination, alternative splicing, so one gene can code for more than one polypeptide.
As the chain grows, a tRNA bringing a new amino acid enters the A site, the tRNA holding the chain sits in the P site, and the empty tRNA leaves from the E site.
Many polypeptides must be modified before they can work. Insulin is made as pre-proinsulin: cutting off the signal peptide gives proinsulin, and cutting out a middle section, the C-peptide, leaves two chains held together by disulfide bonds. Cells also break down proteins all the time in proteasomes and reuse their amino acids, keeping the proteome working.