GCSE · Biology · AQA · Spec 8461

DNA structure (biology only)

Every unit along a DNA strand has the same sugar and phosphate. Only one part changes, in just four kinds, and its order is the code for your proteins.

Inside a strand of DNA

What repeats, and what changes?
chain continueschain continuesPSPSPSPSPSPSphosphatesugarbaseGATCCAOne stretch ofa DNA strand.Six nucleotidesshown; a realstrand is farlonger.

View: Whole strand. Showing 3 layers: Sugar-phosphate strand, Key, Whole strand

View

Explore

switch the view, then tap a part

DNA is a polymer: a long chain made of repeating units called nucleotides. Switch the view to see what repeats and what changes, then tap each part.

P = phosphate group, S = sugar. Each tab is a base.

Reading the code

What does a group of three bases code for?

The order of the bases is the part of DNA that varies. The bases are read in groups of three, and each group of three is called a triplet.

Which is closest to what you think right now?
How sure are you?
Higher

The second strand

Work along the strand one base at a time.

DNA has complementary strands. One strand has the sequence G A T C C A. Which sequence is on the opposite strand, base for base?

Higher

How a protein is made

Each stage depends on the one before it.

  1. The templateA protein is built according to a template: the base sequence of its gene. A copy of that sequence is carried from the DNA to a ribosome in the cytoplasm.
  2. On a ribosomeProteins are synthesised on ribosomes. The ribosome works along the template, reading its bases in order.
  3. Carrier moleculesCarrier molecules bring specific amino acids to the ribosome. The bases being read decide which amino acid is needed next, so the amino acids arrive in the correct order.
  4. The chain growsEach amino acid that arrives is added to the growing protein chain, one after another, until the chain is complete.
  5. FoldingWhen the protein chain is complete, it folds up to form a unique shape. That shape is what lets the protein do its job: as an enzyme, as a hormone, or forming structures in the body such as collagen.
Higher

When the DNA changes

?

Reason it through

How can a change in the DNA base sequence change the protein a gene makes?

Link 1 of 4

First link · your turn

One base in a gene is replaced by a different base. What has changed in the code?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first
Higher

Where a change falls

Does a change in DNA always matter?

Follow a genetic variant. Choose where it lies in the DNA, then what it does.

Where the change is → What the change does

  • Mutations occur continuously. Where a change falls in the DNA decides what it can do.

3 possible routes.

On A genetic variant. 2 branches to choose from.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

DNA is a long chain of one repeating unit. Only one part of each unit changes, and the order of those parts is the code for building proteins.

What you need to know

  • DNA is a polymer made from four different nucleotides, repeated along the chain.
  • Each nucleotide is a common sugar and a phosphate group, with one of four bases attached to the sugar. The four bases are A, C, G and T.
  • The long strands of DNA are alternating sugar and phosphate sections, with one base attached to each sugar.
  • A sequence of three bases is the code for a particular amino acid. The order of bases controls the order in which amino acids are assembled to produce a particular protein.
  • Higher: in the complementary strands, a C is always linked to a G on the opposite strand, and a T to an A.
  • Higher: proteins are synthesised on ribosomes according to a template. Carrier molecules bring specific amino acids to add to the growing chain in the correct order, and the complete chain folds into a unique shape that lets the protein work as an enzyme, a hormone or a structure such as collagen.
  • Higher: mutations occur continuously. Most do not alter the protein, or alter it only slightly. A few code for a protein with a different shape, so an enzyme may no longer fit its substrate binding site or a structural protein may lose its strength.
  • Higher: not all DNA codes for proteins. Non-coding parts can switch genes on and off, so variants there may affect how genes are expressed. Variants in coding DNA may alter a protein's activity.

The big picture

DNA is a polymer of repeating nucleotides. Each nucleotide is a common sugar and a phosphate group with one of four bases, A, C, G or T, attached to the sugar, so the alternating sugar-phosphate strand repeats while the bases vary. Three bases code for one particular amino acid, and the base order controls the order in which amino acids are assembled into a protein. Higher: C links to G and T to A; proteins are built on ribosomes and fold into a unique working shape; a base change can alter that shape, though most mutations do not; and non-coding DNA can switch genes on and off.

Key points

1Everything along a strand of DNA repeats except the base, so the code is in the order of the bases, not in the sugar-phosphate backbone.
2Read the bases three at a time: each triplet codes for one amino acid, and the base order sets the amino acid order.
3Higher: pairing is fixed, C with G and T with A, so one strand sets the sequence of the other.
4Higher: a protein works because of the shape its chain folds into. That is how a change in DNA can change what a protein does, although most changes leave the protein working.
5Higher: a variant can influence phenotype in two ways: in coding DNA by altering a protein's activity, and in non-coding DNA by altering how genes are expressed.

Worked example

Problem

A section of one DNA strand has the bases T G C A A T C G T T A C, and all of it codes for protein. Describe the structure of this section, and work out how many amino acids it codes for.

⚠ Watch out

Saying that one base codes for one amino acid. The code is read in threes: a sequence of three bases codes for one amino acid, so 9 bases code for 3 amino acids, not 9.

🧠

Memory hook

Picture a string of identical beads with a letter tag hanging from each one: A, C, G or T. The string never changes; only the tags do. Read the tags in threes and they spell out the order of amino acids in a protein.

✓

Check yourself

Cover the page. Which part of a nucleotide changes along a strand? How many amino acids do 15 coding bases give? Higher: which base faces T, and why do most mutations leave a protein working?

Flashcards

(14)
What is DNA a polymer of?
Repeating units called nucleotides. There are four different nucleotides.
What three parts make up one nucleotide?
A common sugar, a phosphate group, and one of four bases attached to the sugar.
Name the four bases in DNA.
A, C, G and T.
What are the long strands of DNA made of?
Alternating sugar and phosphate sections, with one base attached to each sugar.
Which part of a nucleotide differs from one nucleotide to the next?
The base. The sugar and the phosphate are the same in every nucleotide.
What does a sequence of three bases code for?
One particular amino acid.
What does the order of the bases control?
The order in which amino acids are assembled to produce a particular protein.
Higher: which base links to C, and which to T, on the opposite strand?
C links to G. T links to A.
Higher: where are proteins synthesised?
On ribosomes, according to a template.
Higher: what do carrier molecules do in protein synthesis?
They bring specific amino acids to add to the growing protein chain in the correct order.
Higher: what happens to a protein chain once it is complete, and why does it matter?
It folds up into a unique shape. That shape lets the protein do its job as an enzyme, a hormone or a structure such as collagen.
Higher: do most mutations change a protein's function?
No. Mutations occur continuously, but most do not alter the protein, or alter it so slightly that its appearance or function is not changed. Only a few change its shape.
Higher: what can happen when a mutation gives a protein a different shape?
An enzyme may no longer fit its substrate binding site, or a structural protein may lose its strength.
Higher: what are the two ways a genetic variant can influence phenotype?
In coding DNA, by altering the activity of a protein. In non-coding DNA, by altering how genes are expressed (switched on and off).

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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