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1.8Cell Structure and Molecular Biology

Molecular genetics — EST 2 Biology Revision Notes & Topical Questions

20 min read 30 parts Lesson 8 of 11 2 quizzes

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Curriculum Equation Key Mendelian Genetics & Punnett Square LawsFast Exam Solution
Monohybrid: Aa × Aa ➔ 1 AA : 2 Aa : 1 aa (Phenotype 3:1) | Dihybrid: 9:3:3:1
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Lesson summary

SAT Biology molecular genetics: trace genetic information from DNA replication through transcription and translation into a polypeptide. DNA is a double helix made of two complementary, antiparallel nucleotide strands.

  • A pairs with T
  • C pairs with G
  • semiconservative

Section 1 of 30DNA structure

DNA structure
DNA is a double helix made of two complementary, antiparallel nucleotide strands. Each nucleotide contains deoxyribose, phosphate and one base: adenine, thymine, cytosine or guanine.
A pairs with T
Hydrogen bonds join complementary bases: A pairs with T and C pairs with G. The base sequence stores hereditary information, while the sugar-phosphate backbone supports each strand.

Section 2 of 30Semiconservative replication

DNA replicates during S phase. The double helix unzips at its hydrogen bonds, and each original strand acts as a template for a complementary new strand.
Semiconservative
Replication is semiconservative because each daughter molecule contains one original strand and one newly made strand. DNA polymerase adds nucleotides and proofreads the new DNA, reducing replication errors.

Section 3 of 30RNA and transcription

RNA is usually single-stranded, contains ribose and uses uracil instead of thymine. Messenger RNA carries a code, transfer RNA carries amino acids, and ribosomal RNA forms part of the ribosome.
Transcription
During transcription, RNA polymerase binds near a gene, separates the DNA strands and constructs complementary mRNA from one template strand. In RNA, A pairs with U and C pairs with G.

Section 4 of 30RNA processing

1Introns
In eukaryotes, the initial RNA transcript is processed before leaving the nucleus. Noncoding introns are removed and coding exons are joined to make mature mRNA.
2
Processing allows the final mRNA sequence to carry the usable coding message through a nuclear pore to a ribosome in the cytoplasm or on rough endoplasmic reticulum.

Section 5 of 30Translation and gene expression

Translation
During translation, the ribosome reads mRNA in three-base codons. A tRNA with a complementary anticodon delivers the corresponding amino acid, and peptide bonds join amino acids into a polypeptide.
A start codon begins translation and a stop codon ends it. Cells regulate which genes are transcribed, allowing different cell types to make the particular proteins needed for their structures and functions.

Section 6 of 30RNA polymerase and mRNA processing

Transcription converts information in a DNA template strand into a molecule of mRNA.

1
RNA polymerase binds to the gene and unwinds (unzips) the DNA, exposing the gene to be transcribed.
2
Free RNA nucleotides align with their complementary exposed bases on the template strand (A–U, C–G).
3
RNA polymerase joins the RNA nucleotides with covalent bonds, building the mRNA in the 5′ → 3′ direction.
4
The mRNA detaches and the DNA rewinds.

In eukaryotes the mRNA is modified by enzymes: a cap and a tail are added and introns are removed while exons are spliced together. Different combinations of exons can produce different versions of the mRNA, which is called alternative splicing.

Section 7 of 30Translation and the genetic code

Translation converts the genetic code in mRNA into a polypeptide.

1
Ribosomes are the site of translation; they are present in the cytoplasm of both prokaryotic and eukaryotic cells and on the rough endoplasmic reticulum of eukaryotic cells.
2
Each mRNA codon, a triplet of bases, corresponds to a specific amino acid, which can be found with a genetic code chart.
3
Many amino acids are coded by more than one codon: for example GAU and GAC both code for aspartic acid.
4
In prokaryotes, transcription and translation can occur simultaneously, with one end of the mRNA still being made while the other end is attached to a ribosome.

Nearly all living organisms use the same genetic code, which is evidence of the common ancestry of all living organisms.

Section 8 of 30Worked example: decoding a gene

Worked examplededuce the amino acids for the DNA sense strand 5′-TTCGAGCATTACGCC-3′.

1
Convert the sense strand into the template strand: 3′-AAGCTCGTAATGCGG-5′.
2
Transcribe: the mRNA matches the sense strand with U for T: UUC GAG CAU UAC GCC.
3
Use the genetic code chart for each codon: UUC is in the top-left box (first base U, second base U, third base C), which is Phe.
4
Repeat: GAG = Glu, CAU = His, UAC = Tyr, GCC = Ala.

The sequence is Phe–Glu–His–Tyr–Ala.

Section 9 of 30Retroviruses and the flow of genetic information

Retroviruses are viruses with RNA as their genetic material.

1Reverse transcriptase
When they infect a host cell their RNA must be converted into DNA by the enzyme reverse transcriptase before transcription and translation can occur.
2
The viral genetic material is inserted into the host cell genome,
  • and the host cell then carries out transcription and translation to make viral proteins.
3
HIV is an example.

The nucleic acid DNA is the primary source of heritable information. Genetic information can also be carried by RNA, as in some viruses. It is packaged differently in different organisms: prokaryotic cells contain DNA as a single, circular chromosome and as small circular plasmids, whereas eukaryotes have linear chromosomes in a nucleus.

DNA and RNA are polymers of nucleotides. DNA nucleotides contain a deoxyribose sugar, a phosphate and a nitrogenous base; RNA nucleotides contain a ribose sugar, a phosphate and a base. Nucleotides join to form polynucleotides with a sugar-phosphate backbone, and single strands can join to form double-stranded molecules.

Section 10 of 30Exam technique: gene expression

Always give the location and the enzyme: transcription in the nucleus by RNA polymerase, splicing in the nucleus, translation at ribosomes in the cytoplasm or on rough ER.

Exam technique: gene expression
When decoding, write the mRNA codons first, check whether you were given the sense or template strand, and then read the chart carefully.
Mutations are explained by changing one base, which may change the codon, the amino acid, the protein shape and so the function.

Section 11 of 30Worked examples

  1. Distinguish between DNA polymerase and RNA polymerase. [2]2 marks

  2. Explain how alternative splicing produces different proteins from one gene. [3]3 marks

  3. Deduce the amino acids for mRNA UUC GAG CAU. [3]3 marks

  4. Why does a retrovirus need reverse transcriptase? [2]2 marks

Section 12 of 30Base pairing: purines and pyrimidines

Base pairing occurs because of the differences in structure between the bases.

Purines
Guanine and adenine have a double-ring structure and are purines; cytosine, thymine and uracil have a single ring and are pyrimidines.
A purine always pairs with a pyrimidine,
  • so the base pairs are all the same length : A pairs with T (or U in RNA) and C pairs with G.
    • (a single-ring base matched to a double-ring base)

Section 13 of 30Semiconservative DNA replication

DNA replication occurs during S phase of the cell cycle, so genetic information is available for transmission between generations, and it ensures DNA is copied accurately so hereditary information is continuous. It is semiconservative: in each new DNA molecule half is from the original and half is new, since each original strand is a template for a new complementary strand.

Helicase
Helicase unwinds the strands and breaks hydrogen bonds, exposing the template strands.
  • This increases supercoiling elsewhere, which topoisomerase relaxes in front of the fork.
RNA primers attach to the template strand.
DNA polymerase
DNA polymerase adds complementary DNA nucleotides to the 3′ end of the growing strand.
Leading strand
The leading strand is made continuously in the same direction as the replication fork; the lagging strand is made in short fragments in the opposite direction.
DNA ligase
The primers are replaced with DNA and DNA ligase joins the fragments.

Do not mix up the enzymes: DNA polymerase joins DNA nucleotides in replication; RNA polymerase joins RNA nucleotides in transcription.

Section 14 of 30Transcription and RNA types

To make a protein, the DNA base sequence is converted first into an mRNA sequence and then into a sequence of amino acids.

Transcription and RNA types
Several types of RNA are involved, whose functions depend on their base sequence and structure.
TRNA
The mRNA sequence is determined by the DNA sequence from which it is copied. tRNA molecules have one end that binds to a specific amino acid and another end with an anticodon that base-pairs with the matching mRNA codon; rRNA forms part of the ribosome.

Transcription converts information in a DNA template strand into a molecule of mRNA.

1
RNA polymerase binds to the gene and unwinds (unzips) the DNA, exposing the gene to be transcribed.
2
Free RNA nucleotides align with their complementary exposed bases on the template strand (A–U, C–G).
3
RNA polymerase joins the RNA nucleotides with covalent bonds, building the mRNA in the 5′ → 3′ direction.
4
The mRNA detaches and the DNA rewinds.

In eukaryotes the mRNA is modified by enzymes: a cap and a tail are added and introns are removed while exons are spliced together. Different combinations of exons can produce different versions of the mRNA, which is called alternative splicing.

Section 15 of 30Translation in brief

Translation converts the genetic code in mRNA into a polypeptide.

1
Ribosomes are the site of translation; they are present in the cytoplasm of both prokaryotic and eukaryotic cells and on the rough endoplasmic reticulum of eukaryotic cells.
2
Each mRNA codon, a triplet of bases, corresponds to a specific amino acid, which can be found with a genetic code chart.
3
Many amino acids are coded by more than one codon: for example GAU and GAC both code for aspartic acid.
4
In prokaryotes, transcription and translation can occur simultaneously, with one end of the mRNA still being made while the other end is attached to a ribosome.

Nearly all living organisms use the same genetic code, which is evidence of the common ancestry of all living organisms.

Section 16 of 30Worked examples

  1. Explain why a purine always pairs with a pyrimidine. [2]2 marks

  2. Describe the role of helicase and topoisomerase. [2]2 marks

  3. Why is replication called semiconservative? [2]2 marks

  4. What is the role of tRNA in translation? [2]2 marks

Section 17 of 30Nucleotides, backbone and base pairs

The nucleic acid DNA is the primary source of heritable information. Genetic information can also be carried by RNA, as in some viruses. It is packaged differently in different organisms: prokaryotic cells contain DNA as a single, circular chromosome and as small circular plasmids, whereas eukaryotes have linear chromosomes in a nucleus.

DNA and RNA are polymers of nucleotides. DNA nucleotides contain a deoxyribose sugar, a phosphate and a nitrogenous base; RNA nucleotides contain a ribose sugar, a phosphate and a base. Nucleotides join to form polynucleotides with a sugar-phosphate backbone, and single strands can join to form double-stranded molecules.

Base pairing occurs because of the differences in structure between the bases.

Purines
Guanine and adenine have a double-ring structure and are purines; cytosine, thymine and uracil have a single ring and are pyrimidines.
A purine always pairs with a pyrimidine,
  • so the base pairs are all the same length : A pairs with T (or U in RNA) and C pairs with G.
    • (a single-ring base matched to a double-ring base)

Section 18 of 30Codons and the genetic code

Translation converts the genetic code in mRNA into a polypeptide.

1
Ribosomes are the site of translation; they are present in the cytoplasm of both prokaryotic and eukaryotic cells and on the rough endoplasmic reticulum of eukaryotic cells.
2
Each mRNA codon, a triplet of bases, corresponds to a specific amino acid, which can be found with a genetic code chart.
3
Many amino acids are coded by more than one codon: for example GAU and GAC both code for aspartic acid.
4
In prokaryotes, transcription and translation can occur simultaneously, with one end of the mRNA still being made while the other end is attached to a ribosome.

Nearly all living organisms use the same genetic code, which is evidence of the common ancestry of all living organisms.

Section 19 of 30Worked example

Worked examplededuce the amino acids for the DNA sense strand 5′-TTCGAGCATTACGCC-3′.

1
Convert the sense strand into the template strand: 3′-AAGCTCGTAATGCGG-5′.
2
Transcribe: the mRNA matches the sense strand with U for T: UUC GAG CAU UAC GCC.
3
Use the genetic code chart for each codon: UUC is in the top-left box (first base U, second base U, third base C), which is Phe.
4
Repeat: GAG = Glu, CAU = His, UAC = Tyr, GCC = Ala.

The sequence is Phe–Glu–His–Tyr–Ala.

Section 20 of 30Retroviruses

Retroviruses are viruses with RNA as their genetic material.

Reverse transcriptase
When they infect a host cell their RNA must be converted into DNA by the enzyme reverse transcriptase before transcription and translation can occur.
The viral genetic material is inserted into the host cell genome,
  • and the host cell then carries out transcription and translation to make viral proteins.
HIV is an example.

Section 21 of 30Exam technique: decoding sequences

Check whether the given DNA strand is the sense (coding) strand or the template strand.

1
The mRNA has the same sequence as the sense strand with U instead of T, and is complementary to the template strand.
2
Group the mRNA bases into triplets from the start codon AUG, read each on the genetic code chart, and write the amino acids with hyphens.
3
A stop codon (UAA, UAG or UGA) ends translation.

Section 22 of 30Worked examples

  1. Deduce the polypeptide from the sense strand 5′-TTCGAGCATTACGCC-3′. [4]4 marks

  2. State two ways prokaryotic DNA differs from eukaryotic DNA. [2]2 marks

  3. Why is the shared genetic code evidence for common ancestry? [2]2 marks

  4. Explain how HIV makes viral proteins. [3]3 marks

Section 23 of 30The types of RNA and their jobs

To make a protein, the DNA base sequence is converted first into an mRNA sequence and then into a sequence of amino acids.

The types of RNA and their jobs
Several types of RNA are involved, whose functions depend on their base sequence and structure.
TRNA
The mRNA sequence is determined by the DNA sequence from which it is copied. tRNA molecules have one end that binds to a specific amino acid and another end with an anticodon that base-pairs with the matching mRNA codon; rRNA forms part of the ribosome.

Section 24 of 30Transcription step by step

Transcription converts information in a DNA template strand into a molecule of mRNA.

1
RNA polymerase binds to the gene and unwinds (unzips) the DNA, exposing the gene to be transcribed.
2
Free RNA nucleotides align with their complementary exposed bases on the template strand (A–U, C–G).
3
RNA polymerase joins the RNA nucleotides with covalent bonds, building the mRNA in the 5′ → 3′ direction.
4
The mRNA detaches and the DNA rewinds.

In eukaryotes the mRNA is modified by enzymes: a cap and a tail are added and introns are removed while exons are spliced together. Different combinations of exons can produce different versions of the mRNA, which is called alternative splicing.

Section 25 of 30Translation and the genetic code

Translation converts the genetic code in mRNA into a polypeptide.

1
Ribosomes are the site of translation; they are present in the cytoplasm of both prokaryotic and eukaryotic cells and on the rough endoplasmic reticulum of eukaryotic cells.
2
Each mRNA codon, a triplet of bases, corresponds to a specific amino acid, which can be found with a genetic code chart.
3
Many amino acids are coded by more than one codon: for example GAU and GAC both code for aspartic acid.
4
In prokaryotes, transcription and translation can occur simultaneously, with one end of the mRNA still being made while the other end is attached to a ribosome.

Nearly all living organisms use the same genetic code, which is evidence of the common ancestry of all living organisms.

Section 26 of 30Genetic information in different organisms

The nucleic acid DNA is the primary source of heritable information. Genetic information can also be carried by RNA, as in some viruses. It is packaged differently in different organisms: prokaryotic cells contain DNA as a single, circular chromosome and as small circular plasmids, whereas eukaryotes have linear chromosomes in a nucleus.

DNA and RNA are polymers of nucleotides. DNA nucleotides contain a deoxyribose sugar, a phosphate and a nitrogenous base; RNA nucleotides contain a ribose sugar, a phosphate and a base. Nucleotides join to form polynucleotides with a sugar-phosphate backbone, and single strands can join to form double-stranded molecules.

Retroviruses are viruses with RNA as their genetic material.

Reverse transcriptase
When they infect a host cell their RNA must be converted into DNA by the enzyme reverse transcriptase before transcription and translation can occur.
The viral genetic material is inserted into the host cell genome,
  • and the host cell then carries out transcription and translation to make viral proteins.
HIV is an example.

Section 27 of 30Exam technique: protein synthesis

Describe the stages in order

  • transcription (DNA to mRNA), processing (eukaryotes), then translation (mRNA to polypeptide).
  • Use the terms template strand, codon, anticodon, RNA polymerase, ribosome, tRNA and peptide bond precisely.
  • Remember that the sequence of amino acids is the primary structure, which folds into the final protein.

Section 28 of 30Worked examples

  1. Describe transcription. [4]4 marks

  2. Describe RNA processing in eukaryotes. [3]3 marks

  3. Explain the role of tRNA. [2]2 marks

  4. Why can prokaryotes make protein faster after a gene is activated? [2]2 marks

Section 29 of 30Key terms

TermMeaning
TranscriptionCopying a DNA template strand into mRNA.
TranslationUsing the mRNA code to build a polypeptide at a ribosome.
CodonA triplet of mRNA bases coding for one amino acid.
AnticodonThree bases on tRNA that pair with an mRNA codon.
SemiconservativeEach new DNA molecule has one old and one new strand.
IntronA non-coding section of a gene removed from the mRNA.
ExonA coding section of a gene retained in mRNA.
RetrovirusA virus with an RNA genome converted to DNA in the host.
TermMeaning
TranscriptionCopying a DNA template strand into mRNA.
TranslationUsing the mRNA code to build a polypeptide at a ribosome.
CodonA triplet of mRNA bases coding for one amino acid.
AnticodonThree bases on tRNA that pair with an mRNA codon.
SemiconservativeEach new DNA molecule has one old and one new strand.
PurineA double-ring base: adenine or guanine.
PyrimidineA single-ring base: cytosine, thymine or uracil.
HelicaseEnzyme that unwinds DNA.
Okazaki fragmentA short fragment of the lagging strand.
TermMeaning
TranscriptionCopying a DNA template strand into mRNA.
TranslationUsing the mRNA code to build a polypeptide at a ribosome.
CodonA triplet of mRNA bases coding for one amino acid.
AnticodonThree bases on tRNA that pair with an mRNA codon.
SemiconservativeEach new DNA molecule has one old and one new strand.
PlasmidA small circular DNA molecule in prokaryotes.
Sense strandThe DNA strand with the same sequence as the mRNA (with T for U).
Template strandThe DNA strand read to make mRNA.
TermMeaning
TranscriptionCopying a DNA template strand into mRNA.
TranslationUsing the mRNA code to build a polypeptide at a ribosome.
CodonA triplet of mRNA bases coding for one amino acid.
AnticodonThree bases on tRNA that pair with an mRNA codon.
SemiconservativeEach new DNA molecule has one old and one new strand.
mRNAMessenger RNA carrying the code from DNA to ribosomes.
tRNATransfer RNA carrying an amino acid.
SplicingJoining exons together after introns are removed.

Section 30 of 30Exam technique

1
Mixing up DNA polymerase and RNA polymerase.
2
Saying translation occurs in the nucleus of eukaryotes.
3
Saying each amino acid has only one codon.
4
Forgetting U replaces T in mRNA.
1
Saying replication is conservative.
2
Saying DNA polymerase starts a new strand without a primer.
3
Saying the lagging strand is made continuously.
4
Confusing codon (mRNA) with anticodon (tRNA).
1
Reading the template strand instead of the sense strand when deducing mRNA.
2
Forgetting that three bases code for one amino acid.
3
Saying plasmids are linear.
4
Saying all viruses have DNA.
1
Saying transcription occurs at ribosomes.
2
Saying tRNA has codons rather than anticodons.
3
Saying mRNA contains thymine.
4
Saying all mRNA is translated immediately in eukaryotes.

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