DNA: the code of life

PAPER 2 (27 MARKS)

SET ON PREVIOUS PAPERS

EXAMINABLE CONTENT ONLY

Set&COMPILED BY MOLOI NL
2023

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NTRODUCTION

The human body is made up of numerous cells, the nucleus of each cell contains 46 chromosomes, 23
paternal& 23 maternal. The chromosomes are made up of the DNA which carries genetic information/
material. On the DNA there are specific segments/ parts which codes for a particular characteristic
called the genes. The position of a gene on a chromosome is called a locus (plural: loci).

NB: To be learnt on the next chapter MEIOSIS

STRUCTURE AND LOCATION OF DNA

• DNA stands for DeoxyribuNucleicAcid.

• DNA is a nucleic acid
• DNA is a double stranded molecule
• The natural shape of DNA is double helix
• Each strand of a DNA is made up of monomers/ building blocks called nucleotides.
• A nucleotide has three components; Phosphate group, Sugar (Deoxyribose in DNA); Nitrogenous
base (Adenine &Thymine; Cytosine &Guanine)
NB: Thymine is only found in DNA
• A chain of Nucleotides forms a strand of a DNA
• The two strands of DNA are joined together by the weak hydrogen bonds to form a helical shape

LOCATION OF DNA

• In animal cells, the DNA is found mainly in the nucleus (nuclear DNA). Some of the DNA is found
in the mitochondria (mitochondrial DNA/mtDNA).
• In plant cells, the DNA is found in the nucleus and chloroplast.

FUNCTIONS OF DNA

• DNA controls protein synthesis/ contains coded information for protein synthesis.
• DNA passes/ transmits genetic information from parents to offspring

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DISCOVERY OF DNA

-Many studies were carried out to discover the DNA and its structure, but only few studies will be
focused on.

• Maurice Wilkins carried out his studies at King`s College London but he was working along with
Rosalind Franklin
• Rosalind Franklin produced the X-ray picture of DNA
• Wilkins disliked Franklin, he stole the X-ray picture and showed it to James Watson and Francis
Crick
• Watson and Crick were able to create the DNA model using Franklin findings
• Watson, Crick, and Wilkins received the Nobel Prize for discovering the structure of DNA
• Unfortunately, Rosalind wasn’t honored because she died in the 1958, age of 37 due to cancer.
NB: in an exam you may be asked the scientists who discovered the structure of DNA, a
candidate is expected to choose Watson and Crick
The scientist who produced the X-RAY picture of DNA, a candidate is expected to choose
Franklin.

The above picture shows

The above picture shows
Maurice Wilkins, New The above picture shows the
Rosalind Franklin, a British
Zealand-British Watson and Crick model.
chemist
biophysicist.
(25-07-1920 to 16-04-1958)
(15-12-1916 to 5-10-2004)

X-RAY diffraction produced by

Rosalind Franklin

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DNA REPLICATION

-A process whereby DNA makes extra copies of itself that are identical to the original strand.

-DNA replication takes place in the nucleus during interphase which is a phase of a cell cycle before cell
division.

Process of DNA replication

-The double helix unwinds/ untwists√

-the weak hydrogen bonds break√

-the two strands unzip/ separate√

-each strand serves as a template to pick up free DNA nucleotides√ in the nucleus according to the
complementary base rule (Adenine pairs with Thymine; Guanine pairs with Cytosine) √

-this process is controlled by enzymes. √

Significance of DNA replication

• Doubles genetic information√, so that each daughter cell at the end of meiosis has enough
genetic information. √

DNA PROFILING/ FINGERPRINTING

-Arrangement of DNA fragments/ black bars showing the gene sequence.

Uses of DNA Profiling

• To investigate crimes
• To identify organisms from their remains
• To identify family relationships other than paternity
• To establish matching tissues for organ transplants

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RNA

• RNA stands for Ribonucleic acid

• RNA is a single stranded molecule
• RNA is a nucleic acid
• The strand of RNA is made up monomers/ building blocks called nucleotides
• Nucleotide of RNA consists of three components; Phosphate group, Sugar (Ribose in RNA),
Nitrogenous bases (Adenine pairs with Uracil; Guanine pairs with Cytosine)
• Three types of RNA; transfer RNA (tRNA) and messenger RNA (mRNA) and ribosomal RNA (rRNA)
• The base triplets on mRNA are called codons and the base triplets on tRNA are called
anticodons.

NB: Candidates must only focus on mRNA and tRNA.

DIFFERENCES BETWEEN DNA AND RNA

DNA RNA
-Double stranded -Single stranded
-Double helix -Straight
-Sugar Deoxyribose -Sugar Ribose
-Nitrogenous base Thymine -Nitrogenous base Uracil
-Long molecule -Short molecule

NB: A candidate must be able to provide 3 differences

DIFFERENCES BETWEEN MONOMERS OF DNA AND RNA

DNA monomer RNA monomer

-Sugar Deoxyribose -Sugar Ribose
-Has Thymine -Has Uracil

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PROTEIN SYNTHESIS

-A process in a cell where proteins are formed/ produced.

-it takes place in two stages Transcription and Translation.

-Transcription takes place in the nucleus and translation in the ribosome of the cytoplasm.

Process of Transcription

-The double helix unwinds/ untwists√

-weak hydrogen bonds break√

-the stands unzip/ separate√

-Only one strand serves as a template to pick up free RNA nucleotides√ according to the complementary
base rule/ (A pairs with Uracil; G pairs with C) √

-to form the mRNA√

-this process is controlled by enzymes. √

Process of Translation

-The mRNA leaves the nucleus and enters the ribosome√

-if the anticodon on the tRNA matches the codon on mRNA √

-the tRNA brings the required amino acid√

-to the ribosome√

-amino acids are joined by peptide bonds√

-to form the required protein

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DIFFERENCES BETWEEN DNA REPLICATION AND TRANSCRIPTION

DNA REPLICATION TRANSCRIPTION

-Both strands act as templates√ -One strand acts as a template√
-DNA nucleotides are picked√ -RNA nucleotides are picked√
-A pairs with T√ -A pairs with U√

NB: A candidate must be able to provide 2 differences & a compulsory mark is given for tabulating.

HOW GENE MUTATION INFLUENCES THE STRUCTURE OF A PROTEIN

-A mutation is a change in the nucleotide/nitrogenous base sequence of a DNA molecule.

Scenario 1

-Gene mutation results in the change of mRNA/codons√

-As a result, different tRNA molecules√

-carrying different amino acids√ will be required

-The sequence of amino acids changes √

-resulting in the formation of a different protein√.

Scenario 2

-If the same amino acid√ is coded for

-There will be no change√ in the protein structure.

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