Docsity
Docsity
Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity

Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan

Guidelines and tips
Guidelines and tips
Sell on Docsity
Sell on Docsity
Docsity AI
Docsity AI
Log inSign up

Prepare for your exams

Study with the several resources on Docsity

Find documents

Prepare for your exams with the study notes shared by other students like you on Docsity

Search for your university

Find the specific documents for your university's exams

Docsity AINEW

Summarize your documents, ask them questions, convert them into quizzes and concept maps

Explore questions

Clear up your doubts by reading the answers to questions asked by your fellow students

Earn points to download

Earn points by helping other students or get them with a premium plan

Share documents
download point icon20 Points

For each uploaded document

Answer questions
download point icon5 Points

For each given answer (max 1 per day)

All the ways to get free points
Get points immediately

Choose a premium plan with all the points you need

Study Opportunities

Choose your next study program

Get in touch with the best universities in the world. Search through thousands of universities and official partners

Community

Ask the community

Ask the community for help and clear up your study doubts

Free resources

Our save-the-student-ebooks!

Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors

AQA A level Biology DNA topic study notes, Study notes of Biology

Biology

Study notes for the a level topic in title.

Typology: Study notes

2023/2024

Uploaded on 11/22/2025

jace-walker-1
jace-walker-1 🇬🇧

1 document

1 / 10

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
DNA, genes, chromosomes and
protein synthesis (3.4.1-3.4.2)
Specification Reference: 3.4.1 DNA, genes and chromosomes
Content
•In prokaryotic cells, DNA molecules are short, circular and not associated with
proteins.
•In the nucleus of eukaryotic cells, DNA molecules are very long, linear and
associated with proteins, called histones.
◦Together a DNA molecule and its associated proteins form a chromosome.
•The mitochondria and chloroplasts of eukaryotic cells also contain DNA which,
like the DNA of prokaryotes, is short, circular and not associated with protein.
•A gene is a base sequence of DNA that codes for:
◦the amino acid sequence of a polypeptide
◦a functional RNA (including ribosomal RNA and tRNAs).
•A gene occupies a fixed position, called a locus, on a particular DNA molecule.
•A sequence of three DNA bases, called a triplet, codes for a specific amino
acid.
•The genetic code is universal, non-overlapping and degenerate.
•In eukaryotes, much of the nuclear DNA does not code for polypeptides.
◦There are, for example, non-coding multiple repeats of base sequences
between genes.
◦Even within a gene only some sequences, called exons, code for amino acid
sequences.
◦Within the gene, these exons are separated by one or more non-coding
sequences, called introns.
pf3
pf4
pf5
pf8
pf9
pfa

Partial preview of the text

Download AQA A level Biology DNA topic study notes and more Study notes Biology in PDF only on Docsity!

DNA, genes, chromosomes and

protein synthesis (3.4.1-3.4.2)

Specification Reference: 3.4.1 DNA, genes and chromosomes

Content

  • In prokaryotic cells , DNA molecules are short , circular and not associated with proteins.
  • In the nucleus of eukaryotic cells , DNA molecules are very long , linear and associated with proteins , called histones. ◦ Together a DNA molecule and its associated proteins form a chromosome.
  • The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short , circular and not associated with protein.
  • A gene is a base sequence of DNA that codes for: ◦ the amino acid sequence of a polypeptide ◦ a functional RNA (including ribosomal RNA and tRNAs ).
  • A gene occupies a fixed position , called a locus , on a particular DNA molecule.
  • A sequence of three DNA bases, called a triplet , codes for a specific amino acid.
  • The genetic code is universal , non-overlapping and degenerate.
  • In eukaryotes , much of the nuclear DNA does not code for polypeptides. ◦ There are, for example, non-coding multiple repeats of base sequences between genes. ◦ Even within a gene only some sequences, called exons , code for amino acid sequences. ◦ Within the gene, these exons are separated by one or more non-coding sequences, called introns.

Specification Reference: 3.4.2 DNA and protein synthesis

Content

  • The concept of the genome as the complete set of genes in a cell and of the proteome as the full range of proteins that a cell is able to produce.
  • The structure of molecules of messenger RNA (mRNA) and of transfer RNA (tRNA).
  • Transcription as the production of mRNA from DNA.
  • The role of RNA polymerase in joining mRNA nucleotides. ◦ In prokaryotes , transcription results directly in the production of mRNA from DNA. ◦ In eukaryotes , transcription results in the production of pre-mRNA ; this is then spliced to form mRNA.
  • Translation as the production of polypeptides from the sequence of codons carried by mRNA.
  • The roles of ribosomes, tRNA and ATP. Students should be able to:
  • relate the base sequence of nucleic acids to the amino acid sequence of polypeptides, when provided with suitable data about the genetic code
  • interpret data from experimental work investigating the role of nucleic acids. Students will not be required to recall in written papers specific codons and the amino acids for which they code.
  • The start of the sequence for a polypeptide is always the same codon (Methionine) ◦ note that methionine can be removed from the molecule if it is not need in the final protein
  • There are three codons (UAG, UGA, UAA) which do not code for an amino acid, and instead signal translation to stop. ◦ These are known as STOP codons or TERMINATION codons (TERM for short)
  • Genetic code is ‘ non-overlapping ’ which means that each base in a sequence is part of only one codon, and only read once ◦ An easy way to think about this is to imagine a sequence AGGCGG ◦ This sequence will be read as AGG then CGG, not AGG, GGC, GCG, CGG
  • Genetic code is mostly universal ◦ This means that across almost all organisms, the same codon codes for the same amino acid ◦ This provides some indirect evidence for evolution In eukaryotes, a lot of the DNA does not code for anything, and instead is responsible for many regulatory functions. For example, there are non-coding sections of DNA between genes which consist of multiple repeats of base sequences. These sections are commonly referred to as untranslated regions (or UTRs). Within genes there are also non-coding sections. The sections that code for amino acids are called exons , and the non-coding sections are called introns. Also, remember that some genes code for rRNAs (ribosomal RNAs) and tRNAs (transfer RNAs). The genome is the complete set of genes in a cell, which includes those present in mitochondria/chloroplasts. The proteome is the full range of proteins coded for in the genome. Sometimes, this is called the complete proteome. When this is the case, the word proteome instead refers to the full range of proteins produced by a given cell or cell type in given conditions. Don’t forget that uracil takes the place of thymine in RNA.

Eukaryotes vs Prokaryotes

In prokaryotes, DNA molecules are shorter, circular and not associated with proteins (therefore they cannot form chromosomes). Prokaryotic DNA also does not contain any introns. In eukaryotes, DNA is longer, linear and associated with proteins called histones, allowing them to form chromosomes. There is also DNA present in the mitochondria and chloroplasts of eukaryotic cell which is similar to that of prokaryotes in nature (short, circular and not protein bound). See section 3.2 for all differences between eukaryotes and prokaryotes.

Chromosomes

DNA associates with histones, forming a DNA-histone complex. This DNA-histone complex coils tightly, and then those coils loop and pack together to form chromosomes. Chromosomes are only visible as distinct structures of DNA during mitosis, when the genetic material condenses. The general structure of a chromosome consist of thread-shaped sections of DNA (called chromatids) attached together by a specialised region called a centromere. Chromosomes allows a large length of DNA to be packed very tightly. The number of chromosomes in the members of one species is usually the same, but the number typically varies from species to species. Homologous chromosomes are pairs of chromosomes that always carry the same genes in the same loci, but not necessarily the same alleles. One chromosome (the paternal chromosome) in the pair is inherited from the father, and the other (the maternal chromosome) is inherited from the mother. The number chromosomes in the homologous pairs in a genome is referred to as the diploid number (46 in humans). An allele is an alternate form of a gene. Genes exist in several different forms, with different base sequences, called alleles. Each allele codes for a different sequence of amino acids. Different inherited allele combinations leads to different proteins being produced (different phenotypes).

RNA Structures RNA is a single stranded polynucleotide chain, with each molecule containing the following:

  • The pentose sugar ribose
  • A nitrogenous base (adenine, uracil, cytosine or guanine)
  • A phosphate group Since genetic code is stored in the nucleus and translation occurs in the cytoplasm, the genetic material must be transported out of the nucleus. This happens in a process called transcription, where the DNA is transcribed onto smaller RNA molecules to act as messengers (hence the name, messenger RNA). These molecules are small, so fit easily through the nuclear pores.

mRNA, Transcription and Splicing

mRNA consists of thousands of nucleotides in a chain, twisted to form a single α-helix. It is complementary to the template strand of the gene it was transcribed from, thereby being essentially identical to the coding strand. mRNA interfaces with ribosomes to act as a template for protein synthesis. It is adapted for its function as it is a linear sequence of codons which can bind to the small subunit (SSU) of a ribosome to act as a template for protein synthesis. Transcription is the process of making pre-mRNA (or mRNA in prokaryotes, but he this section focuses of eukaryotes) strands from template strands of DNA. The process occurs as follows:

  1. An enzyme catalyses the breakdown of hydrogen bonds between base pairs in a specific region of DNA, exposing the base pairs.
  2. The free activated nucleotides (nucleoside triphosphates) in the nucleoplasm align with the bases on the template strand (3’ to 5’)
  3. The enzyme RNA polymerase catalyses the formation of phosphodiester bonds between the free nucleotides in the 5’ to 3’ direction, forming a pre-mRNA molecule
  1. As RNA polymerase catalyses the formation of the pre-mRNA molecule, the strands of the DNA molecule behind it close up, so that only about 12 bases are exposed at one time.
  2. When a STOP codon is reached, the production of the pre-mRNA is complete. However, this pre-mRNA molecule (premature messenger RNA) still contains introns, which do not code for amino acids. (note that in prokaryotes, transcription results directly in mRNA, and splicing is not needed) They are removed in a process called splicing, resulting in a mRNA molecule (mature messenger RNA) The mRNA then leaves the nucleus by a nuclear pore, as it is too big to diffuse, and is attracted to ribosomes.

tRNA and Translation

tRNA is relatively small (around 80 nucleotides), single stranded and folded into a clover shape. One end of the chain extends beyond the other, acting as an amino acid attachment site. There is base pairing in tRNA, but it is still only a single stranded molecule. The section with the three complementary bases to a codon is called the anticodon loop. The anticodon is complementary to one codon, and the attachment site is complementary to only on amino acid. This means that the right amino acid can be attached to the codon that codes for it. There are around 60 tRNAs, each with a unique anticodon. This means each amino acid can attach to multiple different tRNAs. Once mRNA reaches a ribosome, it acts as instructions for protein synthesis by translation, which occurs as follows:

  1. A ribosome attaches to the start codon (AUG) of a mRNA at the binding site in the SSU (small subunit).
  2. The tRNA with the complementary anticodon and an attached amino acid (Met) pairs up with the codon.
  3. A tRNA with a complementary anticodon to the next codon pairs up with the next codon, carrying another amino acid.
  4. The ribosome moves along the mRNA, bringing together the two tRNA molecules.
  5. A peptide bond forms between the two amino acids. This is catalysed by an enzyme, and the energy required comes from the hydrolysis of ATP.