DNA, Genes and Chromosomes and The Genome
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of DNA, genes and chromosomes from GCSE. You can test your knowledge on these below.
What is a gene?
A section of DNA found at a particular locus. The sequence of bases within a gene determines the sequence of amino acids during protein synthesis.
Where is DNA found in eukaryotic and prokaryotic cells?
- In eukaryotic cells, DNA is located in the nucleus, (There is also some circular DNA in chloroplasts and mitochondria).
- In prokaryotic cells, DNA is circular and found freely in the cytoplasm (not in a nucleus).
What is a chromosome?
A chromosome is a long, coiled structure of DNA and proteins that carries genes.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains DNA, genes and chromosomes and the genome or read the full notes below. Once you've gone through the whole note, try out the practice questions!
DNA
- DNA (deoxyribonucleic acid) is the genetic material that carries the code for protein synthesis.
- It is a double-stranded helix made of many nucleotides containing:
- A deoxyribose sugar.
- A phosphate group.
- A nitrogenous base (A, T, C, G).
- The sequence of bases in DNA determines the order of amino acids in a polypeptide.
Gene
- Gene: A section of DNA found at a particular locus. The sequence of bases within a gene determines the sequence of amino acids during protein synthesis.
- Each gene contains a sequence of bases, read in sets of three, called triplets (or codons in mRNA). Each triplet codes for an amino acid.
- A gene has a specific location, or locus, on a chromosome.
Chromosomes
- Chromosomes are long, coiled molecules of DNA that contain genes.
- In eukaryotic cells, DNA is tightly wound around histone proteins to fit inside the nucleus. The DNA and proteins form the chromosomes.
- We can say chromosomes are made of chromatin. Chromatin = DNA + histone proteins.
- Humans have 46 chromosomes (23 pairs) in a body cell, with one set inherited from each parent.
Genome
- The genome is the complete set of DNA in an organism.
- The human genome contains approximately 3 billion base pairs and 20,000 genes.
Differences Between Prokaryotic and Eukaryotic DNA
| Feature | Eukaryotic DNA | Prokaryotic DNA |
| Structure | Linear (forms chromosomes). | Circular (plasmids & main circular DNA called the nucleoid). |
| Location | Found in the nucleus. | Free in the cytoplasm. |
| Associated Proteins | Wrapped around histone proteins. | Not associated with histones. |
| Introns | Contains introns (non-coding regions). | No introns (only coding regions). |
| Plasmids | Not present. | Often present (small circular DNA). |
Remember, eukaryotic cells will also have DNA in their mitochondria and chloroplasts (if present). This DNA will be similar to prokaryotic DNA. For example:
- This DNA is circular.
- This DNA is not associated with histone proteins.
- There is much less DNA in the organelles than in the nucleus.
Key Terms
- DNA (Deoxyribonucleic Acid): The molecule that carries genetic instructions.
- Gene: A section of DNA found at a particular locus. The sequence of bases within a gene determines the sequence of amino acids during protein synthesis.
- Chromosome: A long, coiled DNA molecule containing many genes.
- Genome: The entire genetic material of an organism.
- Histones: Proteins that help package and organise eukaryotic DNA.
- Plasmids: Small, circular DNA molecules found in prokaryotic cells.
- Chromatin: DNA + histone proteins that make up the chromosomes.
No answer provided.
Exam Tips
When comparing the structure of Prokaryotic vs. Eukaryotic DNA, focus on:
- Circular vs. linear.
- Presence of histones.
- Presence of introns.
- Do not talk about the location of the DNA if the question asks about structure.
Learn the exam specific definitions for terms such as Gene and Genome.
No answer provided.
Describe the differences between prokaryotic and eukaryotic DNA. (5 marks)
- Eukaryotic DNA is linear, whereas prokaryotic DNA is circular (1)
- Eukaryotic DNA is associated with histone proteins, while prokaryotic DNA is not. (1)
- Eukaryotic DNA contains introns (non-coding sequences), whereas prokaryotic DNA does not. (1)
- Some prokaryotic cells contain plasmids, which are small circular DNA molecules, whereas plasmids are not found in eukaryotic cells. (1)
- Eukaryotic DNA is found enclosed within a nucleus whereas prokaryotic DNA is in the cytoplasm. (1)
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!