DNA Structure
What is the shape of DNA called
Double Helix
Double Point Bonus: Draw and label a DNA molecule.
What does "epigenetics" mean?
Changes in gene expression without altering the DNA sequence.
Double point bonus: name two processes that alter gene expression
How many daughter cells are produced in mitosis and meiosis?
Mitosis produces 2 genetically identical diploid cells. Meiosis produces 4 genetically unique haploid cells.
Double Point Bonus: What is the difference in daughter cells produced during spermatogenesis vs oogenesis?
When does DNA replication occur in the cell cycle, and why is this important?
DNA replication occurs during the S phase of interphase. It ensures that each daughter cell receives an identical copy of DNA after cell division.
Where in the cell does transcription occur?
Transcription occurs in the nucleus, where DNA is used as a template to produce a complementary mRNA strand.
Name the four nitrogenous bases found in DNA.
Adenine, Thymine, Cytosine, Guanine.
Give one example of an environmental factor that can influence gene expression.
Environmental factors such as diet, stress, exposure to toxins, or physical activity can alter the chemical modifications of DNA or histones, thereby influencing which genes are expressed and how strongly.
During which phases of the cell cycle do chromosomes condense and spindle fibres form, and why is this important?
Chromosomes condense and spindle fibres form during prophase (mitosis) or prophase I/II (meiosis). This is important because condensed chromosomes are easier to move and spindle fibres ensure accurate chromosome segregation.
Explain why DNA replication is described as "semi-conservative".
Each new DNA molecule consists of one original (parental) strand and one newly synthesised strand, which preserves half of the original DNA in each molecule.
Name the two main types of RNA involved in protein synthesis and describe the role of each.
mRNA (messenger RNA): Carries the genetic code from DNA in the nucleus to the ribosome in the cytoplasm.
tRNA (transfer RNA): Brings specific amino acids to the ribosome and matches them to codons on the mRNA via its anticodon.
How does complementary base pairing work in DNA compared to RNA?
A-T and C-G in DNA. A-U and C-G in RNA.
How does DNA methylation affect gene expression?
DNA methylation adds methyl groups to cytosine bases, usually near gene promoters. This often represses transcription by preventing the binding of transcription factors, effectively silencing the gene.
What is crossing over and in which stage of meiosis does it occur?
Crossing over is the exchange of DNA segments between homologous chromosomes during Prophase I of meiosis. It increases genetic variation in gametes.
What is the role of helicase in DNA replication?
Helicase unwinds and unzips the double helix by breaking hydrogen bonds between complementary bases, creating two single-stranded templates for replication.
What is a codon, and how does it relate to amino acids?
A codon is a sequence of three nucleotides on the mRNA. Each codon specifies a particular amino acid, which is brought to the ribosome by a tRNA with a complementary anticodon, ensuring the amino acids are added in the correct order to build a protein.
Which type of bond holds the two DNA strands together and why is this type important for DNA function?
Hydrogen bonds between complementary bases. They are weak enough to allow strand separation in replication and transcription, but strong enough to hold strands together.
Why might identical twins sometimes look or act differently as they age?
Identical twins have the same DNA sequence, but environmental factors and lifestyle differences can cause changes in epigenetic marks over time. These changes can alter gene expression patterns, resulting in physical, behavioural, or health differences.
Compare the genetic outcomes of mitosis and meiosis.
Mitosis results in two genetically identical diploid cells for growth and repair. Meiosis produces four genetically unique haploid cells, reducing chromosome number by half and increasing genetic diversity through crossing over and independent assortment.
What is the role of DNA polymerase, and why is directionality important in replication?
DNA polymerase adds complementary nucleotides to the 3' end of the growing strand and proofreads for errors. DNA strands are antiparallel, so replication occurs continuously on the leading strand and in fragments on the lagging strand.
Distinguish between transcription and translation.
Transcription is the process of synthesising mRNA from a DNA template in the nucleus. Translation occurs in the cytoplasm at the ribosome, where mRNA codons are read, and tRNA brings corresponding amino acids to build a polypeptide.
DNA strands are described as antiparallel. Explain what this means.
Antiparallel means the two DNA strands run in opposite directions, one from 5' to 3' and the other from 3' to 5'
Explain how histone acetylation can affect gene expression.
Histone acetylation involves adding acetyl groups to histone proteins, which reduces the positive charge on the histones. This weakens their interaction with the negatively charged DNA, causing the DNA to become less tightly wrapped. As a result, the DNA is more accessible to transcription factors and RNA polymerase, increasing gene expression.
Explain how independent assortment contributes to genetic variation.
Independent assortment occurs during metaphase I of meiosis, when homologous chromosome pairs line up randomly. This random orientation results in different combinations of maternal and paternal chromosomes in gametes, increasing genetic variation.
Describe the steps of DNA replication in order, including the roles of key enzymes.
Helicase unwinds the double helix.
Single-strand binding proteins stabilise unwound DNA.
Primase synthesises RNA primers.
DNA polymerase adds nucleotides to the 3' end.
Okazaki fragments are joined on the lagging strand by DNA ligase.
DNA polymerase proofreads and corrects errors.
A DNA sequence is TAC GGA CTA. Write the mRNA strand and the amino acids produced.
mRNA strand: AUG CCU GAU
Amino acids: Methionine – Proline – Aspartic acid
Why is the 5' to 3' direction important in DNA replication?
DNA polymerase can only add nucleotides to the 3' end, so replication is directional.
Describe how epigenetic changes can be inherited even though the DNA sequence does not change.
Some epigenetic marks, such as DNA methylation patterns or histone modifications, can be maintained through cell division and passed to offspring via gametes. This allows traits influenced by the environment or parental experience to affect gene expression in the next generation without altering the underlying DNA sequence.
Explain how errors in meiosis can lead to genetic disorders, giving an example.
Errors like non-disjunction (failure of chromosomes to separate properly) can produce gametes with extra or missing chromosomes. Fertilisation with these gametes can cause genetic disorders, e.g., Down syndrome (trisomy 21), Turner syndrome (monosomy X), or Klinefelter syndrome (XXY).
Explain how errors during DNA replication can lead to mutations, and describe cellular mechanisms that reduce these errors.
Mistakes during replication can change the nucleotide sequence, producing mutations that may alter protein function. DNA polymerase proofreads and removes incorrect nucleotides, and mismatch repair mechanisms correct remaining errors, reducing the mutation rate.
Explain how a mutation in DNA might change the protein that is produced, and give one possible consequence.
A mutation changes the DNA sequence, which may alter the mRNA codons during transcription. This can change the amino acid sequence during translation, potentially altering the protein’s shape and function. Consequences may include loss of function, gain of function, or diseases such as cystic fibrosis or sickle cell anaemia.