4.1 DNA, genes and chromosomes Flashcards
What is a gene?
A section of DNA that contains a specific sequence of bases that code for a polypeptide.
What is a locus?
Location of a gene on a chromosome.
What is an allele?
An alternative form of a gene.
How many chromosomes do humans have?
46 (23 pairs).
What is a homologous chromosome?
2 copies of the same chromosome (1 from the mother, and 1 from the father).
Homologous chromosomes are the same __________ and have exactly the same genes?
Size, genes.
What can be different about homologous chromosomes?
They may have different alleles.
How is DNA stored?
As chromosomes in the nucleus.
What is the shape of chromosomes in eukaryotic cells?
Linear.
How is DNA arranged in chromosomes?
Coiled & wrapped around histones (proteins).
Describe the DNA in prokaryotic cells.
- Stored in CIRCULAR chromosomes
- DNA is shorter
- DNA is NOT associated with proteins.
Do mitochondria and chloroplasts have their own DNA?
YES.
What is the DNA in mitochondria and chloroplasts used for?
Codes for enzymes that are essential for aerobic respiration + photosynthesis, respectively.
How does mitochondria and chloroplast DNA compare to the DNA found in the nucleus of cells?
Mitochondria and chloroplast DNA is shorter and circular, and is NOT histone bound.
What are 3 features of the DNA code?
- Degenerate
- Universal
- Non-overlapping.
What is the purpose of start and stop codons?
- The start codon marks the site at which translation into protein sequence begins, and the stop codon marks the site at which translation ends.
- Ensures that cells produce the correct sequences of amino acids.
Degenerate:
Multiple codons code for the same amino acid.
Universal:
The same triplet of bases code for the same amino acid in all organisms.
Non-overlapping:
- Each base in a gene is only part of ONE triplet of bases that codes for ONE amino acid.
- Therefore, each codon, or triplet of bases, is read as a discrete unit.
What is an advantage of the genetic code being non-overlapping?
If a point mutation occurs, it will only affect 1 codon, and therefore only 1 amino acid.
Introns:
- Non-coding regions of DNA.
Are introns found in eukaryotic and prokaryotic DNA?
NO - ONLY eukaryotic.
What happens to introns after transcription?
Removed (spliced) out of mRNA molecules.
Exons:
The sections of DNA that DO code for amino acids.
Genome:
An organism’s complete set of DNA in one cell.
Proteome:
The full range of proteins in one cell.
Does the genome ever change?
NO.
Does the proteome change?
Yes -> it is constantly changing, depending on which proteins are currently needed.
Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule.
- Condensation reaction
- Between phosphate and deoxyribose
- Catalysed by DNA polymerase.
Describe how the separation of strands occurs in semi-conservative DNA replication.
- DNA helicase
- Breaks hydrogen bonds between complementary base pairs.
Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus.
- Chloroplast DNA is shorter
- Chloroplast DNA has fewer genes
- Chloroplast DNA is circular (nucleus DNA = linear)
- Chloroplast DNA not associated with proteins (nuclear DNA is).
Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide. Give two reasons why.
- Triplets code for the same amino acid (degenerate)
- Occurs in introns / non-coding sequence.
Explain how the organic bases help to stabilise the structure of DNA.
- Hydrogen bonds between the base pairs holds two strands together
- Many hydrogen bonds provides strength.
Describe two differences between the structure of a tRNA molecule and the structure of an mRNA molecule.
- tRNA is ‘clover leaf shape’, mRNA is linear;
- tRNA has hydrogen bonds, mRNA does not;
- tRNA has anticodon, mRNA has codon.
Describe and explain a difference in the structure of mRNA and pre-mRNA.
- mRNA has fewer nucleotides
- mRNA has no introns (only exons)
- Because of splicing.
Why is the genetic code described as being universal?
In all organisms, the same triplet codes for the same amino acid.
What is the process that removes base sequences from pre-mRNA to form mRNA?
Splicing.
What is the evidence from the diagram that DNA is a polymer?
Repeating units / nucleotides / monomer / molecules.
