Module 4: Genetic Information, Variation & Relationships Flashcards
Describe prokaryotic DNA?
- Short, circular DNA molecules
- Not associated with histones
- Free floating in the cytoplasm
- Majority is coding DNA
Describe eukaryotic DNA?
- Long, linear DNA molecules
- Associated with histones
- Majority is non-coding DNA
- Located in the nucleus
What is a gene?
A gene is the base sequence of DNA that codes for a polypeptide or functional RNA.
What is a locus?
The fixed position of a gene on a particular DNA molecule.
What is a triplet?
A sequence of three DNA bases.
What are three features of the genetic code?
- Degenerate (some amino acids are coded for by more than one base triplet).
- Universal (each triplet codes for the same amino acids in all organisms).
- Non-overlapping (each base triplet is read once in the sequence & separate from the triplet before it and after it).
What is a codon?
Sequence of three bases on DNA.
What is an anticodon?
A group of three bases on a tRNA molecule.
What is an intron?
A non-coding sequence of bases within a gene.
What is an exon?
A sequence of bases within a gene that code for an amino acid.
What are multiple repeats?
Regions of non-coding DNA base sequences that are repeating.
What is the genome?
The complete set of genes in a cell.
What is the proteome?
The full range of proteins a cell is able to produce.
What is transcription?
The process where the DNA code is copied into mRNA.
What is translation?
The process where mRNA joins with a ribosome for protein synthesis to occur
Describe the structure of mRNA
- Linear shape
- Single polynucleotide strand
- A group of three adjacent bases form a codon
Describe the structure of tRNA
- Clover shape
- Single polynucleotide strand
- Hydrogen bonds present between specific base pairs
- Has an amino acid binding site at one end
- Specific sequence of three bases form an anticodon at the other end
Where does transcription occur in eukaryotes?
The nucleus
Where does transcription occur in prokaryotes?
The cytoplasm
What is the process of transcription?
- RNA polymerase attaches to DNA double helix.
- DNA helicase on RNA polymerase breaks hydrogen bonds between the two DNA strands, unwinding the DNA to expose the bases.
- One DNA strand acts as a template.
- The free RNA nucleotides are attracted to the exposed bases on the template strand & bind via complementary base pairing.
- RNA polymerase reforms the hydrogen bonds & the strands coil back into a double helix, forming pre-mRNA.
- RNA polymerase detaches from the DNA strand when it reaches a ‘stop’ triplet.
What is produced during transcription in a prokaryote?
mRNA
What is produced during transcription in a eukaryote?
pre-mRNA (containing introns & exons).
What is splicing?
The process of removing introns & joining exons in a pre-mRNA strand to form mRNA.
What is the process of translation?
- mRNA binds to the ribosome at the start codon.
- A tRNA molecule carries a specific amino acid (with a complimentary anti codon) binds to the codon on the mRNA molecule.
- A second tRNA molecule binds to the next codon on the mRNA.
- The two amino acids attached to the tRNA molecules are joined by a peptide bond (hydrolysis of ATP provides the energy).
- The first tRNA molecule moves away leaving behind its amino acid.
- The ribosome moves along the mRNA to produce the polypeptide chain
Where does translation occur in prokaryotes & eukaryotes?
At the ribosomes in the cytoplasm.
What are alleles?
Alleles are different versions of the same gene with a slightly different base sequence. They code for different versions of the same polypeptide.
What is a gene mutation?
A gene mutation is a change in the DNA base sequence of chromosomes that can occur spontaneously during DNA replication.
What is a substitution mutation?
A mutation where one base is substituted with another base (e.g. ATGCCT becomes ATTCCT as G is swapped for T).
What is a deletion mutation?
A mutation where one base is deleted (e.g. ATGCCT becomes ATCCT as G is removed).
Why don’t all substitution mutations result in changes to the amino acid sequence.
Due to the degenerate nature of the genetic code, some amino acids are coded for by more than one DNA triplet. Therefore, some substitutions will still code for the same amino acid.
Why does a deletion substitution always result in changes to the amino acid sequence.
Because the deletion of a base changes the number of bases present which causes a frame shift in all the base triplets read after it.
What is a mutagenic agent?
- A mutagenic agent causes an increase in the rate of gene mutations & increases the probability of a mutation occurring.
What are chromosome mutations caused by & how do they occur?
- Chromosome mutations are caused by errors in meiosis (cell division).
- They occur when the cell produced has variations in the number of chromosomes (e.g. they don’t possess 23 whole chromosomes).
What can chromosome mutations lead to?
They can lead to inherited conditions.
What are some examples of mutagenic agents?
- Ultraviolet radiation
- Ionisation radiation
- Some chemicals
- Some viruses
Why do chromosome mutations lead to inherited conditions?
Because the errors are present in the gametes (the hereditary cells).
What is a non-disjunction chromosome mutation?
- Non-disjunction is the failure of the chromosomes to separate properly.
- Non-disjunction of chromosome 21 can lead to Down’s syndrome.
Why is random fertilisation important?
- Because it produces zygotes with different combinations of chromosomes.
- It increases genetic diversity within a species.
Where does meiosis occur?
It occurs in the reproductive organs of multicellular, eukaryotic organisms.
Explain the process of meiosis?
- The DNA unravels & replicates to produce two copies of each chromosome (chromatids).
- DNA condenses to form double-armed chromosomes (each made from two sister chromatids joined by a centromere).
- Meiosis I - the chromosomes arrange themselves into homologous pairs & then separate, halving the chromosome number.
- Meiosis II - the pair of sister chromatids that make up each chromosome are separated.
- Produces four haploid cells that are genetically different.
Name the two main events during meiosis that lead to genetic variation.
- Crossing over of chromatids.
- Independent segregation of chromosomes.
Explain the process of crossing over.
- Occurs during meiosis I when the homologous chromosomes pair up.
- The chromatids twist around each other & bits of chromatids swap over.
- The chromatids still contain the same genes but now possess a different combination of alleles.
Explain how independent segregation of chromosomes leads to genetic variation.
- When the homologous pairs of chromosomes separate during meiosis I, it’s completely random which chromosome ends up with which daughter cell.
Compare the differences between mitosis & meiosis
Mitosis:
- Produces cells with the same number of chromosomes as the parent cell.
- Daughter cells are genetically identical to each other & to the parent cell.
- Produces 2 daughter cells.
- 1 division occurs.
Meiosis:
- Produces cells with haploid number of chromosomes as parent cell.
- Daughter cells are genetically different to one another & parent cell.
- Produces 4 daughter cells.
- 2 divisions occur.