DNA Flashcards
Role of DNA
Carries genetic information to determine our inherited characteristics
What is a gene?
Section of DNA that codes for a polypeptide and a functional RNA
What is a locus?
Location of a particular gene on a chromosome
What is a chromosome?
- DNA is stored in chromosomes
- 23 pairs = 46 in total
What is a homologous pair of chromosomes?
Pair of chromosomes with the same genes but different alleles
How is DNA stored?
- Stored as chromosomes in nucleus
- Chromosomes are linear so to fit tightly coiled DNA in nucleus = DNA wound around histone proteins = nucleosome
Difference between prokaryotic/eukaryotic DNA
EUKARYOTIC:
- Nucleus
- Linear
- Associated with proteins = chromosomes
- Mitochondria/chloroplast contain DNA
PROKARYOTIC:
- Free- floating
- Short + circular
- Not associated with proteins
What is the genetic code?
-The order of bases on DNA
- Consists of codons
Features of genetic code
- Degenerate
- Universal
- Non-overlapping
Describe degenerate and it’s advantage
- More than 1 triplet base codes for the same amino acid
- Advantageous: If mutation occurs and triplet base is different it may still code for the same amino acid = have no effect
Describe universal and it’s advantage
- Same triplet bases code for the same amino acids in all organisms
- Advantageous: Allows genetic engineering e.g. inserting human insulin gene in bacteria for mass produce
Describe non-overlapping and it’s advantage
- Each base is only part of 1 triplet code
- Codons are read as discreet units
- Advantageous: If mutation occurs it only affects 1 codon = 1 protein
What is an intron?
- Sections of DNA that do not code for amino acids
- Only in eukaryotic cells
- Spliced out of mRNA
What is an exon?
Sequences of bases that do code for amino acids
Describe genome
- Organisms full set of DNA contained by a cell
- Should never change
- Vary widely from organism to organism
Describe proteome
- Organisms full range of proteins that can be produced by a cell
- Constantly changing depending on which protein is needed
Describe structure of mRNA
- Long single strand
- Complementary to DNA
- 3 bases are called codons
- Carries the genetic code of 1 gene to the ribosome
Describe structure of tRNA
- Single strand folded into a cloverleaf shape by H-bonds
- Found in cytoplasm
- Amino acid binding site at top
- 3 complementary bases to mRNA codons called anticodon at the bottom
- Transfers amino acids to ribosome
Describe structure of rRNA
Combines with protein to make ribosomes
Compare RNA + DNA
MONOMERS:
- DNA has thymine RNA has uracil
- DNA pentose sugar = deoxyribose RNA pentose sugar = ribose
POLYMERS:
- DNA is larger as it has the whole genome RNA is shorter
- DNA double stranded RNA single stranded
2 stages of protein synthesis
1) Transcription: Gene of DNA copied into mRNA
2) Translation: mRNA joins with ribosome + corresponding tRNA brings specific amino acids
Explain transcription
- DNA helicase breaks H-bonds between bases = helix unwinds to expose bases
- 1 chain of DNA acts as a template
- Free mRNA nucleotides in nucleus align opposite complementary DNA bases
- RNA polymerase bonds together RNA nucleotides = new pre-mRNA
- introns spliced out = only exons = mRNA leaves nucleus via nuclear pores
Explain translation
- Modified mRNA attaches to ribosome at start codon
- tRNA with complementary anti-codons align and held in place by ribosome
- Ribosome moves along mRNA = another complementary tRNA to next mRNA codon
- Once 2 amino acids have been bought by tRNA = form peptide bond through enzyme +ATP
- Continues until stop codon reached = ribosome detaches = translation ends
- Polypeptide chain moves to Golgi apparatus = folding + modify
What is meiosis?
- Cell division = 4 genetically different haploid cells
- Gametes
How is variation introduced in meiosis 1?
- Independent segregation
- Crossing over
Describe independent segregation in meiosis 1
- Homologous pairs line opposite each other at equator
- Paternal/maternal chromosomes align randomly
- Pairs are separated so each homologous pairs ends up in a daughter cell
How to calculate combinations of chromosomes from independent segregation
2^n
n= number of homologous pairs
Describe crossing over in meiosis 1
- Homologous pairs line up opposite at equator
- Form chiasma
- Chromatids can become twisted = tension = chromatids break
- Lengths of chromatids exchanged
- Broken parts recombine = new combination of alleles
Compare mitosis/meiosis
MEIOSIS:
- 2 divisions
- Haploid cells
- Genetic variation
MITOSIS:
- 1 division
- Diploid cells
- Genetically identical
How to identify meiosis in a life cycle
The shift from diploid (2n) to haploid (n)
How does fertilization increase variation?
- Fertilization is random
- It is random which egg will fuse with which sperm = increased variation
What is a mutation?
Alteration of DNA base sequence that happens during DNA replication
Describe addition mutation
- 1 extra base is added
- Causes a frame shift
- Could be harmful = code for very different amino acids = different protein
Describe deletion mutation
- Delete 1 base
- Causes a frame shift = could code for a very different amino acid = different protein
Describe substitution mutation
- Base is switched with another
- No frame shift = less damage = degenerate code may code for same amino still
Describe inversion mutation
- Section of base detach and re-join inverted = different amino coded for different region
What is chromosome non-disjunction?
Chromosomes fail to separate correctly during meiosis resulting in more or less chromosomes than normal
Describe polyploidy
- Changes in whole sets of chromosomes
- End up with 3 or more sets instead of 2 when gametes fuse
Describe aneuploidy
- Change in number of individual chromosomes
- End up with 3 or 1 instead of a pair = when fertilization zygote will have more or less chromosomes
Why might a gene mutation not lead to change?
- Genetic code is degenerate = might code for the same amino acid
- Mutation is in intron
Examples of mutagenic agents
Factors that increase rate of gene mutation:
- X-rays
- Gamma rays
- UV
- Chemicals
