4.1-4.3 Flashcards
Compare and contrast the structure of DNA in eukaryotic cells with DNA in prokaryotic cells:
Similarities:
Nucleotide structure is identical
Adjacent nucleotides joined by phosphodiester bonds, complementary bases joined by H bonds
DNA in mitochondria/chloroplasts similar in structure to DNA in prokaryotes
Differences:
Eukaryotic DNA is longer
Eukaryotic DNA is linear, prokaryotic DNA is circular
Eukaryotic DNA is associated with histone proteins, prokaryotic DNA is not
Eukaryotic DNA contains introns, prokaryotic DNA does not
What is a chromosome?
Long, linear DNA and its associated histone proteins
In the nucleus of eukaryotic cells
What is a gene?
A sequence of DNA (nucleotide) bases that codes for:
- amino acid sequence of a polypeptide
- functional RNA (rRNA or tRNA)
What is a locus?
Fixed position a gene occupies on a particular DNA molecule
Describe the nature of the genetic code:
Triplet code: A sequence of 3 DNA bases codes for a specific amino acid
Universal: Same base triplets code for the same amino acids in all organisms
Non-overlapping: Each base is part of only one triplet so each triplet is read as a discrete unit
Degenerate: An amino acid can be coded for by more than one base triplet
What are non-coding base sequences and where are they found?
DNA that does not code for amino acid sequences/polypeptides
Between genes=non coding multiple repeats
Within genes=introns
What are introns and exons?
Exon- base sequence of a gene coding for amino acid sequences
Intron- base sequence of a gene that doesn’t code for amino acids, in eukaryotic cells
Define genome and proteome:
Genome: the complete set of genes in a cell
Proteome: the full range of proteins that a cell can produce
Describe the two stages of protein synthesis:
Transcription: production of mRNA from DNA in the nucleus
Translation: production of polypeptides from the sequence of codons carried by mRNA at ribosomes
Compare and contrast the structure of tRNA and mRNA:
Similarities:
Both single polynucleotide strand
Differences:
tRNA is folded into clover leaf shape, whereas mRNA is linear/straight
tRNA has H bonds between paired bases, mRNA doesn’t
tRNA is shorter, fixed length, whereas mRNA is a longer, variable length
tRNA has an anticodon, mRNA has codons
tRNA has an amino acid binding site, mRNA doesn’t
Describe how mRNA is formed by transcription in eukaryotic cells:
H bonds between DNA bases break
Only one DNA strand acts as a template
Free RNA nucleotides align next to their complementary bases on the template strand
- In RNA, uracil is used in place of thymine
RNA polymerase joins adjacent RNA nucleotides
This forms phosphodiester bonds via condensation reactions
Pre-mRNA is formed and this is spliced to remove introns, forming mRNA
Describe how production of mRNA in a eukaryotic cell is different from the production of mRNA in a prokaryotic cell:
Pre-mRNA is produced in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells
Because genes in prokaryotic cells don’t contain introns so no splicing in prokaryotic cells
Describe how translation leads to the production of a polypeptide:
mRNA attaches to ribosome and the ribosome moves to a start codon
tRNA brings a specific amino acid
tRNA anticodon binds to complementary mRNA codon
Ribosome moves along to next codon and another tRNA binds so 2 amino acids can be joined by a condensation reaction forming a peptide bond
- using energy from hydrolysis of ATP
tRNA released after amino acid joined polypeptide
Ribosome moves along mRNA to form the polypeptide, until a stop codon is reached
Describe the role of ATP, tRNA and ribosomes in translation:
ATP:
Hydrolysis of ATP to ADP+Pi releases energy
So amino acids join to tRNAs and peptide bonds form between amino acids
tRNA:
Attaches to/transports a specific amino acid, in relation to its anticodon
tRNA anticodon complementary base pairs to mRNA codon, forming H bonds
2 tRNAs bring amino acids together so peptide bond can form
Ribosomes:
mRNA binds to ribosome with space for 2 codons
Allows tRNA with anticodons to bind
Catalyses formation of peptide bond between amino acids
Moves along/translocation
What is a gene mutation?
A change in the base sequence of DNA
Can arise spontaneously during DNA replication
What is a mutagenic agent?
A factor that increases rate of gene mutation
Explain how a mutation can lead to production of a non-functional protein/enzyme:
Changes sequence of base triplets in DNA so changes sequences of codons on mRNA
So changes sequence of amino acids in the polypeptide
So changes position of H/ionic/disulfide bonds
So changes protein tertiary structure of proteiin
Enzymes- active site changes shape so substrate can’t bind, E-S complex can’t form
Explain the possible effects of a substitution mutation:
Base/nucleotide in DNA replaced by a different base/nucleotide
This changes one triplet so changes one mRNA codon
So one amino acid in polypeptide changes
- Tertiary structure may change if position of H/ionic/disulfide bonds change
OR amino acid doesn’t change
- due to degenerate nature of genetic code OR if mutation is in an intron
Explain the possible effects of a deletion mutation:
One nucleotide/base removed from DNA sequence
Changes sequence of DNA triplets from point of mutation
Changes sequence of mRNA codons after point of mutation
Changes sequence of amino acids in primary structure of polypeptide
Changes position of hydrogen/ionic/disulfide bonds in tertiary structure
Changes tertiary structure/shape of protein
Describe features of homologous chromosomes:
Same length, same genes at same loci, but may have different alleles
Describe the difference between diploid and haploid:
Diploid- 2 complete sets of chromosomes represented as 2n
Haploid- single set of unpaired chromosomes, represented as n
Describe how a cell divides by meiosis:
In interphase, DNA replicates- so 2 copies of each chromosome, joined by a centromere
Meiosis I separates homologous chromosome
- chromosomes arrange into homologous pairs
- crossing over between homologous chromosomes
- independent segregation of homologous chromosomes
Meiosis II separates chromatids
Explain why the chromosome number is halved during meiosis:
Homologous chromosomes are separated during meiosis I
Explain how crossing over creates genetic variation
Homologous pairs of chromosomes associate/form a bivalent
Chiasmata form
Alleles of sister chromatids exchanged between chromsomes
Creating new combos of alleles on chromosomes
Explain how independent segregation creates genetic variation:
Homologous pairs randomly align at equator- so random which chromosome from each pair goes into each daughter cell
Creates different combos of maternal and paternal chromosomes/alleles in daughter cells
Other than mutation and meiosis, explain how genetic variation within a species is increased:
Random fertilisation/fusion of gametes
Creating new allele combinations/ new maternal and paternal chromosome combinations
Explain the different outcomes of mitosis and meiosis:
Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells
- As one division in mitosis, whereas 2 divisions in meiosis
Mitosis maintains chromosome number whereas meiosis halves the chromosome number
- as homologous chromosomes separate in meiosis but not mitosis
Mitosis produces genetically identical daughter cells but meiosis produces genetically varied daughter cells
- As crossing over and independent segregation happen in meiosis but not mitosis
Explain the importance of meiosis:
Two divisions creates haploid gametes
So diploid number is restored at fertilisation- chromosome number maintained between generations
Independent segregation and crossing over creates genetic variation
Describe how mutations in the number of chromosomes arise:
Spontaneously by chromosome non-disjunction during meiosis
Homologous chromosomes or sister chromatids fail to separate during meiosis
SO some gametes have an extra copy of a particular chromosome and others have none
How can the number of possible combinations of chromosomes in daughter cells following meiosis be calculated?
2^n where n=number of pairs of homologous chromosomes
How can the number of possible combos of chromosomes following random fertilisation of two gametes be calculated?
(2^n)^2 where n=number of pairs of homologous chromosomes
