genetic information, variation and relationship between organisms Flashcards
gene definition
A section of DNA that contains the coding info for making polypeptide and functional RNA
Genome definition
The genome of an organism is the whole of its hereditary information encoded in its DNA
Exon definition
Coding sequence in DNA
Intro def
non coding sequence in DNA
What is a gene
sequences of DNA bases that codes for a polypeptide
sequence determines the sequence of A acids in the primary structure of protein
some also code for functional RNA
e.g.rRNA and tRNA
gene occupies fixed position on particular chromosome called gene locus
homologous chromosomes carry same gene- inherit 1 half of pair from each parent
gene in eukaryotic vs prokaryotic DNA
Eukaryotic-
genes contains both exon and intron
exons code for specific sequence of A acid in primary structure
introns Don’t code for this
introns found in intergenic region between genes
prokaryotic-
genes dont contain intons
Genome to Proteome
genome= all DNA in a cell
includes linear and circular
proteome= full range of proteins produced by cells
during Protein synthesise ( transcription and translation) triplets are used to code for primary structure
DNA triplets are transcribed to comp mRNA codons
Triplet- consists of 3 base pairs e.g. ACT
features of genetic code
universal- same 3 bases code (DNA triplet/mRNA codon) for same A acid in all organisms
non overlapping- each base is only read once in triplet/codon
degenerate- more than 1 triplet/ codon codes for a single amino acids
e.g. only 20 A acids but 64 combinations of triplets
mRNA vs tRNA
mRNA (messenger RNA)-
polymer of RNA nucleotides (ribose sugar and AUGC)
Single stranded
linear
shorter than DNA longet than/ more nucleotides than tRNA
copy of gene that takes info to the ribsomes to make proteins
has Codon
No H bonds
tRNA (transfer RNA)-
polymer of RNA nucleotides (ribose sugar and AUGC)
single strand
folded into cloverleaf shape
held by H bonds between comp base pairs
has A Acid binding site
Has Anticodon
brings specific A acid to ribsome during translation
transcription
production of mRNA from DNA in nucleus
1.DNA helicase breaks H bonds between comp base pairs
2.only 1 strand of DNA acts as template
- free RNA nucleotides align with comp base pairs (H bonds)
- uracil not thymine comp base pair with adenine
5.RNA polymerase joins adjacent RNA nucleotides via phosophodiester bonds
Eukaryotes only:
6.pre-mRNA spliced to remove introns producing mRNA
stop codons
stop translation
result in detachment of polypeptide chain from ribosome
end of polypeptide chain
translation
production of polypeptide
1) mRNA attaches to ribsome
2) tRNA brings specific A acid to ribsome
3)tRNA anticodon binds to comp mRNA codon, 2 tRNA anticodons bind to 2 mRNA codon at any one time in ribsome (form 2 weak H bond)
4) A acids joined by peptide binds (condensation reaction)- catalyzed by ribsome
5)requires energy realses from ATP hydrolysis
6) tRNA is realised after A acids joined in polypeptide
7)ribsome moves along mRNA to form polypeptide- released into cytoplasm
First codon is a ‘start’ codon
eventually ribsome reached ‘stop’ codon and detaches from mRNA ensuring polypeptide produced had correct length
genetic mutation
a random change in the base sequence of DNA, that results in the formation of a new allel
how do mutations effect structure of protein
many change the primary structure (no and sequence of A acids) of polypeptide chain
this is because different codon code for different A acids
this may then change the position of bonds in the tertiary structure as r groups position has changed
may make it non functional or improve it
different types of mutation
substitution
addition
deletion
substitution mutation
typical only changes 1 base in DNA triplet
this changes corresponding mRNA base in codon
possible producing a diff amino acid (only 1 potentially changed in primary stucture)
however due to degenerate code not all subs result in any change as may code for same A acid
in this case no changes to P structure or T structure of Polypeptide (silent mutation)
if diff A acid does arise as result then p and T structures are changed ( as P effects where bonds form in T as it changes position of the R groups)which may make it unable to complete its job or may make it better adapted e.g. more or less comp enzyme
One other type of sub Is formation of stop codon
this signals ribsome to detach from mRNA (stop translation)
causing shorter chain- negative effects
addition and deletion mutations
DNA nucleotides bases are added or deleted within sequence
Frame shift occurs- alteration of the base triplets and codon downstream from the mutation
changing codons, which changes A acid produced (possibly or some may remain same), frame shift change P structure and therefore T structure ( due to changing positions of R groups changing bond positions)
mutagenic agents
increase the rate if gene mutations e.g.
1) high energy radiation- can directly damage the DNA molecules (e.g. x rays)
2)carcinogens- DNA reactive chemicals that can directly and indirectly damage DNA e.g. nitrous acid can convert cytosine in DNA to uracil
3)biological agents- e.g. viruses that can insert there genetic material in host cells genome
Meiosis
produces genetically different daughter cells e.g. gamete
prior: interphase + semi Conservative rep creating copy of each chromosome
Dipolid parent cells (2n) contaun pair of homologous chromosomes which carry same gene diff allels
meiosis 1 : independent seg of homologous chromosomes and crossing over cause genetic diversity
mainly homologous chromosomes are seperated producing 2 diploid cells
meiosis 2: sister chromatids are seperated
over all produces 4 haploid cells
new comb of allels and therefore produces geneti variation in pop
Crossing over
Meiosis 1
rare event (10% of cells undergoing meiosis 1)
occurs during prophase 1
1) homologous pairs of chromosomes associate- bivalent
2)chiasmata form (non sis chromatids wrap around 1 another)
3)equal lengths of non sis chromatids exchanges
4)produces new comb of allels
independent segregation
Meiosis 1
biggest source of genetic variation
happens in Metaphase 1
random assortment or shuffling of homologous chromosomes
during anaphase 1 each pair of H chromosomes is seperated and maternal and paternal chromosomes move to opp poles (2n > n)
daughter cells produced will randomly contain maternal or paternal half of H pair which determined by random assortment
number of possibilities = 2^n where n= no of pairs of homologous chromosomes
e.g. humans 23 pairs so 2^23 = 8388608 random assortments of chromosomes
this is why random fusion of gamete is major source of genetic variation
non disjuction mutations
First or second division
when sister chromatids or homologous chromosomes don’t split
in both cases daughter cells produce extra chromosome (n + 1) or lack of chromosome (n-1)
cause of downsyndrome
Role of phenotypic variation in natural selection
larger pop = more genetically diverse
key points:
Random mutations can produce new allels of a gene
the creates variation in phenotype
selection pressure
phenotype providing advantage more likely to survive and reproduce
these organisms pass on their allels
advantageous allel frequency increases over many gens
2 diff types of selection
stabilizing and directional
