DNA,genes,chromosomes
DNA structure
-Made of DNA nucleotides -joined by phosphodiester bonds
-sugar phosphate backbone
-complementary base pairs joined by H bonds
-double stranded
-associated with histones
DNA in eukaryotes DNA in prokaryotes
associated with histones Not associated with histones
longer shorter
no plasmids Has plasmids
contained in membrane Free in cytoplasm
Endosymbiotic theory
-mitochondria and chloroplasts contain DNA similar to that in prokaryotes
-endosymbiotic theory suggests they were once prokaryotic organisms
Genes and chromosomes
-chromosomes are composed of DNA, DNA and histones are coiled to form in distinct
chromatin
-chromatin is further coiled/condensed to form compact chromosomes for storage in the
nucleus
-chromosomes are only seen as distinct structures during prophase,metaphase and
anaphase
-gene = a base sequence of DNA that codes for the amino acid sequence of a polypeptide
or functional RNA
—> the sequence of bases in a gene determines P structure which determines T. Structure
which determines function of polypeptide made
-functional RNA is non-coding RNA
—> RNA molecules other than mRNA which carry out specific tasks
—> many types (e.g tRNA/rRNA)
-there is one long DNA molecule per chromosome with multiple genes along its length, each
gene occupies a specific position along the DNA molecule called the locus.
—> locus the position of a gene on chromosome DNA molecule
,Homologous chromosomes
-sexually produced organisms are diploid (have pairs of chromosomes called homologous
pairs)
- homologous chromosomes = a pair of chromosomes,one maternal and one paternal that
have same loci and therefore determine same features
—> can have different versions of same gene
—> allele = alternative form of th same gene
-each allele has a different base sequence which codes for different P structure (amino acid
sequence) and therefore produces different polypeptide
-when chromosomes have replicated their DNA they appear as crosses, the 2 copies are
known as sister chromatids which are attached at the centromere.
Traits of the genetic code
-there are 20 amino acids
-3 bases on DNA strand (triplet) together code for specific amino acid
-there are only 4 bases to code for each one of these 20 amino acids so 64 possible
combinations of bases (4x4x4)
—>most amino acids are coded for by more than one codon
—> the genetic code is degenerate
-degenerate = more than one codon codes for a single amino acid
-there are 3 codons (stop codons) that don’t code for an amino acid
-stop codon = mark the end of a polypeptide chain
-DNA has a code that is non-overlapping
-this means every base is only read once
-reduces effect of mutations (change in a nucleotide)
-if overlapping,a single mutation would potentially affect 3 amino acids
-with non-overlapping,this only affects 1
-code is universal
-with some exceptions,each triplet codes for the same amino acid in all organisms providing
evidence for evolution
Exons = base sequences in a gene that do code for amino acid sequences
Introns = base sequences in gene that don’t code for amino acid sequences
Structure of mRNA
-single stranded —> small enough to enter cytoplasm through nuclear pores
-not folded
-longer than tRNA
-single helix shape
-no base pairs or H bonds
, DNA vs RNA
differences DNA RNA
function holds genetic info transfers genetic info
(mRNA) and forms
ribosomes with proteins
(rRNA)
pentose sugar in deoxyribose ribose
nucleotide
nitrogenous base cytosine,guanine, cytosine,guanine,
adenine,thymine adenine,uracil
structure double helix relatively short
2 antiparallel polynucleotide polynucleotide chain
strands held by H bonds
between complementary
base pairs
number of strands 2 antiparallel strands 1 single strand
Protein synthesis
Transcription
-in the nucleus
1) H bonds between bases break
2) 1 strand acts as a template
3) Free RNA nucleotides align by complementary base pairing (A-U, C-G)
4) In RNA uracil is used in place of thymine
5) RNA polymerase catalyses the formation of phosphodiester bonds between
nucleotides by condensation reactions
6) pre-mRNA is spliced (introns are removed)
In prokaryotes:
-mRNA can be produced directly from DNA
-Most prokaryotes do not have introns, so splicing is not needed after transcription. (No pre
mRNA)
DNA structure
-Made of DNA nucleotides -joined by phosphodiester bonds
-sugar phosphate backbone
-complementary base pairs joined by H bonds
-double stranded
-associated with histones
DNA in eukaryotes DNA in prokaryotes
associated with histones Not associated with histones
longer shorter
no plasmids Has plasmids
contained in membrane Free in cytoplasm
Endosymbiotic theory
-mitochondria and chloroplasts contain DNA similar to that in prokaryotes
-endosymbiotic theory suggests they were once prokaryotic organisms
Genes and chromosomes
-chromosomes are composed of DNA, DNA and histones are coiled to form in distinct
chromatin
-chromatin is further coiled/condensed to form compact chromosomes for storage in the
nucleus
-chromosomes are only seen as distinct structures during prophase,metaphase and
anaphase
-gene = a base sequence of DNA that codes for the amino acid sequence of a polypeptide
or functional RNA
—> the sequence of bases in a gene determines P structure which determines T. Structure
which determines function of polypeptide made
-functional RNA is non-coding RNA
—> RNA molecules other than mRNA which carry out specific tasks
—> many types (e.g tRNA/rRNA)
-there is one long DNA molecule per chromosome with multiple genes along its length, each
gene occupies a specific position along the DNA molecule called the locus.
—> locus the position of a gene on chromosome DNA molecule
,Homologous chromosomes
-sexually produced organisms are diploid (have pairs of chromosomes called homologous
pairs)
- homologous chromosomes = a pair of chromosomes,one maternal and one paternal that
have same loci and therefore determine same features
—> can have different versions of same gene
—> allele = alternative form of th same gene
-each allele has a different base sequence which codes for different P structure (amino acid
sequence) and therefore produces different polypeptide
-when chromosomes have replicated their DNA they appear as crosses, the 2 copies are
known as sister chromatids which are attached at the centromere.
Traits of the genetic code
-there are 20 amino acids
-3 bases on DNA strand (triplet) together code for specific amino acid
-there are only 4 bases to code for each one of these 20 amino acids so 64 possible
combinations of bases (4x4x4)
—>most amino acids are coded for by more than one codon
—> the genetic code is degenerate
-degenerate = more than one codon codes for a single amino acid
-there are 3 codons (stop codons) that don’t code for an amino acid
-stop codon = mark the end of a polypeptide chain
-DNA has a code that is non-overlapping
-this means every base is only read once
-reduces effect of mutations (change in a nucleotide)
-if overlapping,a single mutation would potentially affect 3 amino acids
-with non-overlapping,this only affects 1
-code is universal
-with some exceptions,each triplet codes for the same amino acid in all organisms providing
evidence for evolution
Exons = base sequences in a gene that do code for amino acid sequences
Introns = base sequences in gene that don’t code for amino acid sequences
Structure of mRNA
-single stranded —> small enough to enter cytoplasm through nuclear pores
-not folded
-longer than tRNA
-single helix shape
-no base pairs or H bonds
, DNA vs RNA
differences DNA RNA
function holds genetic info transfers genetic info
(mRNA) and forms
ribosomes with proteins
(rRNA)
pentose sugar in deoxyribose ribose
nucleotide
nitrogenous base cytosine,guanine, cytosine,guanine,
adenine,thymine adenine,uracil
structure double helix relatively short
2 antiparallel polynucleotide polynucleotide chain
strands held by H bonds
between complementary
base pairs
number of strands 2 antiparallel strands 1 single strand
Protein synthesis
Transcription
-in the nucleus
1) H bonds between bases break
2) 1 strand acts as a template
3) Free RNA nucleotides align by complementary base pairing (A-U, C-G)
4) In RNA uracil is used in place of thymine
5) RNA polymerase catalyses the formation of phosphodiester bonds between
nucleotides by condensation reactions
6) pre-mRNA is spliced (introns are removed)
In prokaryotes:
-mRNA can be produced directly from DNA
-Most prokaryotes do not have introns, so splicing is not needed after transcription. (No pre
mRNA)
