Topic 4A - DNA, RNA and protein synthesis ARN * Flashcards
DNA, genes and chromosomes RNA and protein synthesis the genetic code and nucleic acids
what kind of DNA do eukaryotic cells contain?
linear DNA molecules in the form of chromosomes, thread-like structures, each made up of one long molecule of DNA, found in the nucleus
how are chromosomes structured to fit inside the nucleus of a eukaryotic cell?
long DNA molecule wound up so it can fit in the nucleus
its wound around proteins called histones
its then coiled very tightly to make a compact chromosome
what do histone proteins do?
they help support the DNA
what is DNA inside chloroplast and mitochondria like?
they have their own DNA, which is similar to prokaryotic data because its circular and shorter and not associated with histone proteins
what is DNA like in prokaryotic cells?
they carry DNA as chromosomes, but the DNA molecules are shorter and circular
the DNA isn’t wound into histones, it condenses by supercoiling
what is a gene?
a section of DNA bases that codes for the sequence of amino acids in a polypeptide or functional RNA
what forms the primary structure of a protein?
the sequence of amino acids
what determines the order of amino acids in a polypeptide?
the order of bases in a gene determines the primary structure, different polypeptides have different numbers and orders of amino acids
what codes for each amino acid?
a sequence of 3 bases in a gene called a triplet or codon codes for an amino acid
what is the first stage of protein synthesis?
DNA is 1st copied into messenger RNA
what do genes code for if not for a polypeptide?
genes that don’t code for a polypeptide code for functional RNA instead
what is functional RNA?
RNA molecules other than mRNA, which perform special tasks during protein synthesis e.g. tRNA and ribosomal RNA, which forms part of ribosomes
what is a genome?
a cell’s genome is the complete set of genes in the cell.
what is a proteome?
a cell’s proteome is the full range of proteins that the cell is able to produce
what are introns?
sections of DNA in a gene that don’t code for amino acids
only in eukaryotes
what are coding parts of a gene called?
exons
what happens to introns?
they’re removed in protein synthesis in a process called splicing- so they don’t affect the amino acid order
what are non-coding repeats?
eukaryotic data contains regions of multiple repeats outside of genes. these are DNA sequences that repeat over and over and don’t code for amino acids either
what are the different forms of a gene called?
alleles
how are alleles different?
the order of bases in each allele is slightly different, so they code for slightly different versions of the same polypeptide
how many chromosomes do humans have?
23 pairs, 46 in total. pairs of matching chromosomes are called homologous pairs
what are homologous pairs?
chromosomes that are the same size and have the same genes, but they could have different alleles. alleles coding for the same characteristic are found at the same locus.
we inherit 1 allele from each parent
what is a locus?
a fixed position on each chromosome in a homologous pair. the location of a gene on the strand of DNA/ chromosome
what happens to DNA when a protein is demand?
the DNA is separated so the genetic code can be ‘read’
only one strand is read the other is just to protect the bases
the base sequence provides the instructions for a proteins primary structure
how many different possible codons are there?
64
4 bases with 3 different positions for them to be in
so 4^3
what are the features of the genetic code?
degenerate each codon read from 5' to 3' every gene begins with TAC there are 3 different 'stop' codons that signify the end of a gene non-overlapping universal
what are the differences between eukaryotic and prokaryotic DNA?
eukaryotes - very long -many molecules of DNA -no plasmids -linear -chromosomes -associated with proteins (histones) prokaryotes - shorter -1 main piece of DNA -plasmids -ring structure -no chromosomes -not associated with histones
what form is DNA in within the nucleus?
in the nucleus of a resting cell DNA exists in the form of chromatin
when is DNA in chromosome form?
after DNA replication occurs and 2 strands of semi-conserved daughter DNA are produced. they are connected at the centromere and referred to as sister chromatids, at this point they need to be condensed into the packaged form of a chromosome (during prophase)
what is the diploid number?
the total of the homologous pairs
what is the haploid number?
half the diploid number
how do new alleles arise?
as a result of random mutations
what are the different types of RNA?
messenger RNA
transfer RNA
when is mRNA made?
during transcription
what is mRNA?
a single polynucleotide strand, groups of 3 adjacent bases are called codons
what does mRNA do?
it carries the genetic code from the DNA to the ribosomes, where it’s used to make a protein during translation
what does tRNA do?
it carries the amino acids that are used to make proteins to the ribosomes in translation
what is tRNA?
a single polynucleotide strand that’s folded into a clover shape. hydrogen bonds between specific base pairs hold the molecule in this shape
what does every tRNA molecule contain?
a specific sequence of 3 bases at one end called an anticodon
and an amino acid binding site at the other end
what is the first stage of protein synthesis?
transcription
when does transcription start?
when RNA polymerase attaches to the DNA double-helix at the beginning of a gene
what happens in transcription?
DNA strands separated by breaking H bonds 1 strand used as template strand complementary base pairing occurs RNA polymerase lines up free RNA nucleotides and joins them DNA re-coils into double helix RNA polymerase reaches stop signal splicing mRNA leaves nucleus
how is the DNA strand separated for transcription?
the hydrogen bonds between 2 DNA strands in gene are broken by DNA helicase attached to RNA polymerase. this separates strands, uncoiling DNA molecule and exposing bases
how are free RNA nucleotides lined up in transcription?
free RNA nucleotides are attracted to exposed bases. specific, complementary base pairing means mRNA strand is complementary copy of template strand
how is mRNA formed in transcription?
RNA polymerase joins up RNA nucleotides after they’re paired with bases on template strand, forming mRNA molecule
the RNA polymerase then moves along DNA, separating strands and assembling mRNA strand
how does double-helix re-coil?
hydrogen bonds between uncoiled strands of DNA re-form once the RNA polymerase has passed, and the strands coil back into a double-helix
in eukaryotes, what happens when RNA polymerase reaches a ‘stop signal’?
it stops making RNA and detaches from the DNA.
the mRNA moves out of nucleus through nuclear pore and attaches to a ribosome in the cytoplasm
what is a ‘stop signal’?
a particular sequence of DNA that tell RNA polymerase to stop making RNA
how is pre-mRNA formed?
introns and exons are both copied into mRNA during transcription
this is called pre-mRNA
what happens to pre-mRNA after transcription?
splicing occurs - removing introns and joining exons - forming mRNA
this takes place in the nucleus before it leaves for the next stage of protein synthesis
how does transcription in prokaryotes differ from eukaryotes?
mRNA is produced directly from the DNA - without splicing taking place
(there are no introns in prokaryotic DNA)
where does translation occur?
at the ribosomes in the cytoplasm
what happens in translation?
mRNA attaches to ribosome tRNA with amino acids attaches to mRNA amino acids joined together this repeats until stop signal reached polypeptide chain now formed
how does tRNA bring amino acids to ribosome?
ATP provides energy for bonds to form between amino acid and tRNA
tRNA with an anticodon complementary to mRNAs codon attaches to mRNA via specific base pairing
bringing amino acid with it
how are amino acids joined together at ribosomes?
2 tRNA molecules attach to codons on mRNA
their amino acids are joined by a peptide bond
1st tRNA leaves and a 3rd one binds to the next codon
its amino acid is bonded to the 2nd one then the 2nd tRNA leaves and so on
exam technique for transcription:
DNA strands separate by breaking H bonds only template strand used to make RNA complementary base pairing occurs adjacent RNA nucleotides joined by RNA polymerase pre-mRNA formed splicing/ introns removed to form mRNA
differences between DNA and mRNA?
DNA - RNA double stranded - single stranded very long - short thymine - uracil deoxyribose - ribose base pairings/hydrogen bonding - no base pairings/ hydrogen bonding has introns - no introns
translation exam technique:
mRNA associates with ribosome ribosome moves to start codon tRNA brings a specific amino acid anticodon complementary to codon ribosome moves to next codon process repeated and amino acids joined by peptide bonds to form polypeptide
differences between mRNA and tRNA?
mRNA - tRNA
longer/ has more nucleotides - shorter
straight molecule - clover-leaf shaped
no base pairings/hydrogen bonds - some base pairings/ hydrogen bonds
no amino acid binding site - amino acid binding site
many different mRNA - only 20/64 tRNA’s
no anticodon - anticodon
what is the genetic code?
the sequence of base triplets in mRNA which code for specific amino acids
what does it mean that the genetic code is non-overlapping?
each base triplet is read in sequence, separate from before it and after it
base triplets don’t share their bases
what does it mean that the genetic code is degenerate?
there are more possible combinations of triplets than there are amino acids. this means that some amino acids are coded for by more than one base triplet
what does it mean that the genetic code is universal?
the same specific base triplets code for the same amino acids in all living things
