4A DNA, Genes and Protein Synthesis Flashcards
How do prokaryotes and eukaryotes store DNA differently?
- eukaryotic cells contain linear DNA
- prokaryotes contain circular DNA
- DNA in eukaryotes exist as chromosomes that are thread-like structures made of DNA and histone proteins found in the nucleus of cells
- in prokaryotes DNA is free-floating and not bound to histones, it fits into the cell by super-coiling
What organelles have their own DNA?
- mitochondria and chloroplasts
- it is similar to prokaryotic DNA
- circular and not bound to histones
What is a gene?
A region of DNA that codes for a protein
What does the order of bases in DNA determine?
The order of amino acids in a protein
What is one amino acid coded by?
3 bases/ codon
Other than amino acids for a protein, what may a gene code for?
- a section of functional RNA
- eg. transfer RNA or ribosomal RNA
What are genomes?
Collection of organism’s genes
What are proteomes?
Collection of organism’s proteins
What are non-coding regions of DNA?
- DNA in eukaryotes that doesn’t code for a protein
- introns
- removed during protein synthesis
Does prokaryotic DNA have introns and/or exons?
No, just exons
What are microsatellites/short tandem repeats?
- eukaryotic DNA contains repeating regions of DNA located outside of genes
- these regions are non-coding
What are alleles?
- multiple forms of genes
- humans usually have 2 alleles of the same gene, one on each chromosome
What can alleles lead to?
- may lead to slightly different proteins being made
- eg . different alleles for eye colour may produce blue and brown proteins
How many chromosomes do humans have in each cell?
- 23 homologous pairs
- 2 copies of the same chromosome in each human cell (one maternal and one paternal)
What is a homologous pair?
- when 2 chromosomes that are the same size and contain the same genes arrange themselves in a homologous pair
- they may have different alleles
What is a karyotype?
- how chromosomes are displayed
- a visual illustration of an organism’s collection of chromosomes
What enzyme unravels DNA double helix to a straight ladder?
Gyrase
What is the function of DNA helicase?
- separates 2 strands of DNA for replication
- it catalyses the disruption of the hydrogen bonds holding the 2 strands together
What is the function of RNA polymerase?
- copies a DNA sequence into a RNA sequence
- links the nucleotides of the new RNA strand in transcription
Where in the cell does transcription occur?
Nucleus
Describe the unwinding of DNA during transcription
- first step of transcription
- DNA unwinds and hydrogen bonds break between complementary bases
- gene is now exposed to be replicated
- complementary copy of the code from the gene is made by building a single-stranded nucleic acid molecule (mRNA)
How is RNA polymerase involved in transcription?
- moves along template strand of DNA
- adds complementary bases to template strand and joins them with phosphodiester bonds
- stops transcribing when it reaches a stop codon
Describe the coding strand involved in transcription
- it is the opposite strand of DNA to the template strand
- the base sequence is identical to the sequence in the new mRNA strand (except it has thymine instead of uracil)
How does the double-stranded DNA molecule reform?
- when the gene has been transcribed the hydrogen bonds between mRNA and DNA break
- hydrogen bonds reform between 2 DNA strands (template and coding)
Where does translation occur in the cell?
- in the cytoplasm
- mRNA attaches to a ribosome
Describe how anticodons are involved with translation
- free molecules of tRNA in cytoplasm have a triplet of unpaired bases at one end called an anticodon
- this is a region where they can bind to codons in mRNA
- they pair with their complimentary codon in the mRNA
Describe how tRNA is involved in translation
- there are at least 20 tRNA molecules each with different anticodons and amino acids
- tRNA molecules bind with their specific amino acid in the cytoplasm and bring them to the mRNA on the ribosome
- 2 molecules of tRNA fit on the ribosome at any one time and a peptide bond is formed between the 2 amino acids
What is required for the joining of amino acids during translation?
- ATP
- provided by mitochondria
When does translation end?
- when a stop codon is reached
- amino acid chain forms final polypeptide
How are proteins made?
- made during protein synthesis
- 2 main stages, transcription and translation
- transcription: DNA transcribed into a molecule of messenger RNA
- translation: mRNA is translated into a protein at a ribosome
Describe eukaryotic RNA
- single-stranded molecule
- contains nucleobase uracil rather than thymine
How is mRNA made?
- during transcription
- synthesised using a DNA template
What is an mRNA codon?
- a 3 base sequence
Why is tRNA important during translation?
- carries amino acids from cytoplasm to ribosome
- amino acids are used to assemble protein
- it is a single polynucleotide folded to resemble 3 leaf clover shape
What is included in pre-mRNA that isn’t in mRNA?
- introns
- regions that don’t code for a protein
What is splicing and describe the process
- introns are removed from pre-mRNA during splicing
- splicing removes introns and the exons are spliced together
- mRNA is then mature mRNA and it leaves the nucleus to associate with a ribosome
Does prokaryotic DNA have introns and exons?
- just exons
- no non-coding regions
- there is no mechanism for splicing in prokaryotes
Describe purines and pyramidines
- adenine and guanine are purine
- cytosine and thymine/uracil are pyramidine
What is the sugar involved in DNA, mRNA and tRNA?
- DNA: deoxyribose
- mRNA: ribose
- tRNA: ribose
Are histones involved in DNA, mRNA and tRNA?
- DNA: yes in euks no in proks
- mRNA: no
- tRNA: no
What are the nucleotides involved in DNA, mRNA and tRNA?
- DNA: ATCG
- mRNA: AUCG
- tRNA: AUCG
What is the shape involved in DNA, mRNA and tRNA?
- DNA: double helix
- mRNA: single linear strand
- tRNA: 3 leaf clover
What is the size of DNA, mRNA and tRNA?
- DNA: big
- mRNA: small
- tRNA: smaller
Describe tRNA structure
- made up of 76 bases always
- all the same in every tRNA molecule except the anticodon
What is the genetic code?
- the means by which DNA and RNA molecules carry genetic information in cells
- it is a sequence of triplet codons that code for specific amino acids
Describe how the genetic code is non-overlapping
- same base is not used for 2 different codons/triplets
- no base can take part in the coding of two or more amino acids
Describe how the genetic code is degenerate
- there is more possible combos of triplet codons than there are amino acids
- there’s 64 codons but only 20 amino acids
- this means more than one codon codes for the same amino acid
- 3 stop codons can signal the ribosome to cease protein production
Describe how the genetic code is universal
- all living things have the same codons coding for the same amino acids
- eg. codon GAC codes for aspartic acid in all organisms
Describe the genetic code with 3 terms
- non-overlapping
- degenerate
- universal
What is a mutation?
- a change in the DNA base sequence
- gene mutations occur in regions of DNA that code for a protein
What is a substitution mutation?
- occurs when one base is substituted for another
What is a deletion mutation?
- occurs when one base is completely removed
What is a deletion mutation?
- occurs when one base is completely removed
What is an insertion mutation?
- occurs when 1 base is inserted
What is an inversion mutation?
- occurs when a section of bases is reversed
How can mutations change the structure of proteins?
- the order of amino acids in a protein is determined by the sequence of bases in the DNA
- mutations alter the sequence of bases so structure is changed
What is a silent mutation?
- if a substitution mutation occurs but the codon still codes for the same amino acid
- has no result on the final polypeptide chain
Describe a frame shift
- when 1 base is deleted the entire reading frame will shift
- results in an entirely new sequence of amino acids
- this can cause many severe genetic diseases eg. cystic fibrosis
How do mutations occur?
randomly
What can increase rate the rate of mutations?
- mutagens
- they may induce errors in DNA replication or they may damage DNA directly
- radiation causes direct damage to DNA
- certain chemicals and viruses can also increase the rate of mutation
Describe how radiation can increase the rate of mutation
- UV and other ionising radiation can disrupt bonds in DNA
Describe how certain chemicals and viruses can increase the rate of mutation
- cyclophosphamide is a common chemotherapy agent that induces mutation in cancer cells
- rous sarcoma virus is linked to increased rates of mutation
Describe how UV radiation can damage cells
- through the formation of thymine dimers
- this changes the structure of the double helix and can lead to errors when the DNA comes to replicate
What is rous sarcoma virus linked to?
- Ewing’s sarcoma
- this is a cancer affecting bones and soft tissue
What may occur if there is an error during meiosis?
- cells may end up with a different number of chromosomes
- this is a chromosome mutation
- some cells may have no copies of a particular chromosome
- other cells may have 2 or more copies of a particular chromosome
Describe what nondisjunction is
- where chromosomes don’t separate correctly
What is an example of nondisjunction?
- Down’s syndrome
- during meiosis one gamete received 2 copies of a chromosome rather than a single copy
- when fertilisation occurs, the zygote will have 3 copies of one chromosome (chromosome 21 in particular)
What are diploid cells?
- cells that have 2 copies of each chromosome
- one inherited from the mother and one from the father
- matching copies of chromosomes are homologous
What are alleles?
- different versions of each gene
What kind of cells are sperm and egg?
- haploid cells
- they contain one copy of each chromosome rather than 2
Describe meiosis briefly
- type of cell division
- takes place in reproductive organs
- events during meiosis cause genetic variation
Give the name of the single diploid cells in mammals, females and males
- mammals: gametocytes
- females: oocytes
- males: spermatocytes
What is the result of meiosis?
- 4 haploid daughter cells that are genetically unique
- these are the gametes
What happens in prophase 1 of meiosis?
- chromatin condenses into visible chromosomes and nuclear membrane breaks down
- crossing over occurs at the chiasmata
What happens in metaphase 1 of meiosis?
- pairs of homologous chromosomes line up in a double line at the equator of the cell
- spindle fibres attach
What happens in anaphase 1 of meiosis?
- spindle fibres pull homologous chromosomes to poles of cell and cytokinesis begins
What happens in telophase 1 of meiosis?
- nuclear membranes reform and cytoplasm completely divides forming 2 new cells
What happens in prophase 2 of meiosis?
- chromosomes condense for second division
- nuclear membrane breaks down
What happens in metaphase 2 of meiosis?
- chromosomes line up at the equator of the cell in a single file line
- spindle fibres attach to centromeres
What happens in anaphase 2 of meiosis?
- spindle fibres pull chromatids away to the poles (centromeres split)
- cytokinesis starts and nuclear membranes begin to reform
What happens in telophase 2 of meiosis?
- chromosomes decondense into chromatin threads wrapped around histones
- nuclear membranes have fully reformed
What are the 2 ways that genetic variation can occur?
- crossing over
- independent segregation
Describe crossing over of chromatids
- during prophase 1 homologous chromosomes come together to form chiasmata
- sections of chromatids are exchanged with one another
- this creates a new combination of alleles
What is the result of crossing over called?
A recombinant chromosome
Describe independent segregation
- each homologous pair of chromosomes consists of one maternal and one paternal
- when homologous pairs are separated during anaphase 1 it’s random which chromosome ends up in which daughter cell
Describe the parasitic protist Plasmodium falciparum
- organism that is responsible for malaria
- only the zygote is diploid, all other cells are haploid
How is meiosis related to Plasmodium falciparum?
- occurs when the zygote divides to form sporozoites
- these affect the salivary glands of the mosquito and are injected into the host when the mosquito has its next blood meal
How is mitosis related to Plasmodium falciparum?
- sporozoites divide by mitosis into haploid merozoites in mammals’ liver
- merozoites divide by mitosis into haploid gametocytes in red blood cells
- another mosquito will feed on the host and ingest the gametocytes
