DNA & Genetic Diversity L6

similarities and differences between DNA and RNA

similarities and differences between DNA and RNA

Sim:

both are polynucleotides (many nucleotides linked together in a long chain)

both contain nitrogenous bases: adenine guanine and cytosine and have phosphodiester bonds

diff:

RNA nucleotides never contain nitrogenous base Thymine (Uracil instead)

RNA contains pentose sugar ribose instead of deoxyribose (that DNA contains)

RNA molecules are only made up of 1 polynucleotide strand (single stranded)

what are nucleotides made from

• pentose sugar ( 5 carbon sugar)

• nitrogen containing organic base

• phosphate base

what bonds are between bases and nucleotides?

hydrogen bonds between bases

phosphodiester bond between nucleotides

structure of DNA and its nucleotides

• polynucleotide

• 2 polynucleotide strands joined together with hydrogen bonds forming a double helix

  Each DNA nucleotide has:

• deoxyribose sugar

• phosphate group

• one of four nitrogenous bases: cytosine, thymine, adenine, guanine

7 structures of DNA and their functions

• sugar phosphate backbone - gives strength

• helix - gives compact shape and protects sequence of bases

• double stranded - each strand serves as a template in replication; protects sequence of bases ; makes molecules more stable

• large molecule- large amount of information can be stored

• many hydrogen bonds - gives stability; individual hydrogen bonds are weak, allowing helix to unzip easily for replication

• sequence of bases- codes for specific sequence of amino acids in polypeptide

• complementary base pairing- enables information to be replicated accurately

What are genes and what do they do?

sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids that in turn make specific polypeptides (proteins)

why can DNA self replicate?

due to complementary base pairing essential for cell division

How does variation occur? (genetic diversity)

random mutations

random fusion of genetically different gamete (fertilisation)

independent segregation of homologous chromosomes

crossing over of homologous chromosomes

mutagenic agents (env

describe stages of semi-conservative mechanism of DNA replication

• As DNA replicates , the enzyme DNA helicase breaks the hydrogen bonds and seperates the two polynucleotide strands

• Each strand acts as a template for formation of 2 new complementary strands

• individual DNA nucleotides align and attach by hydrogen bonding to the exposed bases of each template strand according to specific complementary base pairs

• A-T C-G

• DNA nucleotides in each new strand are joined together by phosphodiester bonds in condensation reactions catalysed by enzyme DNA polymerase

• 2 new DNA molecules are identical to each other and to the original DNA

why does it get the name semi conservative?

Each newly formed DNA molecule contains one original polynucleotide strand and one new strand hence the name

what evidence is there to show replication is semi conservative

two scientists Meselson and Stahl conducted these experiments

• grew bacterium in which 14N was replaced with 15N

• cells were allowed to divide until it was certain that 15N had been incorporated into entire DNA

• bacteria were then transferred to a medium containing only the normal isotope 14N and allowed to divide

• samples of bacteria were then taken after each division (generation) & DNA was extracted and spun into a centrifuge. DNA containing the heavy isotope 15N is slightly heavier than DNA containing the normal 14N isotope.

structure of RNA

• relatively short polynucleotide molecule

• pentose sugar ribose

• organic base uracil replaces thymine

• single stranded (mRNA and tRNA) compared to double stranded DNA

describe structures and functions of mRNA and tRNA.

mRNA:

• linear structure and contains codons (mRNA base triplets)

• single stranded molecule and shorter/ smaller than DNA so it can leave the nucleus through the pore

• exposed unpaired bases

• made up of sugar phosphate backbone

• Uracil bases present instead of Thymine

  function

• found in nucleus and makes copies of the DNA template so that it can leave the nucleus

• involved in protein synthesis and formed in nucleus during transcription

tRNA-

• single stranded molecule

• has a sugar phosphate backbone

• folded clover leaf shape - hydrogen bonds between some of the complementary bases

• amino acid binding site on a specific region of the molecule

• specific anicodon on tRNA (three unattached tRNA nucleotide bases) are complementary to specific codon on mRNA molecule

  function

• found in the cytoplasm and used during translation to deliver specific amino acids to the ribosome to create a polypeptide

• they already exist in the cytoplasm and doesn’t have to be created ( there are 20 different tRNA molecules)

differences between mRNA and tRNA

mRNA

• linear

• posseses codons

• no hydrogen bonds (but there are phosphodiester bonds

• no amino acid binding site

• exposed bases

tRNA

• folded clover leaf shape

• posseses anitocodons (free nucleotides)

• hydrogen bonds present

• amino acids binding site present

DNA in prokaryotes vs eukaryotes

role of histones? and other proteins in eukaryotes?

properties of plasmids ?

eukaryotes:

• DNA molecules associate with proteins called histones to form chromosomes during cell division.

• chromosomes are made of one very long, condensed DNA molecule (in eukaryotic cells)

• histones’ role is to organise and condense the DNA tightly to fit in the nucleus

• other proteins are enzymes used in copying and repairing the DNA

• eukaryotic DNA also contains introns

prokaryotes:

• DNA is double stranded

• prokaryotes possess a single, circular chromosomal DNA molecule

• usually have one or more plasmids (very small circular DNA molecules)

• DNA within prokaryotic cells is not associated with any proteins

• plasmids are small circular DNA and usually only contain few genes; more accessible for proteins required for gene expression and therefore contain genes required often, quickly/ for emergencies; genes for antibiotic resistance are often found in plasmids

what are chromosomes

one very long, condensed DNA molecule associated with histones(proteins)

what are homologous pairs of chromosomes

pair of chromosomes: one inherited from father and one from mother (maternal and paternal)

they carry the same genes in the same loci but not necessarily the same allele

what is a locus

position of a gene on a chromosome. alleles occupy the same locus on each member of a pair of homologous chromosome.

what is a sequence of three nucleotides called and what do they code for

three DNA nucleotide bases : base triplet (template for mRNA)

three mRNA bases: codon (complementary to tRNA) complementary to triplet

three tRNA bases: anticodon (codes for specific amino acids) complementary to codon → same as DNA except for base URACIL which replaces THYMINE

what are the qualities of triplet codes?

• degenerate

• non overlapping

• universal

what does a degenerate triplet code mean

different triplets/ codons coding for the same amino acid

what does non overlapping code mean?

each base only read once as part of a specific triplet

what does universal code mean?

same triplets code for the same amino acids in all organism

what is the purpose if start and stop triplets?

ensures cell reads DNA correctly and produces the correct sequence of amino acids required for the polypeptide to function properly

what are some non coding DNA

Introns

multiple repeats

what do introns and multiple repeats mean?

introns: base sequence present within genes that dont code for amino acids

withIN genes that INterrupt coding

multiple repeats: some base sequences present between genes (contain same base sequences repeated multiple times)

what is an exon?

sequences in genes that code for amino acids (usually separated by 1 or more introns)

what is a genome

complete set of genes (non coding DNA included) in a cell

extra information on genome

always a full genome present in every cell of organism

however not every cell is expressed in every cell

which genes are expressed are dependent on cell type

what is a proteome

full range of proteins that a cell is able to produce

more information on proteome

larger than genome of organism because of large amount of post transitional modification of proteins

each gene is also capable of producing multiple different proteins (alternative splicing)

resulting in larger proteome than genome

Quickly describe transcription and translation

Transcription: DNA is transcribed and an mRNA molecule is produced (occurs in nucleus)

Translation: mRNA is translated and an amino acid sequence is produced (cytoplasm)

Describe and explain transcription

• DNA molecules unwind (hydrogen bonds broken between complementary base pairs) by DNA helicase

• exposes gene to be transcribed

• complementary copy of the code from the gene is made by a single stranded mRNA molecule

• only ONE of the strands acts as a template

• free activated RNA nucleotides pair up via (hydrogen bonds) with their complementary bases (now exposed) on one strand of the unzipped molecule

• sugar phosphate groups of these RNA nucleotides are then bonded together by enzyme RNA polymerase to form the phosphodiester bonds in the mRNA molecule.

• once gene is successfully transcribed, and mRNA molecule is complete, hydrogen bonds between mRNA and DNA strands break and double stranded DNA molecule reforms

• mRNA molecule leaves nucleus via pore in envelope

splicing?

occurs before pre-mRNA leaves nucleus

• non coding sections are removed (introns and multiple repeats)

• coding sections are joined together

• resulting in mRNA molecule carries only coding sequences (exons) of gene

• mRNA contains ONLY exons

describe and explain translation

• mRNA molecules attach to a ribosome

• in the cytoplasm there are free molecules of tRNA

• tRNA have anticodons (unpaired triplet bases) and a region where specific amino acids can attach to

• there are approximately 20 different tRNA molecules with specific anticodons and amino acid binding site

• tRNA molecules bind with their specific amino acid and bring to ribosome and mRNA molecule

• each anticodon on each tRNA molecule pairs with complementary codon on mRNA molecule

• 2 tRNA molecules can fit in a ribosome at a time bringing each amino acid they carry

• peptide bonds form between 2 amino acids

• which requires ATP provided by mitochondria

• process continues until ‘stop’ codon on mRNA is reached and a specific polypeptide is produced

what is genetic diversity?

number of different alleles in a population

what are gene mutations and what types are there?

changes in sequence of nucleotide bases in DNA resulting in altered polypeptide. NEW ALLELES of genes are produced

• Deletion of nucleotides - removal of one it more bases results in frame shift. sequence of amino acids is altered from point of mutation and protein formed nearly always non-functional

• Substitutions: replacement of one or more bases by one or more different bases. only changes amino acid for that triplet ( wont have a knock on effect) non functioning protein may form due to different amino acids in polypeptide

what are alleles?

different form of a gene that usually codes for a similar characteristic buta different type

what is mitosis

mitosis is a type of nuclear division which produces cells that are genetically indentical. Parent cell divides to produce 2 daughter cells. Each contain an identical copy of DNA in parent cell.

mitosis is a part of the cell cycle where the cells undergo a regular cycle of nuclear and cell division seperated by periods of cell growth )

what could the consequence be of an allele forming? and why?

alleles of genes differ in their base sequence and code for different sequences of amino acids and sometimes this change in polypeptide makes the protein produced non functional

• change in amino acid sequence may affects position of hydrogen and ionic bonds ( sometimes disulphide) which in turn changes the tertiary structure

• if this protein coded for an enzyme, due to the change in the allele and in turn the change in tertiary structure, the active site may be altered preventing ES complexes being formed and makes the enzyme non functional

what is meaning of diploid and haploid numbers?

diploid : 2n total number of chromosomes usually found in a normal cell ( 46 in humans)

haploid: n number of chromosomes usually found in gametes after meiosis 2

what is cell cycle divided into?

Interphase- represents the non dividing cell when cell growth occurs

nuclear division (mitosis) (nucleus divides into two)

• prophase

• metaphase

• anaphase

• telophase

cytokinesis (mitosis) - cytoplasm divides into two

Interphase

- Cell carries out its normal functions but during late interphase it prepares for nuclear division by:

• doubling the DNA content (DNA replication)

• increasing protein synthesis

• cells organelles replicating (mitochondria and ATP content increased - cell division is an active process)

prophase

Each chromosome consists of two identical sister chromatids joined together by a centromere (due to DNA replication)

each chromosome shortens and thickens (condensation)

the centrioles (not in plant cells) move to opposite poles of cell and the nuclear membrane breaks down

metaphase

centrioles in animal cells form spindles across cell- spindle consists of protein microtubules

each chromosome moves to the centre of the cell and attaches to spindle fibre via centromere

sister chromatids orientated towards the opposite poles of cell

anaphase

centromere splits and the sister chromatids separate.

sister chromatids are pulled to the opposite poles of the cell by spindle microtubules

telophase

chromatids are at opposite poles of the cell and begin to uncoil and the nuclear membrane reforms- two cells are genetically identical to each other and the original parent cell

cytokinesis

follows nuclear division and involves splitting the cytoplasm into 2. two new cells form as a cell surface membrane forms (cellulose cell wall in plant cells)

what is cancer and how is it treated?

group of diseases caused by uncontrolled cell growth and rapid division of cells. usually caused by damage to genes that regulate mitosis and cell cycle

cancer treatments often use drugs to stop cancerous cells from dividing. drugs may be used to inhibit the enzymes, DNA helicase or DNA polymerase or to inhibit formation of spindle

Describe and explain meiosis. When is it used?

meiosis is a type of cell division that produces cells that are genetically different. During meiosis, a single cell divides twice but DNA replication only occurs once. Produces 4 different daughter cells - haploids

what are differences between mitosis and meiosis?

meiosis

two nuclear divisions

four cells produced

genetically different cells produced

haploid cells are produced- possess half the chromosomes as parent cells

mitosis

one nuclear division

two cells produced

genetically identical cells produced

the cells produced contain the same number of chromosomes as the parent cells

why is meiosis important?

Its relevant in production of gametes (haploids) necessary for fertilisation

what causes genetic diversity during meiosis

independent segregation of homologous chromosomes: alleles of two or more different genes get sorted into gametes independently of one another - homologous chromosomes line up in the centre of cell randomly at metaphase preparing to seperate meaning same parent cell can produce different combinations of chromosomes in daughter cells

crossing over:

process where a chromatid breaks during meiosis and rejoins to the chromatid of its homologous chromosome so that the alleles are exchanged

further explanation

Only occurs in meiosis during prophase of the first meiotic division. two members of each homologous pair lie side by side forming a structure known as bivalent. the chromatids of homologous chromosomes then intertwine. sometimes chromatids break off and equivalent portions are exchanged (crossing over)- resulting in exchange of alleles of same genes and can produce new combination os alleles (recombinants)

this process known as genetic recombination- number of recombinants formed is usually low as crossing over is rare

describe the process of meiosis

in meiosis , 2 nuclear divisions occur : meiosis 1 and meiosis 2 to produce four haploid, genetically different cells

late interphase:

• DNA replication

• build up of ATP

• protein synthesis and replication of cell organelles

meiosis 1:

• seperates members of each homologous pair

meiosis 2:

• seperates the chromatids of each chromosome

how do chromosome mutations occur + explain

chromosome non disjunction : occurs when chromosomes fail to seperate during meiosis

one pair of chromosomes is not sepearted and both are pulled to the same pole of cell

this can lead to gametes with 1 extra and 1 less chromosome than normal.

describe principles of natural selection

• variations already present in population due to random mutations which can result in new alleles of genes

• many mutations are harmful but in some environments the new allele of gene might benefit an organism leading to increased reproductive success

• change in environment results in organisms with advantageous allele for a particular phenotybe being selected for as they are better adapted

• organisms with the allele for this selected phenotype survive and reproduce, passing on the advantageous allele while those without this allele do not - differential reproductive success

• over many generations the frequency of the advantageous allele increases as it is inherited by members of the next generation

what are the adaptations species may have to be better suited for survival?

• anatomical

• physiological

• behavioural

what are selection pressures?

environmental factors that effect chance of survival of an organism

what are the different types of natural selection?

• directional selection

• stabilising selection

describe and explain directional selection

• natural selection that produces a gradual change in allele frequencies over several generations

• usually happens when there is change in environment or selection pressures or a new allele has appeared in population that advantageous

for example: antibiotic resistant bacteria strains are more common due to over use of antibiotics

• presence of antibiotic is a selection pressure

• mutation are occurring in bacteria population randomly

• mutation arises that confers antibiotic resistance- it is a beneficial allele

• bacteria with this new allele are more likelihood survive

• most without this resistance mutation die

• over generation this leads to increase in frequency of beneficial allele that produces antibiotic resistance

describe and explain stabilising selection

• selection acts against extreme phenotypes in a population and occurs when the environment remains stable and selection occurs towards organisms with a phenotype closest to mean

• type of natural selection that keeps the alleles frequencies constant over generations

• things stay the same unless there is a change in the environment

for example optimum human birth weight:

natural selection favours those organisms with alleles for characteristics towards the middle of the range: birth mass

these organisms survive to breed and pass on their alleles to the next generation- the frequency of the alleles for optimal birth mass remains stable over generations.

babies with alleles for the extreme of this range have a higher mortality rate and fewer survive to breed and pass on their alleles to the next generation

very low and very high birth weights are selected against leading to the maintenance of the intermediate birth weight- outcome of their birth changes dependent on their weight

what is natural selection

darwin’s theory to explain mechanism of evolution

its the by which organisms become better adapted to their environment and survive and reproduce and pass on their advantageous alleles to their off spring, while those less adapted fail to do so.

what determines if the two organisms are the same species?

if they can interbreed and still produce fertile offspring

what is phylogeny

an organisms evolutionary history