DNA, genes and chromosomes, topic questions and notes + specification

in prokaryotic cells, DNA molecules are

short and circullar and not associated with proteins

whereas in eukaryotic the DNa molecules are

linear and long and associated with proteins

mitochondria ad chloroplast contain DNA like prokaryotes which are

shorrtt circular and asssociated with proteins ( a link to evolution)

what is a gene

a gene is a base sequence that a DNA codes for

an amino acid

trna

and ribosomal rna

what is a locus

a fixed position that a gene occupies

what is a triplet

a sequence of three DNA bases, that codes for one amino acid

threee features of genetic code

Universal

non overlapping

degenerate

what are exons

coding regions

what is an intron

a coding region

structure of a chromosome

A DNA molecule is amde up of two polynucleotide strands, is twisted into a double helix around associateed histones

dna histone complex is formed Dna histone complex is coiled

coils form into loops

loops coil and pack together to forma chromosome

sister chromatids are held together by a centromere

what is a proteome

the full range of proteins that a cell is able to produce

what is a genome

a complete set of genes in the cell

similaritiies between tRNA and mRNA

single polypeptide chain

has the base uracil

differences between mRNA and tRNA

longer chain than tRNA

trNA has a clover leaf shape , mRNA is linear

tRNA is chemically stable, mRNA is chemically unstable

Transcription as the production of

mRNA form DNA

differences in transcription between prokaryotic and eukaryotic

in prokaryotic mRNA is produced directly form DNA

in eukaryotic, pre-mRNA is produced before it is spliced to form mRNA

what is splicing

the removal of non coding regions from pre-mRNA

protein syntehsis mechansim

DNA itemplate is transcribed into mRNA in the nucleus

mRNA base sequence is then translated into an amino acid sequence in association with tRNA on ribosmes in cytoplasm

the process of transcription

hydrogen bonds break between complementary bases of DNA

one strand acts as a template

freee mRNA nucleotides are attracted to the complementary bases

RNA polymerace joins adjacent pre-mRNA nucleotides together

pre-MRNA is spliced forming mrna

DNA strand joins together again

what is translation

Production of polypeptides from sequence of codons on mRNA

Describe translation

mRNA moves out from nucleus through the nuclear pore to the cytoplasm

mRNA attaches to the ribosome

ribosme moves to the start codon

anticodon on tRNA attaches to the complementary codon on mRNA

tRNA brings the specific amino acid

ribosme moves along the mRNA

and again complementray anticodon bind to codon

energy produced from ATP

form a peptide bond between amino acids

Difference between translation in prokaryotes and eukaryotes

occurs in smaller ribosomes and

does not use the ER because it is a prokaryote

use of ATP in translation

TP hydrolysis provides energy

for peptide formation between amino acids

why is the genetic code referred to as non-overlapping

the same base is not used in a different triplet

Describe the names and roles of the nuclear DNA that does not code for polypeptides

introns

they are removed after splicing

they are multiple repeats between genes

and they are regulatory sequences

what is a haploid cell

a cell that contains a single set of unpaired chromosome

Explain why a small proportion of 20,000 genes are transcribed 3 marks

revolves around the idea that genes are not always needed.

genes are switched off when not needed

because cells are specialised

genes are transcribed when they are needed

why is it important for chloroplasts and mitochondria to have their own DNA 2marks

it contains genes for respiration and photosnthesis

there is not need to import proteins from the cytoplasm

to assess means to

evaluate

differences between mitochondria anf prokaryotes

prokaryotes hve a cell wall, mitochondria does not

so the theory is hard to prove

mitochondrial DNA is inherited by the mother.

suggest a reason why the DNA is not inherited by the fathers

because no mitochondria in the sperm cell enters the eggg duringg fertilisation

explain why knowledge of the cells genome is insufficient to predict the proteome of that cell

not all genes are expresssed

genes are switched off

and introns are removed from the base sequence of mRNA

what is genetic mutation

involves a change in the base sequence of

genetic mutations can arise s

pontaneously during DNA replication

types of genetic mutations

base deletion and base substitution

not all base substitutions cause a change in the sequence of encoded amino acids because of the

the degenerate nature of genetic code

mutagenic agaents can

increase the rate of genetic mutation

mutations in the number of chromosomes can arise

spontaneously by chromosome non-disjunction during meiosis

meoisis

produces 4 haploid daughter cells that are genetically different from each other

genetically different daughter cells result

from the independent segregation of homologous chromosomes

crossing over between homologous chromosomes results in

genetic variation

random fertilization increases

genetic variation

define haploid cell

a cell that contains a single set of uinpaired chromosomes

what does n mean in genetic variation

number of homologous chromosomes

what happens to the base sequece when a base gets deleted

so the base triplet composition of amino acids change along the entire polypeptide chain

what what is natural selection

a process in nature where individuals with advantageous alleles are more liikely to survive, reproduce and pass on their traits

priciples of natural selection

Random mutation results to new alleles of a gene

often times the mutations are neutral

but in a particular environment, an allele becomes advantageous to the individual.

this increases the chance of reproductive success, inherited by members of the next generation.

the advatageous allele will increase in the population over future generation s

types of selection

directional and sttabilising

what is a frameshift

when the base triplet composition for amino acids change along the polypeptide chain because of a base deletion.

bases substitution

A base replaces another base , changing the genetic code for the protein

why would a base substitution not change the reulting polyeptide chain

because genetic code is degenerate

why is the genetic code degenerate

becuase all base triplet combinations only code for 20 amino acids

mutagenic agents succh as

chemicals or radiation

examples of mutagenic agents

ionising radiation

some viruses and microorganisms HPVVVVVVV

unhealthy diet

chemicals like tobacco, asbestos and benzene

in meoisis 1

independent segregation: homologous chromosomes line up side by side at the equator of the cell

crossing over of genes take place

homologous chromosomes seperate

non disjunction can happen in

anaphase 1 when homologous chromosomes refuse to separate or 2, where sister chromatids refuse to separate

compafring mitosis and meiosis

mitosis	meiosis
forms somatic cells	forms gametes
diploid to diploid	haploid to haploid
daughter cells are genetically identical	daughter cells are gentically different
two daughter cells produced	four daughter cells produced
no independent segregation	independent segregation
no crossing over	non sister chromatids can cross over an exchange genetic information

Explain how genetic recombination is achieved by crossing over homologous chromosomes

the chromatids of each pair cross over, forming a chiasma

a section of the chromatids is broken off

broken section rejoins to the homologous partner

Aside from crossing over, explain why meiosis is important in the formation of gametes

meoisis forms haploid cells

which then become diploid after fertilization

independent segregagtion occurs in homologous pairs

gametes are all genetically different leading to genetic variation