4.1-4.3

similarities of DNA in eukaryotic cells and DNA in prokaryotic cells

● nucleotide structure is identical - deoxyribose attached to phosphate and a base

● adjacent nucleotides joined by phosphodiester bonds, complementary bases joined by hydrogen bonds

● DNA in mitochondria / chloroplasts have similar structure to DNA in prokaryotes

○ short, circular, not associated with proteins

differences of DNA in eukaryotic cells and DNA in prokaryotic cells

● eukaryotic DNA is longer

● eukaryotic DNA is linear, prokaryotic DNA is circular

● eukaryotic DNA is associated with histone proteins, prokaryotic DNA is not

● eukaryotic DNA contain introns, prokaryotic DNA does not

what is a chromosome

long, linear DNA and it’s associated histone proteins

in the nucleus of eukaryotic cells

what is a gene

a sequence of DNA bases that codes for

• the amino acid sequence of a polypeptide

• or a functional RNA

what is a locus

fixed position a gene occupies on a particular DNA molecule

triplet code

sequence of 3 DNA bases, called a triplet codes for a specific amino acid

describe the nature of the genetic code (3)

universal

non overlapping

degenerate

universal

the same base triplets code for the same amino acids in all organisms

non overlapping

each base is part of only one triplet so each triplet is read as a discrete unit

degenerate

an amino acid can be coded for by more than one base triplet

what are non coding base sequences

DNA that does not code for amino acid sequences

where are non coding base sequences found

between genes - non coding multiple repeats

within genes - introns

what are exons

base sequence of a gene coding for amino acid sequences

what are introns

base sequence of a gene that doesn’t code for amino acids in eukaryotic cells

what is a genome

the complete set of genes in a cell

what is a proteome

the full range of proteins a cell can produce

what are the stages of protein synthesis (2)

transcription

translation

what is a transcription

production of mRNA from DNA in the nucleus

what is translation

production of polypeptides from the sequence of codons carried by mRNA at ribosomes

similarities of mRNA and tRNA

both single polynucleotide strand

differences between mRNA and tRNA

• tRNA is folded into a clover leaf whereas mRNA is linear

• tRNA has hydrogen bonding between paired bases whereas mRNA doesn’t

• tRNA is a shorter fixed length whereas mRNA is a longer variable length

• tRNA has an anticodon, mRNA has codons

• tRNA has an amino acid binding site, mRNA doesn’t

describe how mRNA is formed by transcription in eukaryotic cells

1. hydrogen bonds between DNA bases break by DNA helicase

2. only one DNA strand acts a template

3. free RNA nucleotides align next to complementary bases on template strand

   • in RNA uracil is used instead of thymine

4. RNA polymerase joins adjacent RNA nucleotides

5. this forms phosphodiester bonds via condensation reactions

6. pre mRNA is formed and this is spliced to remove introns forming mRNA

how is production of mRNA in eukaryotic cells different to production of mRNA is prokaryotic cells

pre mRNA produced in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells

genes is prokaryotic cells don’t contain introns so no splicing

describe how translation leads to production of a polypeptide

1. mRNA attaches to ribosome and moves to a start codon

2. tRNA brings a specific amino acid

3. tRNA anticodon binds to complementary mRNA codon

4. ribosome moves along to next codon and another tRNA binds so 2 amino acids can be joined by a condensation reaction forming a peptide bond

   • using energy from hydrolysis of ATP

describe the role of ATP in translation

hydrolysis of ATP to ADP + Pi releases energy

so amino acids join to tRNA and peptide bonds form between amino acids

describe the role of tRNA in translation

attaches to specific amino acid in relation to its anticodon

tRNA anticodon complementary base pairs to mRNA codon forming hydrogen bonds

2 tRNA bring amino acids together so peptide bond can form

describe role of ribosomes in translation

mRNA binds to ribosomes with space for 2 codons

allows tRNA with anticodons to bind

catalyses formation of peptide bond between amino acids

moves along

what is a gene mutation

a change in base sequence of DNA

can arise spontaneously during DNA replication

what is a mutagenic agent

factor that increases rate of gene mutation

explain how a mutation leads to production of a non functional protein or enzyme

1. changes sequence of base triplets in DNA so changes sequences of codons on mRNA

2. so changes sequence of amino acids in polypeptides

3. so changes position of hydrogen/ionic/disulphide bonds

4. so changes protein tertiary structure

5. enzymes - active site changes shape so substrate can’t bind, E-S complexes can’t form

explain the possible effects of a substitution mutation

1. base in DNA replaced by a different base

2. this changes one triplet so changes mRNA codon

3. so one amino acid in polypeptide changes

   • tertiary structure may change if position of hydrogen/ionic/disulphide bonds change

   • or amino acid doesn’t change due to degenerate nature of genetic code or if mutation is in an intron

explain possible effects of a deletion mutation

1. one base removed from DNA sequence

2. changes sequence of DNA triplets from point of mutation

3. changes sequence of mRNA codons after point of mutation

4. changes sequence of amino acids in primary structure of polypeptide

5. changes position of hydrogen/ionic/disulphide bonds in tertiary structure of protein

6. changes tertiary structure of protein

describe features of homologous chromosomes

same length

same genes at same loci but may have different alleles

diploid cells

has 2 complete sets of chromosomes

2n

haploid

has a single set of unpaired chromosomes

n

stages of meiosis (3)

interphase

meiosis I

meiosis II

interphase

DNA replicates - 2 copies of each chromosome, joined by the centromere

meiosis I

separates homologous chromosomes

chromosomes arrange in homologous pairs

crossing over between homologous chromosomes

independent segregation of homologous chromosomes

meiosis II

separates chromatids

why are the number of chromosomes halved in meiosis

homologous chromosomes are separated during meiosis I

explain how crossing over creates genetic variaiton

homologous pairs of chromosomes form a bivalent

chiasmata form

alleles exchanges between chromosomes

creating new combination of alleles on chromosomes

explain how independent segregation causes genetic variation

homologous pairs randomly align at equator - so random which chromosomes from each pair goes into each daughter cell

creating different combination of maternal & paternal chromosomes/alleles in daughter cells

other than mutation and meiosis explain how genetic variation is increases

random fertilisation

creates new allele combinations

explain the different outcomes of mitosis and meiosis

1. mitosis produces 2 daughter cells whereas meiosis produces 4 daughter cells

   • as 1 division in mitosis, whereas 2 divisions in meiosis

2. mitosis maintains chromosomes number whereas meiosis halves chromosome number

• as homologous chromosomes separate in meiosis but not mitosis

3. mitosis produces genetically identical daughter cells whereas meiosis produces genetically varied daughter cells

• as crossing over and independent segregation occurs in meiosis

explain the importance of meiosis

• two divisions creates haploid gametes

• so diploid number is restored in fertilisation - chromosome number maintained over generations

• independent segregation and crossing over creates genetic variation

how can you recognise mitosis and meiosis in a life cycle

mitosis when chromosome maintained

meiosis when chromosome number halves

describe how mutations in the number of chromosomes arises

spontaneously by chromosomes non disjunction during meiosis

homologous chromosomes or sister chromatids fail to separate in meiosis

gametes have extra copy of a particular chromosome and other have none

calculate number of possible combinations of chromosomes in daughter cells following meiosis

2ⁿ

^{n = number of pairs of homologous chromosomes}

calculate number of possible combinations of chromosomes following random fertilisation of two gametes

(2ⁿ)²