analysing evidence

history of dna

individuals involved - j watson, f crick and roselind franklin

rosalind franklins image of dna - image 51, proved the double helix structure

technique used to image dna - x-ray

genetic structures - overview

a chromosome is made of 1 DNA strand, along this chromosome is multiple genes, these structures are in the nucleus   

dna

• deoxyribonucleic acid

• the genetic material of an organism, all of an organisms body cells will have the same DNA

• classifed as a nucleic acid (specific group of molecules)

dna - structure

double helix strucutre

- two strands

two major bonds in DNA - between the strands, within a strand

monomer

small single simple molecules that can join together

polymer

large molecules made up of the smaller monomer units together in a repeating chain

polynucleotides

• a single strand of DNA that are long chains of nucleotide monomers linked together , known as a polynucleotide - made of subunits called nucleotides

polymer - polynucleotide

monomer - nucleotide

nucleotide

3 parts

• deoxyribose sugar - as its pentose sugar

• negative phosphate group (PO4³-)

• nitrogenous base (varies) : adenine, guanine, cytosine or thymine

bonds within a strand (nucleotides)

strong sugar phosphate bonds join nucleotides to create a polynucleotide strand

• between the negative phosphate group and the deoxyribose sugar

dna backbone

deoxyribose sugar, negative phosphate group

dna - base pairing and bonds

four bases : adenine (A), thymine (T), guanine (G), cytosine (C)

bond on a complementary base pairing system - joined by hydrogen bonds

• A will always bond to T (2 bonds)

• G will always bond to C (3 bonds)

not as strong as sugar phosphate bond

chargaff’s rule

in DNA the number of

• A = T

• G = C

applies when considering both DNA strands, not one

genes

a sequence of DNA bases which codes for your traits/proteins

• proteins control the function and structure of an organism

• all your physical attirbutes are a result of the sequence of bases in you DNA

• traits can be determined by multiple genes

• there are segments of DNA that do not code for a trait

genes are passed onto subsequent generations - genes of offspring are derived from their parents

genome

the complete set of genes in an organism

chromosomes

a chromosome is compacted DNA - found in the nucleus of eukaryotic cells

• eukaryotes - organisms with nucleus and organells

1 chromosome is composed of 1 DNA double helix, humans have 46 chromosomes (23 pairs)

karyotype

an image of an individuals complete set of chromosomes

human chromosomes

• 1 pair of sex chromosomes (X or Y)

• 22 homologous pairs of autosomal chromosomes (1-22)

autosomal chromosomes (autosomes)

non sex-chromosomes

you have 2 of each autosome

sex chromosomes

determine sex of indivudal

• XX = female , 23 homologous pairs

• XY = male , 22 homologous pairs , 2 non homologous sex chromosomes

each parent contributes 1 chromosome to each of the 23 pairs of chromosomes

homologous pairs

chromosomes pairs with the same

• genes

• gene position (locus) / bands

• length

linked genes

genes on the same chromosome

dna replication - importance

DNA must be replicated to produce new cells (new DNA molecules are made from existing DNA) - ensures new cells have correct number of chromosomes

new strands must be identical to the original

by using existing DNA strands as templates, we can ensure that new DNA is identical to the original

dna replication - enzymes

dna helicase - breaks hydrogen bonds between DNA strands / bases (unzipped)

dna polymerase - add nucleotides to template strands to create new strands

dna replication - process

1. DNA helicase moves along the DNA, breaking the hydrogen bonds between bases

2. DNA polymerase adds complementary nucleotides to the template strands to create new strands, 1 nucleotide at a time - helix will reform when strand is complete

semi conservative process

semi - half

conservative - to retain , keep the same

because new DNA is made from existing DNA strands, half of each new DNA helix is from the original

DNA replication - after

before replication - single strand chromosomes

after replication - double strand chromosomes

chromatids

• 1 chromosome strand

• DNA replication produces chromosomes with 2 chromatids

sister chromatids - on the same chromosome, chromatids attached by a centromere with the same genetic information

centromere

joins the chromatids

diploid vs haploid

diploid - 2 sets of chromosomes, body cells

haploid - single set of unpaired chromosomes (1 set for humans), gametes (sex cells, sperm and egg cell)

cell division - mitosis

• process - 1 parent cell divides into 2 diploid daughter cells, identical to the parent (orirginal) daughter cells = new cells

• occurs in somatic cells (body cells)

purpose of mitosis

• growth and development

• replacing old or damaged cells

mitosis seperations

before mitosis

• dna replication must occur first, ensures each daughter cell can have enough chromosomes

during mitosis

• sister chromatids seperate into each daughter cells

cell division - meiosis

process - 1 parent cell divides into 4 unique haploid daughter cells

• occurs in germ line cells to form gametes (sex cells) - diploid to haploid

why gametes have to be haploid

purpose

• to produce gametes for sexual reproduction

• 2 haploid gametes fuse during fertilisation to contribute a set of chromosomes each 23

• form a zygote with the correct number of chromosomes

meiosis seperations

pre meiosis - dna replication must occur first

2 stage process

• meiosis I - seperation of homologous chromosomes, produces 2 haploid daughter cells with sister chromatids

• meiosis II - seperation of chromatids, produces 4 haploid daughter cells with 23 chromosomes (chromatids)

meiosis variations, name

extra processes during meiosis I to ensure the 4 daughter cells are not identical

• independent assortment

• crossing over

variation by independent assortment

• homologous pairs line up randomly when split

variation - daughter cells have different chromosome combinations of parent chromosomes

variation by crossing over

• dna exchange between homologous pairs of non sister chromatids

homologous pairs have the same genes but could have different base sequence

gene expression

the process of producing proteins from genes

• transcription: DNA to mRNA

• translation: mRNA to protein

RNA

• single stranded nucleic acids involved in protein synthesis - made up of nucletides

composed of

• ribose sugar

• phosphate group

• cyclic nitrogenous base

RNA vs DNA differences

RNA has uracil (U) instead of thymine (T)

• A binds with U

• G binds with C

transcription

objective - produce a strand of mRNA from a gene of DNA

location - nucleus

mRNA - messenger RNA

• capable of exiting the nucleus to act as a template for a protein

• produced by RNA Polymerase

transcription process

1. RNA polymerase binds to template strand causing DNA to unwind

2. RNA polymerase runs along the strand to produce mRNA using complementary base pairs

3. at end of gene, RNA polymerase and mRNA are released

4. mRNA exits the nucleus

proteins and amino acids

polymer: protein, proteins are larger complex polymers and are made up of long chains of amino acids

monomer: amino acids, amino acids are the monomer of proteins just as nucleotides are the monomers of DNA and RNA

translation

objective: produce a protein from mRNA (proteins are formed from a sequence of amino acids one at a time)

location: ribosome

tRNA - transfer RNA

• carry amino acids to the mRNA based on the complementary base pairing

codons

sequences of 3 bases on mRNA

translation process

1. tRNA transport amino acids to the mRNA based on completementary base pairing

2. the amino acids are joined as they are added

3. once the stop codon is reached, the amino acid chain (protein) is released

redundancy

multiple codon sequences code for a single amino acid

tips for translation

1. look for start codon (AUG)

2. write full name of amino acid

3. don’t write stop

mutations

a change to the sequence or structure of DNA

typically occur during DNA replication

can be inherited from parents

cause of mutations

mutagens - environmental factors which increase the frequency of mutation

genetic (single gene) mutation - point substitution mutations

point substitution mutations: 1 base is changed

• missense : an amino acid is changed

• silent: no amino acid change

• nonsense: early stop codon is produced

genetic (single gene) mutation - frameshift mutations

frameshift mutations: bases added or removed (changed reading frame of codons)

• insertion; base added to a gene

• deletion: base removed from a gene

amino acid sequence functionality

• changed amino acid sequence = protein no longer functional

• unchanged amino acid sequence = protein still functional

chromosomal structural mutations

segments of chromosome is changed

• deletion - chromosome segment removed

• duplication - chromosome segment repeated

• translocation - chromsome segment moved to another chromosome

• inversion - chromsome segment if flipped

chromosomal number mutations =

total no. of chromosomes changes

aneuploidy

abnormal no. of specific chromsome

• trisomy 21 - down sydrome

polyploidy

abnormal no. of chromosome sets

alleles

• different forms of a gene

classified as

• dominant (uppercase)

• recessive (lowercase)

genotype

someone allele combination for a trait

• homozygous dominant - 2 dominant alleles

• homozygous recessive - 2 recessive alleles

• heterozygous - 1 of each allele (known as a carrier)

dominant allele masks the recessive allele when present

phenotype

the observed traits in an organism (depends on genotype and environment)

• detached (dominant trait) / attached (recessive trait) earlobes

human somatic cells

diploid - 2 sets of chromosomes

• humans have homologous chromosome pairs

• one set inherited from each parent, so each parent contributes 23 chromosomes

• each parent contributed an allele to all of your genotypes

• your alleles/genotype were dependent on your parents genotypes

punnett square

determine genotype of offspring

• genotype ratio - ½ Dd : ½ dd

• phenotype ratio - ½ detached earlobes : ½ attached earlobes

law of dominance - mendels experiments conclusions

dominant alleles mask recessive alleles

law of segregation - mendels experiments conclusions

alleles are seperated during meiosis

law of independent assortment - mendels experiment conclusions

alleles for different traits are inherited independently on one another

x linked traits - sex linked traits

traits coded by a gene on the x chromosome

• alleles can be dominant or recessive

x linked dominant traits

more females affected

• inherit 2 X chromosomes while males only inherit 1

x linked recessive traits

more males affected

• only inherit 1 X chromosome

• no chance of having dominant allele to make the recessive

Y linked traits

traits coded by a gene on the Y chromosome

• affect biological males