Unit 5 Bio - Variation and Selection

DNA

deoxyribonucleic acid: a chemical of which genes are made (a double helix composed of sugar, phosphates and 4 nitrogenous bases)

gene

sections of DNA that codes for a particular feature/protein / control the production of proteins in a cell

each protein contributes towards a particular body feature (visible or not e.g. eye colour vs type of haemoglobin in red blood cells)

how do genes code for certain features?

by instructing cells to produce particular proteins which lead to the development of said feature

where is DNA found?

in the nucleus of a cell, in the chromosomes

DNA is the basis of

inheritance and reproduction

the structure of DNA

DNA consists of 2 molecules that are arranged in a double helix shape called nucleotides

Each nucleotide consists of

nitrogenous bases, pentose sugar, phosphate group

the base-pairing rule

complementing bases always link / bind with each other and never with any other bases

4 DNA bases

Adenine & Thymine

Guanine & Cytosine

DNA is the only chemical able to replicate itself exactly, meaning

it is able to pass genetic information from on generation to the next as genetic code

Genome

all of an organism's genetic material / all the DNA of that organism

what is an amino acid made up of?

of 3 nitrogenous bases

DNA replication

The process in which DNA makes a duplicate copy of itself.

process of DNA replication

1. the DNA molecule separates into two strands

2. DNA polymers assembles nucleotides into 2 new strands according to base-pair rule

3. each strand acts as a template for the formation of a new strand of DNA

4. 2 identical DNA molecules are formed ( each have a strand from the parent DNA and new complementary strand)

then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand.

the template strand

the strand of DNA that codes for the manufactur of proteins in a cell

what proteins are produced by the template strand?

structural

keratin: nails, hair, horns, feathers

collagen: bones, teeth cartilage

actin: muscle protein

functional

• often enzymes that go on to control processes in the cell

or haemoglobin, antibodies and some hormones

a gene

a sequence of triplets that code for all the amino acids in a protein

what is a protein and an amino acid made up of?

chains of amino acids

1 amino acid = a sequence of 3 bases in the template strand of DNA ( meaning DNA code is a triplet code)

how is the DNA code universal?

the triplets that code for individual amino acids are the same in all organisms/living things (meaning that genetic information is transferable between species)

universal nature of the genetic code is why genetic engineering (the transfer of genes from one species to another) is possible

where does protein synthesis take place? How is DNA pulled out of the nucleus?

in the cytoplasm while DNA is in the nucleus meaning to make the protein the genetic code must be copied then transferred out of the nucleus to the cytoplasm by RNA

Differences between DNA and RNA

• DNA is a double helix / has two strands whereas RNA has one strand

• DNA contains Thymine whereas RNA contains Uracil instead

• DNA has deoyribose, whereas RNA has ribose, uracil, and is single-stranded

what are the 2 types of RNA and what are their roles?

mRNA (messenger RNA): forms a copy of the DNA code

tRNA (transfer RNA): carries amino acids to the ribosomes to make the protein

A Summary of Transcription and Translation

transcription: the DNA stays in the nuleus so the code is copied out onto mRNA then exits the nucleus

translation: the tRNA then attaches to ribosomes

Transcription stages

• inside the nucleus mRNA attaches to non-template strand of DNA

• the enzyme separates the 2 strands of DNA

• RNA polymearse moves along the DNA to reach the coding part of the gene

• RNA polymearse adds complementary RNA nucleotides to the template strand

• RNA polymearse link said RNA nucleotides together to form a strand of mRNA

• the strand travels out the nucleus through a nuclear pore

transcription (long explanation)

takes place in nucleus

• in a chromosome, part of the DNA double helix unwinds / unzips so the 2 strands separate exposing the bases along the template strand

• the template strand is a framework upon which a molecule of mRNA is formed

• the mRNA line up along the template strand according to base pairing rules

• the RNA nucleotides link up to form an mRNA molecule

• they form bonds between their ribosome and phosphate groups

they join together to make the backbone of the molecule

• when a section of DNA has been transcribed, mRNA molecule leave the nucleus to the cytoplasm through a hole in the membrane

• DNA helix then zips up again back to normal

Translation definition

The process of converting mRNA to amino acids.

Codon

triplet of bases (each codon codes for 1 amino acid)

translation stages

• in the cytoplasm a ribosome attaches to the mRNA strand

• a tRNA molecule pairs up with each codon

• the ribosome joins together the amino acids carried by the tRNA molecule

• the types and order of amino acids in a chain cause it to fold in a specific shape

results in the formation of a polypeptide

Mutation + mutagen definition

random changes to the base pair sequence (has a higher chance of occurring due to mutagens)

mutagen: an agent that causes or increase the change of mutations, types are

• chemical mutagens (e.g. tobbaco or mustard gas)

• ionising radiation

types of mutation in DNA

duplication

deletion

substitution

inversion

duplication mutation in DNA

when a nucleotide is repeated (inserted more than once) > entire base sequence altered = whole gene is different and a new protein is coded

e.g.

ATT TCC > ATT TTC

deletion mutation in DNA

when a nucleotide is left out = code for new protein

e.g.

ATT TCC > ATT CCG

substitution mutation in DNA

when a nucleotide is replaced by another base = MAY code for a different protein b/c some amino acids use more than one codon

e.g.

ATT TCC > ATG TCC

inversion mutation of DNA

when a sequence of bases in a codon are reversed = may or may not alter the protein structure

e.g.

CTC CTC > CTC TC

or ATT TCC > ATT CCT

what is inside a chromosome?

one double-stranded DNA molecule, the DNA is folded and coiled around proteins called histones so it can be packed in a small space

How many chromosomes do humans have?

46 (23 pairs)

homologous pairs definition

matching chromosome pairs (e.g. XX) > they carry genes for the same features and are arranged in the same positions & sequence

how many chromosome pairs do women have vs men?

women have 23 homologous pairs

men have 22 homologous pairs and 2 that dont form a pair - the x and y chromosomes

what cells dont have the normal number of chromosomes?

red blood cells: have no nucleus so have none

gametes: have half the normal number of chromosomes (23) as they are formed by meiosis = each gamete cell has 1 chromosome from each homologous pair and one of the sex chromosomes

Genome meaning

The complete set of genes in a cell / the entire DNA of an organism

what happens when 2 gametes fuse during fertilisation?

the 2 nuclei join to form 1 diploid cell = the zygote. It has all its chromosomes in homologous pairs and 2 copies of every gene

Phenotype, Geneotype, allele definitions

phenotype: the physical appearance of a living organism / how a gene is expressed due to its geneotype ( some genes have more than one form which results in alternative forms of the feature)

geneotype: alleles an organism has for a certain characteristic

alleles: alternate forms of genes

dominant allele, recessive allele definitions

dominant: one copy of the allele is needed to have an effect

recessive: 2 copies of an allele are needed to have an effect

Heterozygous