DNA and genetic diversity

what is a diploid cell

a cell that has a full set of chromosomes

what is a haploid cell

a cell that has half the number of chromosomes

what are the features of homologous chromosomes

• pair of maternal (mum) and paternal (dad) chromosomes

• two chromosomes that carry the same genes but not necessarily the same alleles of the genes

• chromosomes are same shape and size

• genes are at the same loci

describe the features of DNA in eukaryotes

• enclosed in the nucleus

• long

• linear

• wrapped around histone proteins

• in chromosomes (form)

• contain more genes than prokaryotic DNA

describe DNA found in prokaryotes

• not enclosed in a nucleus

• short

• circular (plasmids and free DNA)

• not wrapped around proteins

• contain fewer genes than eukaryotes

describe DNA found in the mitochondria and the chloroplast

• the mitochondria and chloroplast are prokaryotic cells that were absorbed by eukaryotic cell via endosymbiosis. So their features of DNA are the same as prokaryotic cells

  • short

  • circular

  • not enclosed in a nucleus

  • fewer genes than eukaryotic cells

  • not wrapped around proteins

what is a gene

A section of DNA that codes for the amino acid sequence of a polypeptide chain and functional RNA

what is the locus of a gene

the position/location of a gene on a chromosome

what is an allele

one of a number of alternate forms of a gene

what is a codon

a sequence of three bases in mRNA

what is a triplet

a sequence of three bases in DNA

what is the role of a triplet in DNA

codes for a codon in mRNA which during translation codes for a specific amino acid that makes up a polypeptide chain

what does it mean by the genetic code being universal

each triplet codes for the same amino acid in all organisms

what does it mean by the genetic code being non-overlapping

each base sequence is only read once

what does it mean by the genetic code being degenerate

most amino acids can be coded for by more than one codon or triplet

what is a benefit of the genetic code being degenerate

some mutations may have no effect on the polypeptide formed

why are amino acids coded for by triplets and codons (3 bases)

triplets and codons consist of a sequence of three bases

there are a total of 20 different amino acids that we need to code for

there are a total of 4 base pairs

• if each base coded for an amino acid we could only code for 4 different amino acids (4^1 = 4)

• if we used base pairs to code for an amino acid we could only code for a total of 16 different amino acids 4² = 16

• we use triplets because 4³ = 64

do only prokaryotes or eukaryotes have introns and exons

only eukaryotes have introns and exons

what are introns and exons

exons- coding sections of a gene that code for amino acids

introns- non coding sections of a gene. They are removed (spliced) before protein synthesis occurs

what is the genome

complete sets of genes in a cell, including mitochondria and chloroplasts

what is the proteome

the full set of proteins that a cell is able to produce at a given time

what is the transcriptome

all the mRNA within a cell

how does a cells genome, proteome, and transcriptome relate to each other

what is the structure of RNA

• polynucleotide

• single stranded

• contains ribose sugars

• 4 bases- A U C G

• phosphate group

• phosphodiester bonds

describe the structure of mRNA

• single polynucleotide chain

• linear

• pentose sugar is ribose

• phosphate group

• phosphodiester bonds

• shorter chain than DNA but longer than tRNA

• less stable than DNA and tRNA

describe the structure of tRNA

• single polynucleotide chain

• clover shaped

• pentose sugar is ribose

• phosphate group

• phosphodiester bonds

• shorter chain than DNA and mRNA

• less stable than DNA but more stable than mRNA

what is the order of the processes involved in protein synthesis

• transcription- reading the DNA and copying it into RNA

• splicing

• translation- where the cell uses information from the mRNA to produce proteins

describe the process of transcription

• free nucleotides in the nucleus become phosphorylated via the hydrolysis of ATP to become activatedFree

• RNA polymerase binds to the promotor region

• the two strands of DNA start to unzip and hydrogen bonds between complementary bases on either strand are broken. This is catalysed by DNA helicase

• the phosphorylated nucleotides line up opposite the exposed bases on the template strand according to complementary base pairing

• temporary hydrogen bonds form between the complementary base pairs so that the order of the bases is maintained

• RNA polymerase forms phosphodiester bonds between the adjacent nucleotides

• RNA synthesis stops when RNA polymerase reaches the terminator region

what happens in the process of splicing

• occurs in the nucleus immediately after transcription

• introns are removed/spliced from the gene by a spliceosome enzyme

• only occurs in eukaryotic cells because prokaryotes don’t have introns

after this, the mRNA strand moves out of the nucleus via the nuclear pores into the cytoplasm where it binds to a ribosome

describe the process of translation

• following transcription mRNA leaves the nucleus via the nuclear pores and enters the cytoplasm and attaches to a ribosome

• tRNA molecules carry a specific single amino acid and have an anticodon that codes for the tRNA molecules’ amino acid

• the tRNA molecule with a complimentary anticodon to the start codon on the mRNA strand brings over the first amino acid to the mRNA chain due to complementary base pairing

• when the tRNA molecule brings the amino acid over to the mRNA strand hydrogen bonds form between the tRNA and mRNA molecules

• a second tRNA molecule binds to the adjacent codon on the mRNA molecule and peptide bonds are formed between the two amino acid molecules via a condensation reaction

• then the first tRNA molecule leaves the ribosome

• the ribosome then moves along the mRNA molecule and the process repeats and the length of polypeptide chain increases

• the length of the polypeptide chain increases until the ribosome reaches the stop codon on the mRNA strand

why do we need cells to divide by meiosis

sexual reproduction requires the fusion of male and female gametes (n) too produce an offspring (2n)

what are the stages of meiosis

interphase

cell replicates its DNA (2n→4n)

prophase 1

nuclear envelope breaks down, spindle fibres form and crossing over occurs

metaphase 1

independent assortment occurs here when homologous pairs are lined up along the equator of the cell

anaphase 1

spindle fibres pull homologous chromosomes to opposite poles of the cell

telophase and cytokinesis 1

chromosomes gather at poles of the cell and the cytoplasm splits

prophase 2

new spindle fibres form

metaphase 2

independent assortment occurs again as homologous pairs are lined up along the equator of the cell

anaphase 2 centromeres’s divide and chromatids are pulled to opposite poles of the cell

telophase and cytokinesis 2

nuclear envelope reforms around each set of chromosomes and the cytoplasm divides resulting in 4 haploid cells

what are the 3 ways in which meiosis causes genetic variation in offspring

• independent assortment

• crossing over

• random fertilisation

how does independent assortment cause genetic variation

it is random which way round homologous chromosomes line up during metaphase 1 and 2, so maternal and paternal chromosomes can get mixed up (varied) into daughter cells

how does crossing over increase genetic variation in offspring

occurs during prophase 1 between homologous pairs of chromosomes. Chromatids on each homologous pair become twisted and cross over, this region is called the chiasma. During this process tensions are created and portions of the chromatid break off and rejoin the other homologous chromosome, leading to a new combination of alleles

how does random fertilisation cause genetic variability

random in which male gamete fuses with which female gamete

what is non disjunction

when chromosomes aren’t split into daughter cells correctly during meiosis divisions, leading to incorrect numbers of chromosomes in daughter cells

the incorrect numbers of chromatids may mean that they won’t be able to form chromosome pairs as there are uneven numbers

what is polyploidy

more common in plants

results in changes to chromosome numbers in plant cells