UNIT 2 section 2 genetics
- Created by: Amy Burgess
- Created on: 14-04-16 16:39
dna function and structure
dna contains your genetic info
double helix structure= 2 seperate strands , coiled
strands are polynucleotides= lots of nucleotides joined together
nucleoties join to form polynucleotides between the phosphate group of one nucleotide and the sugar of another = a sugar phosphate backbone
nucleotide structure
a phosphate group, a pentose sugar and a nitrogenous base
the sugar = deoxyribose sugar
same sugar and phosphate on each nucleotide
bases can vary - 4 possible bases , Adenine (A) Thymine (T) Cytosine (C) Guanine (G)
specific base pairing
2 dna polynucleotide strands join together by hydrogen bonds (H-bonds) between the bases
each base can only join with one particular partner = specific base pairing
Adenine + Thymine
Cytosine + Guanine
structure vs function
double helix = dna is very stable - doesnt break down or get easily damaged
dna molecules are v long and are coiled up v. tightly = lot of info in a small space in the nucleus
dna molecules have a paired structure = easier to copy itself = semi conservative replication = important for cell division+ passing on genetic info. from gen to gen
dna storage
Eukaryotic cells-
linear dna molecules - exist as chromosomes
dna is wound around proteins called histones = this helps to support the dna
dna is tightly coiled to make a chromosome
Prokaryotic cells-
short circular dna
does exist as chromosomes
condenses to fit in the cell by supercoiling
what are genes
sections of dna
found on chromosomes
genes code for proteins
making proteins using genes
amino acids make proteins
different proteins have a different number + order of amino acids
order of nucleotide bases in a gene determines the order of amino acids in a protein
each amino acid is coded for by a sequence of 3 bases = triplet code
different base sequences code for different amino acids
dna is degenerate = more than one base triplet can code for the same amino acid
more possible combo's of 3 bases than there are amino acids
non-coding dna
genes in eukaryotic dna contains sections that dnt code for amino acids = INTRONS , non-coding sections of dna
exons = coding sections of dna
introns are removed during protein synthesis = splicing
eukaryotic dna also has sections that contain multiple repeats outside of genes, =these are also non-coding
genes and development
enzymes speed up our metabolic pathway ------ (chemical reactions that occur in the body)
these pathways determine how we grow and develop
all enzymes are proteins , which are built using genes
alleles
a gene can exist in more than one form = alleles
the order of bases in each allele is slightly different,
so they code for slightly different versions of the same gene
homologous chromosomes
our dna is stored as chromosomes in the nucleus of the cell
humans have 23 pairs of chromosomes , 46 in total
paris of matcing chromosomes = homologous chromsomes
in a h.pair both chromosomes are the same genes , although they could have different alleles
alleles coding for the same characteristics will be at the same locus (position) on each chromosome
gene mutations
changes in the base sequence of an organisms dna
mutations can produce new alleles of the gene
if the sequence of a gene changes , a non-functional protein could be produced
if theres a mutation in a gene that codes for a enzyme = this may change the shape of the enzymes active site = non-functional enzyme
gametes and fertilisation
gametes are sex cells - males= sperm , females= eggs
gametes join at fertilisation to from a zygote
normal body cells have diploid number (2N) of chromosomes = each cell contains 2 of each chromosome, one from mum one from dad
gametes have a haploid number (n) of chromosomes = only one copy of each chromosome
fertilisation = haploid sperm + haploid egg ----> diploid zygote
gametes are genetically different = no 2 gametes alike
gametes formed by MEIOSIS
meiosis (basics)
type of cell division
cells that divide by meiosis are diploid
cells formed by meiosis are haploid
without meiosis youde get double chromosomes when the gametes fused
meiosis (stages)
1. dna unravels+replicates so there are two copies of each chromosome =called chromatids
2. the dna condenses to form double armed chromosomes , made from 2 sister chromatids
3. the chromosomes arrange themselves into homologous pairs
4. the homologous pairs are seperated, halving the chromosome number
5. the pairs of sister chromatids that make up each pair are seperated
4 haploid cells (gametes) that are genetically different from each other produced
how to create genetic variation in gametes
1. crossing over chromatids
2. independant segragation of chromosomes
crossing over chromatids
during m1 homologous pairs join together and pair up
the chromatids twist around each other and bits of chromatids swap over
chromatids still contain the same genes but no have diff. combos of alleles
the crossing over chromatids in m1 means that each of the 4 daughter cells formed in m2 contain chromatids with different alleles
normally in the 4 cells produced, the 2 are the same and the other 2 are the same
increases genetic variation
independant segragation
4 daughter cells fromed from meiosis have completely diff. combos of chromosomes
1/2 chromosomes= maternal chromosomes
1/2 chromosomes= paternal chromosomes
when gametes are produced different combos. of maternal and paternal chromosomes go into each cells = independant segregation of chromosomes
what is genetic diversity
variety in dna
the more closely related the 2 species are , the more dna they will have in common
dna within a species varies very little , = as members of the same species have the same genes
not at all the same alleles = its the difference in alleles that creates genetic diversity
g.d. is important = if the enviro. changes . there are likely to be some organisms with the alleles that enables them to survive
factors affecting g.d.
increase g.d.
mutations in the dna= forming new alleles
gene flow = where diff. alleles move between pops. eg. through migration
decreses g.d.
genetic bottlenecks
founder effect
selective breeding
genetic bottlenecks
event causes a big reduction in a pop.
reduces number of diff. alleles in the gene pool = decrease g.d.
survivors reproduce + a larger pop. is created from few individuals
founder effect
when few organisms from a pop. start a new colony
only a small number of organisms contribute to their gene pool , so g.d. is reduced
more interbreeding the new pop. , which can lead to a high increase of genetic disease
result of geographical isolation
eg, amish pop
selective breeding
results in reduced g.d.
selecting which domesticated animals/strains of plants to reproduce to produce useful characteristics eg. high yield
once an organism with desired charatcteristics has been made, only this type of organism will continue to be bred
results in a genetic bottleneck
arguments for+against selective breeding
FOR -
produces high yielding
used to produce animals+plants that have increased resistance to diesease= farmers have to use fewer drugs/pesticides
increased tolerance of bad conditions eg. cold
AGAINST-
can causes health problems eg. cows
reduces g.d. , increased incidence of genetic disease +increased susceptibility to new diseases because of lack of alleles in the pop.
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