Transcript UNIT 4 PART1 MODERN GENETICS

UNIT 4 PART 1: MODERN
GENETICS
• In sexual reproduction
the new individual
develops from the
zygote formed by the
union of two gametes,
one from each parent.
• Because hereditary
material comes from
two different parents,
the offspring is both
similar to, and different
from, each parent.
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• GENETICS is the branch of biology that studies
the ways in which hereditary information is
passed on from parents to offspring.
• Gregor Mendel, an Austrian monk, was the first
to scientifically study heredity in the 1800s. He
studied the inheritance of certain traits in peas
using 1000’s of pea plants.
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• Mendel then thought that each trait was
controlled by a pair of “factors”, and that
each factor could be one of two kinds. For
example, one factor for green pod color
and one for yellow pod color.
• In a cross, the offspring receives one
factor from each parent.
• In a hybrid one factor may be hidden, but
show itself again in later generations when
fertilization brings together two of these
recessive factors. Ex: yellow pod color
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• When more was
learned about cells, it
was noted that the
movements of the
chromosomes during
meiosis and
fertilization would
account for the
separation and
recombination of
Mendel’s factors, so it
was thought that the
chromosomes might
carry the factors.
• The factors were then
called genes.
THE GENECHROMOSOME
THEORY
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ALLELES
• It is now known that genes do carry the
hereditary information, and that they are
on chromosomes in a definite order.
• Each body cell has two copies of the gene
for each trait. These two copies can be the
same or different.
• Each form of a gene that controls a trait is
called an allele. Pea pod color has two
alleles: green and yellow.
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• If the two alleles an organism has for a trait
are the same, like GG, then the individual is
said to be homozygous for that trait. If the
alleles are different, Gg, the individual is said
to be heterozygous.
• The genetic makeup of an individual, the
alleles it has, is called its genotype. The way
the individual looks as a result of its
genotype, its physical trait, is called its
phenotype.
GG = green; Gg = green; gg = yellow
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SEX DETERMINATION
• Scientists noticed that all
the chromosome pairs
were the same in both
males and females except
for one pair.
• This pair determines the sex of the individual.
These are the sex chromosomes. The other
pairs are called autosomes.
• In fruit flies, as well as humans, the two sex
chromosomes look the same in the female
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(XX), but are different in the male (XY).
GENE
• The traits Mendel
studied were all on
different chromosomes.
• Most chromosomes
have 100’s of genes so
many are inherited
together.
• Genes on the same
chromosome are said
to be Linked.
• abcde are linked, so
are ABCDE.
LINKAGE
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LINKED TRAITS
• Linked genes are
inherited together.
• Ex:
– Blond hair & Blue eyes
– Red hair & Freckles
– White cats & deafness
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CROSSING OVER
• Linked traits may be
separated by crossingover of homologous
chromosomes.
• Genes A an a have
crossed over and
formed new linkage
groups:
– Abcde and aBCDE
• Ex:
– Brown hair & Blue eyes
– Red hair & dark skin
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• Genes determine the
characteristics of an
individual and are
carried on
chromosomes.
• It still needed to be
known what a gene
was and how it
worked.
• Through a number of
experiments it was
determined that
chromosomes were
made of DNA and that
DNA was the
hereditary material. 11
DNA STRUCTURE
•
•
A DNA molecule is made up
of thousands of subunits
called nucleotides.
Each nucleotide has three
parts:
1. phosphate group
2. five carbon sugar –
deoxyribose
3. nitrogenous base
• 4 kinds of bases: adenine
(A), guanine (G), cytosine
(C), and thymine (T)
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DNA STRUCTURE
•
•
•
Watson and Crick
discovered the shape
of DNA.
Two chains of sugarphosphate groups
running parallel to
each other with pairs
of bases joining the
chains like rungs of a
ladder.
Twisting this ladder
forms the double
helix.
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DNA STRUCTURE
• Every DNA molecule has a different
sequence of bases.
• The order of bases along one strand of
the double helix determines the
matching bases on the other side:
– A always pairs with T; C always with G
– So if one strand is AGGTAC the other will
be:
TCCATG
• The two strands are said to be
complementary.
• One gene = a sequence of hundreds of
bases.
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DNA REPLICATION
• DNA can make copies
of itself.
• The two strands unzip
at the weak bonds
between the bases.
• Two new molecules
are built by attaching
new nucleotides to
each original strand
which acts as a
template, or pattern.
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DNA REPLICATION
• Each “new” strand will
actually be half old
and half new.
• The copies of the
DNA molecule can be
passed on to the
daughter cells during
cell division.
• This is how the
chromosomes
replicate.
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DNA FUNCTION
• How do the genes
control all body
traits and
functions?
• One Gene  One
Polypeptide
– Each gene codes
for the production
of a different
polypeptide chain.
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• The work of the cell is carried out by the
proteins it assembles.
• Proteins are made up of chains of
amino acids –i.e. polypeptide chains.
• There are 20 different types of amino
acids.
• The sequence of amino acids
determines the protein created and the
shape it takes on.
• Examples of proteins include: enzymes,
hormones, and structural proteins.
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PROTEIN SYNTHESIS BASICS
• One gene makes one polypeptide.
• The order of bases of three adjacent
nucleotides codes for a particular amino
acid. These 3 bases are called a codon.
– e.g. CAG is the DNA codon for glutamine
–
AAA is the DNA codon for phenylalanine
• Some codons indicate where a
polypeptide begins or ends. (Similar to
punctuation in a sentence.)
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•
•
•
Protein Synthesis Helper:
RNA
Polypeptides are synthesized
outside the nucleus, but the
genes are inside the nucleus.
RNA transcribes the genes
from DNA & takes them to
ribosomes outside the nucleus.
RNA is similar to DNA, but it
has 3 main differences:
1. the sugar is ribose
2. Uracil (U) takes the place
of the Thymine (T) base
3. RNA is single stranded
RNA
DNA
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THREE TYPES OF RNA
Messenger
(mRNA)
Transfer
(tRNA)
Ribosomal
(rRNA)
copies the
carries the
genetic code for a instructions
protein from DNA outside the
nucleus
carries amino
Amino acid is
acids to
attached to chain if
ribosomes
anticodon pairs
with codon
in the nucleoli,
site of the
protein and rRNA assembly line in
join together to
the cytoplasm of
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form a ribosome all cells
Summary of Protein Synthesis
• mRNA copies a gene (transcription)
Translation:
• mRNA attaches to
a ribosome
• tRNA brings amino
acids to the ribosome,
base pairing its
anticodon to the
mRNA codon
• Amino acids connect
to each other
• When finished, a
polypeptide is released
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INSIDE THE
NUCLEUS
OUTSIDE THE
NUCLEUS
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GENE EXPRESSION
•
•
•
•
All your body cells originated from a single
cell, so they all contain the same genes.
However, each cell only turns on the
genes it needs; it uses only some of the
genetic material it contains.
This is called expression.
Expression causes differentiation- the
formation of special types of cells.
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DIFFERENTIATION
• Chemical signals from
within the cell or from
other cells may
activate certain
genes. Ex: hormones
• The proteins that are
assembled in each
type of cell are
different so the cells
themselves become
different.
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GENE EXPRESSION CAN BE
AFFECTED BY THE ENVIRONMENT
•
•
The Himalayan rabbit is
white with some black
fur on some cooler
parts. If the skin on the
back is shaved & an ice
pack applied, the new
fur will also be black.
Alligators sex is determined by the egg temp.:
>34oC for males, <30oC
for females.
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