Biology 1010 - Unit 2

genetics

study of the inheritance of observable traits from one generation to the next, and their effect on populations and species

molecular biology

the study of the molecular processes involved in the transfer of genetic information from genotype to phenotype of an organism

expression of a gene may be affected by…

environment

genotype determines…

phenotype

phenotype

an organism’s physical and biochemical traits

genotype

an organism’s genetic makeup, the genetic information contained in genes

_____ backbone forms the “ribbons” of DNA?

sugar-phosphate

_____ form the “rungs of the ladder”

nitrogenous bases

structure of DNA

double helix

each chromosome is…

one long DNA molecule

name the parts of the chromosome

homologous chromosomes

2 chromosomes, same size, same shape, same genes — but not identical

locus

a specific place along the length of a chromosome where a given gene is located

• eg. gene for eye colour in fruit flies

alleles

alternative versions of the same gene

• eg. each chromosome can have a different version of the eye colour gene

  • red or white eye allele

when do homologous chromosomes exist?

G₁ (before DNA starts to replicate)

sister chromatids

2 DNA strands with identical nucleotide sequence, one copied from the other to prepare for mitosis, joined at the centromere

homologous chromosomes are not to be confused with…

sister chromatids

when are sister chromatids not present? when are they?

Not- G₁

Are- when the cell prepares to divide

S-phase

DNA replication

what is this

a single replicated chromosome (aka mitotic chromosome), consisting of 2 sister chromatids

G₂

pre-mitosis check

what happens during G₂ - pre-mitosis check

• sister chromatids scrambled everywhere

• chromosomes (duplicated and uncondensed)

what happens during mitosis prophase

• nuclear membrane starts to break apart

• 2 replicated chromosomes (2 sister chromatids) condense

• centrosomes move apart

what happens during mitosis prometaphase

• nuclear envelope disappears

• spindles interact with chromosomes

what happens during mitosis metaphase

• alignment of chromosomes on metaphase plate

what happens during mitosis anaphase

• daughter chromosomes move to opposite ends of the cell

• homologous chromosomes separate (sister chromatids stay together)

what happens during mitosis telophase

• nuclear envelope reforms

• chromosomes condense

mammalian somatic cells ___ sets of homologous chromosomes. They are _______ (they have ___ chromosomes)

2

diploid

2n

only _____ _________ have only one set of homologous chromosomes. they are _________ (they have ___ chromosomes)

sex cells

haploid

n

human karyotype

display of condensed chromosomes arranged in pairs

application of human karyotype

• identification of chromosomal abnormalities

  • abnormal number, abnormal arrangements

• these abnormalities are associated with certain congenital disorders

  • trisomy 21/ down syndrome

histones

the proteins that complex DNA

DNA + protein = __________

chromatin

several conformations (types) of chromatin exist in the…

nucleus

hierarchical structure of a chromosome

• “naked” DNA

• nucleosomes

• chromatin fiber

• chromatin loops

• sister chromatid

• metaphase chromosome

purpose of mitosis

to ensure that an exact copy of the parent cell’s DNA is passed on to the 2 new daughter cells

mitosis is a ________ process

continuous

• humans decided to separate that process into ‘phases’ because we like

  to categorize things

• there are actually several inter-dependent

  processes occurring in parallel

what is mitosis

cells split into 2 daughter cells with equivalent DNA

types of asexual reproduction

• budding (hydra)

• fragmentation (plants, some non-vertebrate animals)

If all the cells in an organism have the same genes, can the nucleus from a differentiated animal cell be used to create a new organism (a clone)

The potential of a cell nucleus to produce a whole organism decreases as the cell becomes more differentiated, presumably due to changes in the nucleus

gametes (eggs & sperm) of an organism contain a single basic complement (______) of chromosomes

haploid

fusion of haploid gametes to form a new diploid cell is…

fertilization

fertilization

the fusion of haploid gamete to form a new diploid

what is produced by fusion of egg and sperm

zygote

what does a zygote contain?

in each cell 2 copies of each chromosome (diploid)

what is meiosis?

the cellular process of reducing the diploid complement of chromosomes to a haploid complement of chromosomes to produce sex cells (gametes), an essential process for sexual reproduction

diploid: 2n or 1n

2n

haploid: 2n or 1n

1n

what are is the key difference between mitosis and meiosis?

mitosis: separation of the sister chromatids

meiosis 1: separation of the homologous chromosomes

2n = 6 → breakdown

n = 3 (maternal)

n = 6 (paternal)

synapsis

pairing of homologous chromosomes

• crossing over may occur

crossing over

exchange of alleles

• number of genes in each chromosome stays the same

what are the different chromosome arrangements in mitosis, meiosis l, and meiosis ll

mitosis - sister chromatids

meiosis l - homologous chromosomes

meiosis ll - sister chromatids x2

why sex and not just asexual reproduction?

new alleles and new combination of alleles

what is the purpose of meiosis

1. divide the number of chromosomes in half

   • still want one copy of all the genes

   • diploid (2n = 2 sets)

   • haploid (n = 1 set)

2. variation: new alleles or new combinations of alleles

   • gametes randomly receive one chromosome of each homologous pair

   • crossing over shuffle genes within homologous chromosomes

lecture 1 summary

hybridization

crossing of 2 true-breeding varieties

Mendel’s first law: Segregation of Alleles

2 alleles for a heritable characteristic segregate (separate from each other) during gamete formation and end up in different gametes

___________ __________ of genes account for variations in inherited characters. Mendel’s Observations For each character, an organism inherits ____ _____ (that is, two alleles) of a gene, one from each parent. If the two alleles at a locus differ: ______ _____ determines the organism’s appearance ______ _____ has no noticeable effect

Alternative versions

two copies

dominant allele

recessive allele

genotype determines…

phenotype

heterozygous

an organism with two different alleles for a character

homozygous

an organism with a pair of identical alleles

Mendel’s second law: Independent Assortment of Genes

during gamete formation, a pair of alleles for one gene will segregate independently of a pair of alleles for another gene

deviations from Mendel’s Laws

1. some traits not on nuclear chromosomes

   • eg. mitochondrial or chloroplast chromosomes

2. traits on the same chromosome

3. traits carried on sex-chromosomes

   • eg. mammals XX female, XY male

other deviations from Mendel’s Laws

1. incomplete dominance

2. multiple alleles

3. co-dominance

4. lethal alleles

incomplete dominance

red + white = pink

multiple alleles & co-dominance

blood types

A allele produces the enzyme __________, which catalyzes the transfer of…

GaINAc residues

B allele encodes the enzyme __________, which catalyses the transfer of…

galactose residues

the i allele ______ both enzymatic activities

lacks

lethal alleles

ratio of offspring is not 3:1, but 2:1

what is giemsa stain?

nucleic acid stain that attaches preferentially to A-T bonds

There are way more genes on a chromosome than there are Giemsa stain bands

autosomes

22 pairs of homologous chromosomes exactly the same in both male and female karyotypes

sex chromosomes

one pair of chromosomes (mammals)

female - XX / male - XY

classifications of genetic disorders

• multifactorial

• chromosomal

• single gene

multifactorial disorders

• congenital malformations

• cancers

• coronary artery disease

chromosome or cytogenetic disorders examples

• down syndrome

  • trisomy 21

• XYY syndrome

  • extra Y

nondisjunction

nondisjunction gives rise to:

• monosomics

• trisomics

monosomy

lack a single copy of a chromosome

trisomy

carry an additional copy of a chromosome

what does a down syndrome karyotype look like?

portion of chromosome 21 translocated to chromosome 14

Incidence of Down Syndrome (and other trisomies) ________ dramatically with the age of the mother

increases

nondisjunction of sex chromosomes

sex chromosome disorders

• Klinefelter syndrome - sterile - male w/female characteristics

• turner syndrome - sterile - female, short, immature sex organs

• XXX syndrome - learning difficulties

• XYY syndrome - behavioural difficulties

single gene disorders

caused by a mutant gene allele

in single gene disorders, where is the mutant allele found?

may be present on only one chromosome of a homologous pair (dominant) or on both chromosomes (recessive) to exhibit the disease

• usually obvious pattern of inheritance in a pedigree. most defects are rare

single gene disorder examples

• cystic fibrosis

• Duchenne muscular dystrophy

• Huntington’s disease

• sickle cell anemia

cystic fibrosis

• common autosomal recessive disease

• very rare in aisians

Duchenne Muscular Dystrophy

• common X-linked recessive disorder

• rare in females

Huntington’s disease

• autosomal dominant, late onset

• much higher in small isolated populations

sickle cell anemia

• autosomal recessive

• ~1/400 african americans affected

male

female

sex unspecified

number of children of sex specified

affected

heterozygotes for autosomal trait

• aka carriers

extramarital mating

divorce