Bio1020 Unit 3: Genetics

Chromosomal basis of inheritance, molecular basis of inheritance, mutation, complexity + biotechnology.

Chromosome

Condensed form of proteins and nucleic acids, carrying gneetic information, visible only during cell division

Chromatin

Relaxed + spread out mass of nucleic acids and proteins when cell is not dividing.

Characterise each chromosome by shape.

Metacentric, acrocentric, telocentric

Haploid

Organisms that have one set of chromosomes of which each is unique

Diploid

Organisms that have two sets of chromosomes of similar shapes

Euploidy

The right number of chromosomes for an organism

Aneuploidy

An incorrect chromosome number

Which organisms use XY sex determination system?

Plants, vertebrates, insects

Which is male and female in the XY sex determination system?

XY is male, XX is female

Homomorphic chromosomes

Chromosome pairs that are similar in shape, size and gene content

Heteromorphic chromosomes

Chromosome pairs that differ in shape size and gene content

Pseudoautosomal Regions (PARs)

Homologous sequences of nucleotides found at the tips of sex chromosomes

PARs in humans

PAR1, PAR2

Function of SRY and Eif2s3y genes

Determining male phenotype, morphological development + guiding gametes

Turner syndrome cause

X

Turner syndrome symptoms

Female phenotype

Klinefelter syndrome cause

XXY

Klinefelter syndrome symptoms

Male phenotype

Describe sex determination in fruit flies (Drosophila melanogaster)

The absence of a second X determines male sex.

Which organisms use ZW sex determination system?

Birds, reptiles

Which is male and female in the ZW sex determination system?

ZZ is male, ZW is female

Which organisms use haplodiploid sex determination system?

Insects - ants, bees

Which is male and female in the haplodiploid sex determination system?

2n is female, n is male

Stages of interphase

G1, S, G2

Outline the G1 stage of interphase.

Period of cell growth before the DNA is duplicated

Outline the S phase of interphase

Period when DNA (chromosomes) are duplicated

Outline the G2 stage of interphase

Period after DNA is duplicated; cell prepares for division

Stages of mitosis

Prophase, pro-metaphase, metaphase, anaphase, telophase

Describe prophase

DNA condenses into discrete chromosomes

Describe metaphase

Chromosomes align along metaphase plate, spindle fibres attach to kinetochores

Describe anaphase

Spindle fibres separate chromosomes into sister chromatids, daughter chromosomes pulled to opposite poles of the cell

Describe telophase

Chromosomes de-condense, new nuclear membranes form, the two daughter cells separate

What is the product of meiosis I?

2 haploid daughter cells

What is the product of meiosis II?

4 haploid daughter cells

Consequences of meosisi

Haploid gametes, increased genetic diversity

What events in meiosis increase genetic diversity

Random assortment, crossing over

Mendel’s Observations

1. Phenotype is determined by heritable factors (genes)

2. Alternate forms of genes (alleles) responsible for variation

3. An organism inherits 1 copy of the gene from each paretn

4. If two different alleles are present, one is dominant and one is recessive

5. The two alleles for each character segregate during gamete production

Albinism cause

Lack of melanin production, the enzyme responsible for melanin production (tyrosinase) breaks.

Albinism inheritance type

Recessive

Tyrosinase gene location

Chromosome 11

Phenylketonuria cause

Metabolic disorder, block in phenulketoneuria prevents production of tyrosine.

Phenylketonuria inheritance type

Recessive

Beta thalassemia cause

Blood disorder, beta-globin not formed properly, oxygen and carbon dioxide cannot be carried at same concentration/efficiency levels than needed

Incomplete dominance

3 distinct phenotypes are evidence

Co-dominance

In heterozygotes, both alleles are expressed

Hemizygote

An organism that has only one copy of a specific gene rather than the usual two

Describe prometaphase

Nuclear envelope breaks down, freeing the sister nucleotids from the nucleus; centrosomes move away from each other, development of kinetochore

Locus

Specific location of a gene along the length of a chromosome

Karyotype

Display of condensed chromosomes arranged in pairs by size + shape

Karyotype use

Screen for defective chromosomes or abnormal numbers of chromosomes

How are karyotypes prepared

Prepared from isolated somatic cells (treated with a drug to stimulate mitosis, then isolated + stained when chromosomes are most highly condensed). Digital software arranges pairs according to appearance.

Why do chromosomes in karyotype consist of two sister chromatids?

Chromosomes photographed during M phase.

Alternation of generations

A life cycle in which there is both a multicellular diploid form (sporophyte) and a multicellular haploid form (gametophyte)/ The sporophyte generation produces a gametophyte as its offspring and vice versa.

Third type of life cycle (fungi + some protists)

After gametes fuse and form a diploid zygote, the zygote undergoes meiosis to produces haploid cells that then divide by mitosis into unicellular descendants or a haploid multicellular adult organism.

Describe crossing over.

1. DNA molecule of two non-sister chromatids are broken at precisely corresponding points, then the chromatin condenses.

2. Homologues attach to each other, forming a synaptonemal complex.

3. During synapsis, the DNA breaks are closed up when each broken end is joined to the corresponding segment of the nonsister chromatid, producing crossovers.

4. After the synaptonemal complex disassembles, the homologues move slightly apartt from each other but remain attached. The points of attachment show up as chiasmata.

Synapsis

The pairing and physical connection of one duplicated chromosome to its homolog during prophase I of meiosis

Synaptonemal complex

A zipper-like structure composed of proteins which connects a chromosome to its homolog tightly along their lengths during prophase I of meiosis

Is crossing over compulsory?

At least one crossover per chromosome must occur in order for the homologous pair to stay together as it moves to the metaphase I plate

Chiasma

Result of crossing over and sister chromatid cohesion along the arms.

Which organism and trait did Mendel study?

White and purple pea flowers

Law of segregation

The two alleles for a heritable character segregate during gamete formation and end up in different gametes

Law of independent assortment

Each pair of alleles segregates independently of any other pair of alleles during gamete formation

Pleiotropy

The ability of a single gene to have multiple effects

Epistasis

When the phenotypic expression of one gene alters that of another independently inherited gene

Polygenic inheritance

An additive effect of two or more genes on a single phenotypic character

Multifactorial

A phenotypic character that is influenced by multiple genes and environmental factors

Amniocentesis

Amniotic fluid is tested for the presence of genetic disease

Chromosome theory of inheritance

Genes are located at specific loci on chromosomes and the behaviour of chromosomes during meiosis account for inheritance patterns

Describe X Inactivation in Females

The inactive X in each cell of a female condenses into a Barr body

Linked genes

Genes located close enough together on a chromosome that they tend to be inherited together

What did Griffith’s experiment prove?

Bacteria can transfer genetic information through a process of ‘transformation’

What did Griffith’s experiment demonstrate?

A cellular ‘transforming principle’ could pass from dead, virulent bacteria to live, harmless ones, permanently changing their traits

Results of Avery, MacLeod and McCarty experiment?

Enzymes that destroyed proteins or RNA did not stop transformation, however enzymes that destroyed DNA did.

What did Avery, MacLeod and McCarty’s experiment imply?

DNA is the transforming principle

Results of Hershey and Martha Chase’s experiment

Radioactively labelled sulfur was not passed down to offspring, however radioactively labelled phosphate was.

What did Hershey and Martha Chase’s experiment prove?

DNA, not protein, is the genetic material.

Is genotype determined by DNA or proteins?

DNA

Is phenotype determined by DNA or proteins?

Proteins

Name 3 ways in which proteins determine phenotype

1. The molecules visible in the phenotype

2. The enzymes that make molecules visible in the phenotype

3. The signals to make visible changes in phenotype

Semi-conservative replication

The parent DNA strands separate and individually serve as a template for a new strand

Conservative replication

Original parent DNA model stays entirely intact and serves as a template to create a completely new, separate DNA model made of two new strands

Dispersive replication

Resulting helices contain patchwork of old and new DNA segments

DNA polymerase III function

Extends the RNA primer by catalysing DNA polymerisation in the 5’ to 3’ direction after primase provides a semi-complete template.

Replication bubbles

Location of DNA replication

DNA helicase function

Unzip DNA from replication origins

Primase function

Produce RNA primer

Okazaki fragments

Start-stop regions of DNA replication on the lagging strand because DNA helicase and DNA pol III travel in opposite directions

DNA ligase

Seals DNA fragments and DNA backbone by forming covalent bonds.

DNA polymerase I function

Replace RNA primers with DNA and leaves a blunt end.

Telomere

End of a chromosome

RNA polymerase II function

Catalyses transcription from 5’ to 3’

Transcription steps

1. Initiation

2. Elongation

3. Termination

Describe the initiation step of transcription

RNA polymerase binds to a promotor (special DNA sequence)

Describe the elongation step of transcription

Addition of complementary rNTPs

Describe the termination step of transcription

Release of RNA polymerase and completed RNA from the DNA template at the terminator (unit of DNA with a particular sequence)

Redundant genetic code

More codes than amino acids

Name the tRNA binding sites

A (amino acyl) site, P (peptidyl) site, E (exit) site

What does the ribosome catalyse

Joining of amino acids by covalent bonds.

Describe the ribosome’s A (amino acyl) site

The next amino acid/tRNA waits for peptide bond formation

Describe the ribosome’s P (peptidyl) site

Holds the growing protein protein attached to a tRNA