Unit 2 - Genetic Processes (Bio Exam Review)

Chromatin

Chromatin

Strand-like form of DNA when cell is not dividing (every chromatin = 1 DNA strand)

Chromosomes

Chromatin condenses in late prophase, forms chromosomes

• humans have 46 chromosomes or 23 pairs of chromosomes

• vary in number, size, shape, depending on the organism

• tend to occur in sets

Sister Chromatids

Chromosomes are made up of two sister chromatids which are genetically identical

Centromere

A portion of genetic material that holds sister chromatids together

Centrioles

Assist in nuclear division by developing spindle fibres

Spindle fibres

Attach to the centromeres of chromosomes, where they split them apart and guide them to either side of the cell

Nucleus

A structure that houses genetic material of a cell, site of DNA duplication, RNA processing, etc…

Nuclear membrane

Separates the nucleus from the cytoplasm of the cell, protects valuable genetic info

Cytoplasm

Responsible for suspending + protecting organelles_, also gives cells their shape

Haploid

Only one chromosome in a set - n = 23

(human gametic cells are haploid)

Diploid

Two chromosomes in a set - 2n = 46

(human somatic cells are diploid)

Polyploid

Three or more chromosomes in a set

(some plant cells are polyploid)

Karyotype

Shows all chromosomes

• taken in mitosis, preferably in metaphase when all chromosomes are condensed and visible

Arranged according to:

• size

• structure

• position of centromere

• pattern of banding

Autosomes vs. Sex Chromosomes

All chromosomes are autosomes EXCEPT for 1 pair of sex chromosomes

• XX - females

• XY - males

Sex chromosomes determine the biological sex of a human)

Somatic Cells

• human body cells

• 46 chromosomes, 23 pairs, one chromosome from each parent

  • are diploid (2n=46)

Gametic Cells

• human reproductive cells

• 23 chromosomes, allow for zygote formation (2 gametes combine to make a diploid cell with 46 chromosomes)

• are haploid

  • egg + sperm cells

Meiosis - definition + purpose

• a pair of nuclear divisions

  • PMAT 1

  • PMAT 2

• prevents chromosomes from infinitely doubling when gametes are combined

Functions:

1. Genetic REDUCTION

   • ensures each haploid gamete only has 1 complete set of chromosomes

2. Genetic RECOMBINATION ‘

   • promotes genetic diversity

Interphase + Prophase 1

Interphase

• DNA + organelles replicate

Prophase 1

• chromatin → chromosomes, centrioles develop spindle fibres + move to opposite poles

• SYNAPSES - the aligning of homologous chromosomes to form a tetrad of chromatids

• CROSSING OVER - chiasma (exchange points) form between non-sister chromatids

  • GENETIC EXCHANGE - genes are shuffled, chromosomes are now recombinant chromosomes

Metaphase 1

• spindle fibres attach and move in tetrads (homologous pairs) to the centre

  • orientation of paternal/maternal chromosomes is random

Anaphase 1

• spindle fibres shorten

• homologous pairs are separated

• one chromosome from each pair moves to each pole

Telophase 1 + Cytokinesis

• chromosomes begin to uncoil

  • cell division occurs: 2 haploid cells form

Prophase 2

• chromosomes condense + become visible

Metaphase 2

• spindle fibres attach to the centromeres

  • chromosomes line up in the centre of the cell

Anaphase 2

• spindle fibres shorten

• chromatids break apart at the centre

  • now chromosomes, they move the oppposite ends of the cell

Telophase 2 + Cytokinesis

• chromosomes reach opposite poles and begin to uncoil

• cell division occurs → 2 gametic cells formed

  • in total: 4 haploid cells

Genetic Recombination

Ensured in two ways:

SYNAPSES + CROSSOVER

• segments of DNA exchanged, new combo of genes

• increases genetic diversity

INDEPENDENT ASSORTMENT

• during metaphase 1, chromosomes in homologous pairs are arranged on the cell’s equator

• varying combinations are created depending on how chromosomes line up

Importance of genetic diversity

Increases survival by increasing chance of adaptation and evolution

• also lends itself to disease resistance

Errors in Meiosis

Chromosomal abnormalities that may:

• produce gametes that do not survive

  • fuse to create a zygote with a mutation, all somatic cells +future offspring will carry that mutation

Mutation

Any change in the DNA of a cell

Causes:

• smoking

• carcinogens

• radiation

• viruses

Changes in chromosome structure

Mistakes may happen during synapsis and crossover:

• Deletion

• Duplication

• Inversion

• Translocation

Deletion

• segment is missing from chromosome (lost)

• larger segments = more severe

• small deletions may not have an effect, not all DNA has genetic info

• can result from:

  • breaking without joining

  • crossover with inversion

  • fragile spots

  • radiation

  • unequal cross over

Duplication

• segment is repeated

• repeat may not be harmful, 3+ can cause severe disturbances

• can help with evolution (more genes available for mutation/specialization)

  • disrupts gene dosage balance (proper copies of a gene for regular function)

Inversion

• reverses a fragment of the original chromosome

• can cause missing or deleted DNA, parts of a chromosome break off and invert during crossing over

• chromosomes are functional, issues when they undergo synapsis

Issues with inversion

When inverted chromosomes undergo meiosis, the altered order of genes causes a shape known as an inversion loop.

Person with inverted chromosomes possess necessary genes for survival, but misalignment leads to deleted or duplicated genes → inviable gametes + infertility

Translocation

• fragment of one chromosome attaches to a non-homologous chromosome

• creates a hybrid gene (2 different genes combine and form an abnormal protein)

  • possible cause: irregular enzymes

Non-Disjunction

• an error in chromosome number

• chromosomes did not separate properly

  • results in extra chromosomes → inviable gametes or developmental disabilities

Nondisjunction - Meiosis 1

• during anaphase 1 - synapses can cause an issue with separation

• entire pair is pulled to the same pole

  • gametes now have 22 + 24 chromosomes (should be 23 each)'

• results in monosomy + trisomy

Nondisjunction - Meiosis 2

• during anaphase 2 - chromatids may fail to separate

• entire chromosome is pulled toward the same pole

  • gametes now have 22 + 23 + 24

• results in monosomy + regular + trisomy

Trisomy

Extra chromosome gained (47 total)

• usually lethal

(n+1)

Monosomy

Loss of a chromosome (45 total)

• likely only in sex chromosomes

(n-1)

DNA

contains coded instructions for proteins (structural and functional molecules)

Nucleotides

DNA subunit molecules, contains:

• pentose (deoxyribose) sugar

• phosphate group

• nitrogenous bases (code)

DNA molecule

2 long strands held together by H-bonds to form a double helix

DNA structure

Composed of an alternating sugar-phosphate backbone (protects nitrogenous bases) connected by nitrogenous base pairs

Complimentary Base Pairs

Each strand is connected with its respective pair:

• thymine with adenine (T-A) 2 bonds

• guanine with cytosine (G-C) 3 bonds

Proportion of each base in a pair is equal

Trait

Inherited characteristics coded by genes

Heredity

Passing of traits from parents → offspring

Monohybrid Cross

Crossing organisms that differ by one trait

Dihybrid Cross

Crossing organisms that differ by two traits, inheritance pattern of both traits are observed

Cross pollination

Use a brush to transfer pollen between male/female plants for cross-breeding

Allele

Different versions of a gene

Homozygous Dominant

2 dominant alleles

Homozygous recessive

2 recessive alleles

Heterozygous

1 dominant, 1 recessive

*are HAPLO-SUFFICIENT

• only possesses half the amount of protein, but still enough to make the trait appear

Genotype

Combination of alleles Ph

Phenotype

expression of a genotype - visible characteristics

Pure Bred

Only possessing that one allele

Gene Linkage

Genes are closely located - crossing over does not occur

• Genes are inherited together

Complete Dominance/Discrete Inheritance

• simple pattern of inheritance

• 1 allele from each parent 3 possible combos

• recessive only shown in homo-recessive genotype

Incomplete Dominance

Neither two alleles can completely conceal the presence of the other

• Intermediate expression

Notation has capital for the trait, superscript for the descriptor

Co-Dominance

Both alleles for a trait are fully expressed

• Expression of both dominant traits \

Ex. BLOOD TYPES

• i - type O

• I^A - type A

• I^B - type B

Multiple Alleles

More than two alleles exist - but only two are inherited

Same notation as Co-dominance; dominance hierarchy exists

Polygenic Inheritance

Broad range of slightly different phenotypes - additive effect of multiple genes (ex. skin colour)

• often incompletely dominant alleles

• more genes involved = smoother curve

  • midrange tends to appear the most frequently

X-Linked Dominant

Dominant trait on the x-chromosome

if mother is:

• homo-dom: all offspring have it

• hetero: 50% of offspring will have

If father has it:

• all daughters would have (must get one X from dad)

• no sons would have (inherit y from father)

Hemizygous

Only one copy of a gene

Describes makes since they have only one X chromosome

X-Linked recessive

Affects more males_, since they only have 1 X - no other X to conceal it

If mother is:

• hetero (carrier): 50% sons have, 50% daughters are carriers

• homo (possesses disorder): 100% sons have, 100% daughters are carriers

If father has it (hemizygous): 0% sons have it from him, 100% daughters are carriers

Autosomal Recessive

• affects males and females equally

• must be homo-recessive

  • often skips generations

Autosomal Dominant

• affects males and females equally

If:

• one hetero parent: 50% will have

• one/two homo parents: 100% will have

Pedigree

Flowchart that shows inheritance patterns

• uncovers genotypes

  • detect probability of a genotype or phenotype

Gel electrophoresis - Purpose

• technique used to separate DNA fragments by size

• can help identification of a person, looks at STRs (which tend to be unique as they are inherited from both parents)

• different sizes move at different speeds

  • Smaller fragments - move more/faster

  • Larger fragments - move less/slower

STRs

Short tandem repeats

• 2-10 base pairs in length

• typically in non-coding regions

Gel Electrophoresis - Procedure

1. samples of DNA cast in wells

2. gel is immersed in conducting fluid, electrical field applied

3. molecules in the sample with charge will move through the gel

   • DNA has negative charge from phosphate group

   • same direction, different rate

4. separates fragments by size