Unit 2 - Genetic Processes (Bio Exam Review)

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Chromatin

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72 Terms

1

Chromatin

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

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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

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Sister Chromatids

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

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Centromere

A portion of genetic material that holds sister chromatids together

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Centrioles

Assist in nuclear division by developing spindle fibres

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Spindle fibres

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

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Nucleus

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

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Nuclear membrane

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

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Cytoplasm

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

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Haploid

Only one chromosome in a set - n = 23

(human gametic cells are haploid)

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Diploid

Two chromosomes in a set - 2n = 46

(human somatic cells are diploid)

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Polyploid

Three or more chromosomes in a set

(some plant cells are polyploid)

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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

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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)

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Somatic Cells

  • human body cells

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

    • are diploid (2n=46)

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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

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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

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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

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Metaphase 1

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

    • orientation of paternal/maternal chromosomes is random

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Anaphase 1

  • spindle fibres shorten

  • homologous pairs are separated

  • one chromosome from each pair moves to each pole

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Telophase 1 + Cytokinesis

  • chromosomes begin to uncoil

    • cell division occurs: 2 haploid cells form

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Prophase 2

  • chromosomes condense + become visible

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Metaphase 2

  • spindle fibres attach to the centromeres

    • chromosomes line up in the centre of the cell

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Anaphase 2

  • spindle fibres shorten

  • chromatids break apart at the centre

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

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Telophase 2 + Cytokinesis

  • chromosomes reach opposite poles and begin to uncoil

  • cell division occurs → 2 gametic cells formed

    • in total: 4 haploid cells

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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

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Importance of genetic diversity

Increases survival by increasing chance of adaptation and evolution

  • also lends itself to disease resistance

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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

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Mutation

Any change in the DNA of a cell

Causes:

  • smoking

  • carcinogens

  • radiation

  • viruses

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Changes in chromosome structure

Mistakes may happen during synapsis and crossover:

  • Deletion

  • Duplication

  • Inversion

  • Translocation

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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

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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)

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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

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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

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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

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Non-Disjunction

  • an error in chromosome number

  • chromosomes did not separate properly

    • results in extra chromosomes → inviable gametes or developmental disabilities

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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

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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

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Trisomy

Extra chromosome gained (47 total)

  • usually lethal

(n+1)

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Monosomy

Loss of a chromosome (45 total)

  • likely only in sex chromosomes

(n-1)

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DNA

contains coded instructions for proteins (structural and functional molecules)

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Nucleotides

DNA subunit molecules, contains:

  • pentose (deoxyribose) sugar

  • phosphate group

  • nitrogenous bases (code)

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DNA molecule

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

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DNA structure

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

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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

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Trait

Inherited characteristics coded by genes

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Heredity

Passing of traits from parents → offspring

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Monohybrid Cross

Crossing organisms that differ by one trait

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Dihybrid Cross

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

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Cross pollination

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

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Allele

Different versions of a gene

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Homozygous Dominant

2 dominant alleles

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Homozygous recessive

2 recessive alleles

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Heterozygous

1 dominant, 1 recessive

*are HAPLO-SUFFICIENT

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

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Genotype

Combination of alleles Ph

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Phenotype

expression of a genotype - visible characteristics

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Pure Bred

Only possessing that one allele

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Gene Linkage

Genes are closely located - crossing over does not occur

  • Genes are inherited together

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Complete Dominance/Discrete Inheritance

  • simple pattern of inheritance

  • 1 allele from each parent 3 possible combos

  • recessive only shown in homo-recessive genotype

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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

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Co-Dominance

Both alleles for a trait are fully expressed

  • Expression of both dominant traits \

Ex. BLOOD TYPES

  • i - type O

  • IA - type A

  • IB - type B

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Multiple Alleles

More than two alleles exist - but only two are inherited

Same notation as Co-dominance; dominance hierarchy exists

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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

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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)

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Hemizygous

Only one copy of a gene

Describes makes since they have only one X chromosome

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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

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Autosomal Recessive

  • affects males and females equally

  • must be homo-recessive

    • often skips generations

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Autosomal Dominant

  • affects males and females equally

If:

  • one hetero parent: 50% will have

  • one/two homo parents: 100% will have

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Pedigree

Flowchart that shows inheritance patterns

  • uncovers genotypes

    • detect probability of a genotype or phenotype

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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

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STRs

Short tandem repeats

  • 2-10 base pairs in length

  • typically in non-coding regions

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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

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