GENETICS 2ND LONG EXAM

Genetic linkage

the tendency of genes that are located proximally to each other on a chromosome to be inherited together during meiosis

Linkage

the tendency of genes present in the chromosome to stay intact and transfer to the next generation

crossing over

the exchange of chromosomal sections to disrupt built links and form new linkages

crossing over (homologous recombination)

This is a process that happens at the very beginning of meiosis, in which homologous chromosomes randomly exchange matching fragments.

Complete Linkage

When genes are so closely associated that they are always inherited together, linkage between them is complete

Incomplete Linkage

Thomas Hunt Morgan pointed out that the genes were arranged in a linear order along the length of the chromosome and linkage was an indication of their relative proximity to each other

Thomas Hunt Morgan (1910)

The non random segregation of linked genes was attributed to their location on the chromosomes

AB or cis form

indicates the coupling phase

Ab/aB

trans form

Trans Form

repulsion phase

Trans Form or Repulsion phase

the linked genes being located on the separate chromosome, enter the zygote separately

Recombination

process of producing new combinations of alleles by the recombination of DNA molecules

also referred to as genetic recombination

Sexuality Theory

every cell of sexually reproducing organism was bisexual and had the full potentials for the male and female sex

This theory helped explain bisexuality, hermaphroditism, sex reversal and intersexuality in plants and animals

Genetic Sex Determination

A. Sex determined by specific genotypes

B. Multiple allelic series for sexuality – insects like Hymenopterans

C. Multiple genes

D. Haplo-Diploidy – among bees and ants

Environmental Sex Determination

Some animals sex determination depends on environmental circumstances

example: marine worm, coral reef fish

Chromosomal Sex Determination

McClung showed the association of the sex characteristics and the presence or absence of a particular chromosome

XX-XY Mechanism

found in most mammals

XO

male

XX

female

XX(mammals)

female

XY (mammals)

male

ZW (birds, moths, and butterflies)

female

ZZ (birds, moths and butterflies)

male

X-linked inheritance

for genes located on the X-chromosome

Y-linked inheritance

for those on the Y-chromosome

X-Linked Inheritance

• Color-blindness

• Musculardystrophy

• Hemophilia

• Congenitaldeafness

• Mentaldeficiency

• Parkinsonism

• Albinism-deafnesssyndrome

• Brown teeth

Y-Linked Inheritance

The Y-chromosomes carry very few genes other than those associated with sex determination

example: webbed toes, hypertrichosis of the ear

Gene

a segment of DNA that contains instructions for building proteins or functional RNA

Gregor Mendel (1860)

proposed “factors” controlling traits, now called genes

Wilhelm Johannsen

coined the term gene for Mendel’s unit of heredity

defined genotype and phenotype

Modern Genetics

combine Mendelian inheritance with molecular biology

DNA

molecule made of nucleotides (A,T,C,G)

Watson and Crick

discovered the double helix structure of DNA

Coding regions (exons)

sequences that determine proteins

Non-coding regions (introns/regulatory sequences)

control how and when genes are expressed

Gene expression

the process of using DNA information to produce RNA and proteins

The Flow of Genetic Information

DNA (replication) - RNA (transcription) - Protein (translation)

Hereditary information

store and pass on traits

Protein coding

provide instructions to build enzymes, hormones, and structural proteins

Regulation

control growth, development, and cellular activities

Structural genes

Code for proteins that perform essential functions

Regulatory genes

Control activity of other genes

Non-coding genes

Produce RNA molecules (rRNA, tRNA, miRNA) that do not become proteins

Inheritance

Explain why children resemble parents

Variation & Evolution

Mutations create genetic diversity

Nucleotides

these are the fundamental units that make up DNA

Phosphate Group A

chemical group that provinces the negative charge of DNA

Deoxyribose Sugar

a five carbon sugar that forms part of the DNA backbone

Nitrogenous Base

Adenine, Guanine, Cytosine, Thymine

Sugar Phosphate Backbone

nucleotides are linked together by alternating sugar and phosphate groups, forming to chain

Base Pairing

these two strands of the DNA are held together by hydrogen bonds between complementary nitrogenous bases

Double Helix

these paired bases form the “rungs” of a twisted ladder like structure

Proteins

provide a structural stability and also regulate the function of genes, ensuring the cell operates properly

Chromatin

made of DNA and proteins, primarily the positively charged histones, and a small amount of RNA

Core Histones

H2A, H2B, H3, and H4

Histones

essential for compacting the massive length of DNA to fit inside the cell nucleus and for regulating gene expression

evolutionary conserved proteins that form an octamer of eight histones attached together

Linker Histone

H1 or H5

Chemical Composition of Chromosomes

made up of DNA, proteins and RNA

DNA serves as the genetic blueprint

Proteins provide structure and regulatory functions

RNA ensures the expression of genes through proteins synthesis

Chemical Analysis

determined that DNA was a weak acid rich in phosphorus

Chemical Components of Chromosomes

1. Nucleic acids

2. Proteins

3. Lipids

DNA as a genetic material

capable of replication

should be structurally and chemically stable

must have the property to changes due to mutation

expressed in the form of Mendelian Characters

Frederick Griffith

performed his experiment in 1928

working with Streptococcus pneumoniae

Identified two strains of bacteria: r strain and s strain

R strain (rough colony)

lack polysaccharide coat, it gives rough colony and is non-virulent

S strain (smooth colony)

have a polysaccharide coat which give rise to smooth and shiny colony and is virulent and can cause pneumonia

Avery, Macleod & Mccarty Experiment

they determined the biochemical nature of the “transforming principle” identified by Griffith

They concluded that genetic material is DNA, not RNA or protein

Hershey and Chase Experiment

they used bacteriophages to experiment

concludes that the DNA is the genetic material transferred from virus to bacteria, but not protein

Bacteriophages

a type of virus that infect bacteria

Purines

bases that have a double-ring structure

adenine and guanine

Pyrimidines

bases that have a single-ring structure

thymine and cytosine

Nucleoside

fundamental structural subunit of nucleic acids (DNA and RNA)

Nucleotide

when a phosphate group is also attached to the sugar

Erwin Chargaff

developed a chemical technique to measure the amount of each base present in DNA

Homologous Chromosome

matched pairs of chromosomes in a diploid organisms

Homologous

“same knowledge”

H1

binds the linker DNA to stabilize the nucleosome

DNA replication (DNA synthesis)

occurs during the S phase of the cell cycle, before cell division otherwise semi-conservative, meaning each new DNA molecule contains one old (parental) strand and one newly synthesized strand

Initiation

DNA synthesis is initiated at particular points within the DNA strand known as ‘origins’, which have specific coding regions. These origins are targeted by initiator proteins, which go on to recruit more proteins that help aid the replication process, forming a replication complex around the DNA origin. Multiple origin sites exist within the DNA’s structure; when replication of DNA begins, these sites are referred to as replication forks

Elongation

Once DNA Polymerase has attached to the two unzipped strands of DNA (i.e. the template strands), it is able to start synthesising new strands of DNA to match the templates. DNA polymerase is only able to extend the primer by adding free nucleotides to the 3’ end

Termination

The process of expanding the new DNA strands continues until there is either no more DNA template strand left to replicate (i.e. at the end of the chromosome) or two replication forks meet and subsequently terminate. The meeting of two replication forks is not regulated and happens randomly along the course of the chromosome

Semi-conservative Model

This model was proposed by Watson and Crick in 1953 and confirmed by the Meselson-Stahl experiment in 1958

Conservative Model

The parental DNA remains completely intact, and an entirely new molecule is made.

One molecule has two old strands, and the other has two new strands

Dispersive Model

The parental DNA is broken into segments, and new DNA is formed in pieces.

The resulting DNA molecules have intermixed old and new sections on both strands

Experimental Evidence ( Meleson-Stahl Experiment)

Conducted in 1958 using E. coli bacteria grown in heavy nitrogen (¹⁵N) and normal nitrogen (¹⁴N)

Helicase

the unzipping enzyme

breaks through the hydrogen bond and holds the DNA bases together

DNA Polymerase

the builder

replicates DNA molecules to actually build a new strand of DNA

Primase

the initializer

makes the primer so that DNA polymerase can figure out where to go, to start to work

Ligase

the gluer

helps glue DNA fragments together

Parental or template DNA

Functions as the master copy upon which the synthesis of daughter DNA molecules occurs

Deoxyribonucleotide triphosphates

(dATP, dGTP, dTTP, dCTP) function as substrates

DNA helicase (Helix-unwinding protein)

responsible for the unwinding of the parental strands to create two template strands

Single Strand DNA binding protein (SSBP)

responsible for preventing the separated parental strands from reannealing. This is accomplished by binding of SSBP to the single-stranded DNA at the replication fork

DNA topoisomerase or DNA gyrase

This is done by breaking a phosphodiester bond in one of the parental strands ahead of the replication fork, creating a swivel point on the opposite strand. Rotation occurs around the swivel point, thus removing the supercoiling created by strand separation. After the tension is removed, the phosphodiester bond is restored

DNA polymerase III or  DNA replicase

Catalyzes the synthesis of daughter DNA. It is a multimeric enzyme with a molecular mass of about 900,000 daltons in its holoenzyme or complete form

Primase

Initiates the synthesis of RNA primer strands. RNA primer is composed of 10 to 60 nucleotides

DNA polymerase I

Exonucleases activity cleaves the RNA primer from the elongating DNA strand

DNA ligase

a joining enzyme catalyzing the formation of a covalent phosphodiester bond between adjacent nucleotides that have been separated by a nick

Linear DNA replication

This is done by the formation of replication “bubbles”

Circular DNA (Theta conformation)

Specific replication points on the circular DNA are identified. Replication is initiated by the formation of a replication “bubble” at this point and as replication proceeds toard both directions around the chromosome, this replication bubble grows in size

Circular DNA (Rolling circle conformation)

The rolling circle, a current model for the replication of the single stranded DNA viruses, as well a the “looped” rolling circle model. The whole process can be divided into three main stages with reference to the viral life cycle

Mismatch Repair (MMR) System

Is a DNA repair process that corrects errors, such as base misincorporations and small insertions or deletions, that occur during DNA replication