Ramp-Up Biology: DNA Replication & Inheritance of Traits

DNA Replication

the process by which the genome’s DNA is copied in cells

• before a cell divides, it must first copy (or replicate) its entire genome so that each resulting daughter cell ends up with its own complete genome

• each DNA template strand is read from 3’ to 5’ to make a complementary DNA strand from 5’ to 3’

Cell Cycle

a series of events that takes place in a cell as it grows and divides

• a cell spends most of its time in what is called interphase (G1 → S → G2) and during this time it grows, replicates it chromosomes, and prepares for cell division

• the cell leaves interphase, and undergoes mitosis (M phase of the cycle), and completes its division

• the resulting cells (daughter cells) each enter their own interphase and begin a new round of the cell cycle

Mitosis

the process by which a cell replicates its chromosoms and then segregates them, producing two identical nuclei in preparation for cell division

• mitosis is generally followed by equal division of the cell’s content into two daughter cells that have identical genomes

• this is inherent to our growth, repair, and maintenance

Meiosis

a type of cell division in sexually reproducing organisms that reduces the number of chromosomes in gametes (the sex cells, or egg and sperm)

• in humans, body/stomatic cells are diploid (containing two sets of chromosomes, one from each parent)

• to maintain this state, the egg and sperm that unite during fertilization must be haploid with a single set of chromosomes

• each diploid cell undergoes two rounds of division to yield 4 haploid daughter cells - the gametes

Inheritance

the passing of genetic information from one generation to the next (parents to child) through the genes in sperm and egg cells

• also called heredity

Pedigree

the diagram of family history that uses standardized symbols

• shows relationships between family members and indicates which individuals have certain genetic pathogenic, variants traits, and diseases within a family as well as vital status

• can be uses to determine disease inheritance patterns within a family

Template DNA

a DNA strand that is read from 3’ to 5’ to make a new strand, either DNA (during replication) or RNA (during transcription) that is complementary and antiparallel to it

Deoxyribonucleotides (dNTPs)

monomers of DNA

• each monomer has a nucleotide base (ATGC) covalently bonded to deoxyribose sufar which is also bonder to a triphosphate

DNA Polymerase Enzyme

polymerizes the new strands of DNA by reading the template DNA strand during replication

• a primer dependent enzyme (can add complementary bases only to the 3’ end of the primer)

RNA Primers

made by primase enzymes

• short RNA sequence complementary and anti-parallel to the template DNA strand

• have the 3’ end to which DNA polymerase adds dNTPs during DNA replication

DNA Helicase

an enzyme that unwinds the DNA double helix during DNA replication

Primase

an enzyme that makes RNA primers in a cell

• these primers are essential for DNA polymerase to start making a new strand of DNA

RNAase

an enzyme that degrades RNA primers later during DNA replication

Ligase

an enzyme that stitches the two strands of DNA together to make them continuous

Semiconservative mode of DNA Replication

the result of DNA replication is 2 DNA molecules consisting of one new and one old chain of nucleotides

• why DNA replication is described as semi-conservative, hald of the chain is part of the original DNA molecule, and half is brand new

Replication Fork

a region where a cell’s DNA double helix has been unwound and separated to create an area where DNA polymerase and the other enzymes involved can use each strand as a template to synthesize a new double helix

DNA Polymerase

the enzymes that replicate DNA in living cells

• add individual dNTPs to the 3’ end of a growing strand of DNA

• uses a complementary, single strand of DNA as a template

Origin of DNA Replication (Ori Site)

the point where the 2 strands of DNA start to unwind to start DNAA replication

• an AT base pair rich region

• 2 replication forks originate from each origin of replication and they move in opposite directions to each other

Leading DNA Strand

the strand that is synthesized continuouly by reading the DNA template during DNA replication

Lagging DNA Strand

the strand that is synthesized as small Okazaki fragments by reading the DNA template during DNA replication

• these fragments are finally stitched together by DNA ligaze enzyme to make the strand continuous

Gene

unit of heredity

• a specific nucleotide sequence that codes for a gene product (protein, rRNA, tRNA, etc) which regulates the characteristics/traits in an organism

Alleles

different variants/forms of a gene

• in a somatic cell, there are 2 alleles (maternal allele and paternal allele) or each gene

• gametes have only one allele (either maternal allele or paternal allele) of a gene

Genotype

the combination of maternal and paternal allele of a gene

• dictates the trait or characteristic associated with that gene

Phenotype

the trait or characteristic associated with a gene(s)

• regulated by the corresponding genotype

Heterozygous Organism

the organism has 2 different alleles of a gene (has a genotype Aa for gene A)

Homozygous/Pure Breed/True Breeding Organism

the organism has 2 identical allele of a gene (has genotype AA or aa for gene A)

Dominant Phenotype

the trait that appears in a heterozygous (Aa) organism

Recessive Phenotype

the trait that does not appear in a heterozygous (Aa) organism

Chromosomes

linear segments of DNA and proteins

Somatic Cell

a cell that has a paternal and maternal copy of each chromosome (diploid - 2n)

Gametes

a cell that has either a paternal or maternal copy of each chromosome (haploid - n)

Ploidy

the total number of genomic DNA copies that a cell has (n, 2n, 4n, etc)

Homologous Chromosomes

a pair of chromosomes made of maternal and paternal homologs

• the homologs have the same location of a gene but the alleles of the gene at that location may be same or different

Sister Chromatids

2 exactly similar copies of paternal or maternal chromosome homologs

• have the same alleles of the genes

Non-sister Chromatids

come from homologous chromosomes

• not identical

• may contain same or different alleles of the same gene at the same location

Genome

all the chromosomes in the nucleus of a cell

Steps of Mitosis

1. prophase

2. metaphase

3. anaphase

4. telophase

5. cytokinesis

Prophase

nuclear membrane disintegrates, chromosomes start to shorten and thicken

Metaphase

chromosomes are shortest and thickest, they align at the center

Anaphase

sister chromatids pilled apart to the 2 ends of a cell

Telophase

nuclear membrane reforms and chromosomes become thin and less distinct once again

Cytokinesis

cell physically divides to form 2 daughter cells

Mendel’s Law of Segregation

when 2 organisms’ ate to produce an F2 offspring, only one of the 2 alleles of a gene in an organism is distributed to each gamete (egg or sperm cell) that it makes, and the allocation of the gene copies is random

• when an egg and a sperm join in fertilization, they form a new organism, whose genotype consists of the alleles contained in the gametes

Test Cross

created by Gregor Mendel; an experimental cross of an individual organism of dominant phenotype but unknown genotype and an organism with a homozygous recessive genotype (and phenotype)

Monohybrid Cross

a breeding experiment between P generation organisms that differ in a single given trait and are rue breeding or homozygous for alleles of the gene in question