(ATAR, QCAA) Biology, Unit 4, Topic 1: DNA, Genes and the Continuity of Life

deoxyribonucleic acid (DNA)

thread-like chain of nucleotides carrying the genetic instructions in a double helix of antiparallel strands

ribonucleic acid (RNA)

thread-like chain of nucleotides carrying the genetic instructions for forming a protein in a cell

sugar-phosphate backbone

the structural framework of nucleic acids, composed of alternating sugars and phosphate groups

condensation polymerisation

the formation of a polymer by the reaction between two monomers, with the loss of a small molecule

antiparallel

two biopolymers that run parallel to each other but with opposite alignments

nucleosome

a length of DNA coiled around a core of eight histone protein molecules

epigenetic factors

a chemical tag that determines the degree of coiling of the DNA around the nucleosome and thus gene expression

gene

a region of DNA, made up of nucleotides, that encodes molecules that have a function

genetic code

the code the body uses to convert the instructions contained in the DNA into the proteins essential for life

DNA helicase

an enzyme that breaks down the hydrogen bonds holding two DNA strands together

semiconservative replication

replication of DNA resulting in two copies that each contain one of the original strands and one new strand

3' (three-prime) end

one end of a DNA strand in which the C carbon atom of the sugar molecule in the DNA's sugar backbone has a 'free' OH (hydroxyl) group that is not linked to another chemical group

5' (five-prime) end

one end of a DNA strand in which the C carbon atom of the sugar molecule in the DNA's sugar backbone has a 'free' phosphate group that is not linked to another chemical group

DNA polymerase

a type of enzyme that is responsible for assembling nucleotides to form new copies of DNA

origin of replication

a particular sequence in the DNA molecule at which replication is initiated

replication fork

structure with two branching sections that is created when DNA helicases break the hydrogen bonds holding the two DNA strands together at a certain point

replication bubble

an unwound and open region of DNA in which DNA replication occurs, created when the enzyme DNA helicase separates the two strands of DNA

leading strand

DNA template from the middle of the replication bubble (point of origin) to the fork

lagging strand

DNA template from the fork to the middle of the replication bubble (point of origin)

Okazaki fragments

fragments of DNA that are formed on the lagging strand of replicating DNA

DNA ligase

a specific type of enzyme that facilitates the joining of DNA strands together

termination sequence

a section of a nucleic acid sequence that marks the end of a gene or replication site

homologous chromosomes

chromosomes of the same type; usually a pair

diploid

cellular condition in which there are two of each type of chromosome present in the nucleus

haploid

cellular condition in which there is one of each type of chromosome present in the nucleus

mitosis

nuclear division resulting in daughter cells having the same number and type of chromosomes as the parent cell

meiosis

nuclear division resulting in daughter cells having half as many chromosomes, but the same types, as the parent cell; a reduction division, from the diploid to the haploid condition

chromatid

a replicated chromosome, still attached to the original at the centromere

tetrad

homologous chromosomes, each consisting of two chromatids, lying side by side

chiasma (plural chiasmata)

connection between non-sister chromatids of homologous chromosomes during meiosis, where interchange occurs during crossing over

autosome

non-sex chromosome

sex chromosomes

chromosomes carrying information that determines the sex of the individual

X chromosome

female sex chromosome in vertebrates and some other animals

Y chromosome

male sex chromosome in vertebrates and some other animals

gametogenesis

formation of gametes

spermatogenesis

formation of spermatozoa in animals

oogenesis

formation of ova in animals

spermatogonium (plural spermatogonia)

germ (or stem) cell in a layer lining the tubules in the testes

primary spermatocyte

diploid cell formed from mitosis of a spermatogonium, which undergoes meiosis I to produce two haploid secondary spermatocytes

secondary spermatocyte

haploid cell that undergoes meiosis II to form spermatids

spermatid

immature spermatozoon

Sertoli cell

cells in the epithelium of tubules of the mammalian testes that protect and nourish the developing spermatozoa

spermatozoon (plural: spermatozoa)

small, motile male gamete

ovum

haploid female gamete

oogonium

a small diploid cell in the ovary that forms a primary oocyte in a female foetus

primary oocyte

a diploid cell developed by an ovarian germ cell in mammals, which may later develop into an ovum

primary follicle

a single-layered structure in the mammalian ovary containing the primary oocyte

secondary oocyte

a large haploid cell produced during meiosis I of the primary oocyte

polar body

small haploid cell produced from both meiosis I and II during oogenesis as a result of uneven cell division

ovulation

release of the secondary oocyte that has started meiosis II from the ovary

gene linkage

location of genes on the same chromosome

allele

one or two or more forms of a gene located in equivalent positions on homologous chromosomes

homozygous

describes a condition in which both alleles for the gene are the same

heterozygous

each allele for the characteristic exhibit a different expression

recessive allele

an allele found on a chromosome that can be masked by its dominant form in the heterozygous condition; can be autosomal or sex-linked

dominant allele

an allele for a gene that overrides the effects of the recessive allele in the heterozygous condition; can be autosomal or sex-linked

dominant trait

the particular trait of a characteristic that is expressed in the phenotype of a heterozygous individual

genotype

the genetic makeup of an individual

phenotype

actual expression of the genotype

law of segregation

Mendelian law that states that genes for a characteristic occur in pairs in an individual, one inherited from each parent, and are separated when the reproductive cells are formed

law of independent assortment

Mendelian law that states each allele pair segregates independently during gamete formation; applies when genes for two traits are located on different pairs of homologous chromosomes

crossing over

breaking and rejoining, with exchange of DNA, between non-sister adjacent chromatids of homologous chromosomes during meiosis I

recombiant gametes

gametes produced as a result of crossing over of the chromatids of homologous chromosomes during meiosis

genome

the complete set of nucleotide sequences encoded in the total DNA of an organism

structural gene

sections of DNA that carry the instructions for production of a protein

gene expression

the ability of a gene to be transcribed

non-coding DNA

the greater part of the DNA molecule that does not contain structural genes

regulatory gene

a non-coding segment of DNA that produces transcription factors for gene expression

mRNA

messenger RNA; RNA molecule formed during transcription

transcription

first step of gene expression, in which a particular segment of DNA is copied into mRNA

codon

triplet of nucleotides on mRNA specifying an amino acid

split gene

a gene that contains sections of non-coding DNA called exons (expressed as RNA and protein) interrupted by sections of DNA called introns

intron

a non-coding section of DNA within a split gene

exon

a coding section of DNA within a split gene

trimming

removal of non-coding sections at the beginning and end of mRNA

capping

the addition of methyl-guanine at the start of the trimmed mRNA

tailing

the addition of a long tail of adenines at the end of mRNA

tRNA

small segments of RNA that transport specific amino acids to the mRNA attached to a ribosome during translation

anticodon

triplet of nitrogen bases found on tRNA

rRNA

RNA component of a ribosome

polysome

chain of ribosomes that 'read' mRNA

translation

production of a polypeptide sequence from a sequence of mRNA codons

transcription factor

a protein produced by regulatory genes that controls gene expression

epigenome

a system of gene control outside of the DNA

heterochromatin

chromatin tightly coiled around histone proteins in the nucleosome

euchromatin

'relaxed' chromatin that contains genes that can be transcribed

histone modification

addition of chemical tags that vary the width between nucleosomes and determine whether or not a gene can be transcribed

DNA methylation

the addition of a chemical tag to the start of a structural gene to block transcription

dizygotic twins

each twin develops from a separate egg and each egg is fertilised by a separate sperm cell

fraternal twins

an alternative name for dizygotic twins

monozygotic twins

twins that develop from one zygote, which splits and forms two embryos; since they have the same genotype, they are considered to be identical

Hox protein

a transcription factor produced by a Hox gene

SRY gene

the gene on the Y chromosome responsible for initiating male sex determination in humans

mutation

small permanent change in the DNA of an organism

point mutation

a change in a single nucleotide in the DNA code that may result in translation of one different amino acid in a polypeptide sequence

somatic mutation

a mutation in the somatic tissue of an organism that affects the specific cell type by is not inherited

germ line mutation

a heritable change in the DNA that occurs in a germ cell or the zygote at the single-cell stage and so is incorporated in every cell of the body

frameshift mutation

the deletion or insertion of a single or non-multiple of three nucleotides into the DNA

chromosome mutation

a change in the chromosome structure or number, often due to an error in pairing during the crossing over stage of meiosis

homologs

two chromosomes that are homologous; during meiosis, the set of pairing maternal and paternal chromosomes; have the same genes at the same loci but may have different alleles