dna and genetics y10

heredity

the process by which traits are passed from parents to offspring

structure of dna

• Deoxyribonucleic acid is a double stranded molecule that forms a double helix shape

• dna is a polymer made up of many monomer units called nucleotides

nucleotides

• have three components:

• a pentose, deoxyribose sugar

• a nitrogenous base (ATGC)

• a phosphate group

complementary base pairings

• Adenine binds with thymine ( 2 hydrogen bonds)

• cytosine binds with guanine (3hydrogen bonds)

DNA

• provides a code for protein that organisms require to live

• deoxyribose

• ATGC - bases

• Stores and transfers genetic information

• found in nucleus, mitochondria

Gene

• a section of DNA which codes for the synthesis of proteins

Chromosome

a thread like structure of tightly wound DNA and proteins

RNA

• ribose

• AUGC (base)

• Messenger RNA, transfer RNA, ribosomal RNA

• odes for amino acids and makes proteins

• found in nucleus, cytoplasm

protein synthesis

• process of building proteins that perform a wide variety of functions in living organisms using the DNA code

• each gene is a section of DNA that codes for the production of a specific protein

• protein synthesis stars at a specific gene and can be split into two main stages - transcription, translation

Transcription

1. RNA polymerase breaks the hydrogen bonds between the two DNA strands (unzips)

2. template DNA strand is copied by another enzyme which builds a complementary mRNA

3. The DNA zips back up

4. the m RNA strand leaves the nucleus

Dna is far too large to leave the nucleus so only part of the DNA (gene) is unzipped and then copied into another nucleic acid

occurs inside the nucleus of cell

Translation

1. The mRNA molecule attaches to a ribosome

2. ribosomes read three bases at a time (3 bases = 1 codon)

3. each codon in the mRNA specifies which amino acid is added to the polypeptide chain

4. transfer RNA molecules pick up the specific amino acid in the cytoplasm and join them to the polypeptide chain at the ribosome (complementary to mRNA)

5. at the end(when the stop codon is reached) the mRNA will detach from the ribosome as will the finished protein

occurs in the cytoplasm

amino acid chart

• mRNA strands always should start with ‘start’ codon (A, U or G) which codes for the amino acid - methonine

• There are several different stop codons all of which don’t code for an amino acid

homologous chromosomes

• there are 46 chormosomes in every human body ( somatic) cell, 23 pairs

• in these cells, each chromosome contains a mathcing pair

• they are pairs of mathcing chromosomes as they have matching bonding patterns, genes and size

• for each homologous pair there is 1 inherited from mum and 1 inherited from dad.

chromosome

allele

• a version of a gene

autotsomes

• body chromosomes

• any chromosome that is not a sex chromosome

• any chromosome that is in pairs 1 to 22

heterosomes

• sex chromosomes

• chromosomes that determine the sex of an organism

• the 23rd pair in humans ( xx in femals, xy in males)

down syndrome

• an extra chromosome on the 21st pair

• trisomy 21 means an individual has 3 chromosomes instead of two for a homologous pair

replication in cells

• all oragnisms replicate ( increase in number) in order to grow

• DNA replication occurs in every cell before it divides

• it doubles the amount of DNA required before the cell divides

• sister chromatids ( which are genetically identical to each other) are joined at the centromere of the 46 chromosomes

• after DNA replication occurs, the cell may enter into cell division. (mitosis or MEIosis)

how does DNA replicate

1. DNA unwinds and unzips - catalysed by DNA helicase

2. ne nucleotides are added following complementary base pairing rules - catalysed by DNA polymerase

3. result is two identical strands of DNA

mitosis

• cell division of somatic cells

• somatic cells are diploid (2n) as they contain 2 sets of chromosomes, 23 from mum, 23 from dad

• occurs in humans for - growth, reapir and healing, tissue replacement ( from aging cells )

meiosis

• cell division in animals and plants that produces gametes ( sex cells )

• male gametes are sperm cells

• female gametes are ova (ovum singular)

• meiosis produces 4 daughter cells

• the new daughter cells have only one set of chromosomes in each - therefore they are know as haploid (n)

heredity 2

• the passing on of physical or mental characteristic genetically from one generation to another

• homologous chromosomes have the same genes located at the same location of the chromosome ( location of gene is called the locus)

• homologous chromosomes may have a different form of the gene (allele)

• e.g. alleles for brown eyes or blue eyes

genotype

• the combination of alleles an organism has for a particular gene

• if 2 identical alleles are present in a persons genotype, they are homozygous

• e.g. brown hair allele from mother and father

• if 2 different alleles are present for the same trait, the person is heterozygous for that trait

phenotype

• the observable characteristics of an organism

dominant 

• if this allele is present on either homologous chromosome, it’s physical appearance will be expressed ( masking the other allele)

recessive

• this allele must be present on both homologous chromosomes in order to be physically expressed ( otherwise it will be masked by the dominant allele)

what are the three possible genotype

• homozygous dominant

• heterozygous

• homozygous recessive

homozygous dominant

both chromosomes contain the dominant allele

BB

heterozygous

one chromosome contains the dominant allele, the other contains the recessive

Bb

homozygous recessive

both chromosomes contain the recessive allele

bb

sex linkage

• all eggs=x

• sperm=x or y

• males are at a much greater risk for inheriting sex-linked disorders because they only inherit one x, so if the x has the allele for the disorder, they will suffer from the disorder

codominance

• both alleles are expressed equally in the phenotype

• e.g. black chicken + white chicken =speckled

pedigree

• a pedigree chart shows the relationships between family members and indicates which individuals have certain genetic traits

autosomal dominance

males and females can be equally affected, yet all of the affected individuals must have at least one parent affected

autosomal recessive

• two unaffected parents can have an affected child

• males and females are equally affected

stages of mitosis

• interphase - chromosomes replicate( DNA replication), where cell spends most of its time, its job

• prophase- chromosomes shorten and condense and become visible

• metaphase - chromosomes align in the middle of cell ( equator), spindle fibres attach to the centromere

• anaphase - chromatid moves to opposite side of cell ( poles)

• telophase - nucleus of cell reforms

• cytokenesis - cell membrane reforming, 2 daughter cells that are genetically identical